diff --git a/.gitignore b/.gitignore
index a547bf3..9275b8b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,6 +12,14 @@ dist
 dist-ssr
 *.local
 
+# Supabase self-hosted
+docker-compose.yml
+.env
+.env.*
+!.env.example
+volumes/
+logs/
+
 # Editor directories and files
 .vscode/*
 !.vscode/extensions.json
diff --git a/ANALYSIS.md b/ANALYSIS.md
new file mode 100644
index 0000000..f7ada86
--- /dev/null
+++ b/ANALYSIS.md
@@ -0,0 +1,668 @@
+# padhle — Codebase Analysis & Feature Tracker
+
+## File Map
+
+| File | Purpose |
+|---|---|
+| `frontend/src/App.jsx` | Main orchestrator — owns all state, SSE chat logic, session CRUD, auth gating |
+| `frontend/src/styles/global.css` | Global CSS variables (colors, fonts, spacing, radius, shadows) |
+| `frontend/src/main.jsx` | React entry point — wraps App in `AuthProvider` |
+| `frontend/src/lib/auth/SupabaseAuth.jsx` | Auth context — `signIn`, `signUp`, `signOut`, `getToken` (all via backend) |
+| `frontend/src/lib/auth/backendAuth.js` | Backend auth client — `signup`, `signin`, `signout`, `me`, `check` |
+| `frontend/src/lib/auth/SignIn.jsx` | Sign-in/Sign-up modal form |
+| `frontend/src/lib/auth/SignIn.css` | Modal styles |
+| `frontend/src/components/selector-flow/SelectorFlow.jsx` | Generic step-based selector — grade, subject, chapter (grid layout, conditional icons) |
+| `frontend/src/components/selector-flow/SelectorFlow.css` | Grid layout for grade options |
+| `frontend/src/components/top-nav/TopNav.jsx` | Header with grade/subject/chapter dropdown pills + search/notifications/user avatar |
+| `frontend/src/components/top-nav/TopNav.css` | Pill buttons, dropdown menus, avatar, sign-out styles |
+| `frontend/src/components/sidebar/Sidebar.jsx` | Brand, New Chat button, subjects list, chat history list, user info, sign-out, upgrade card |
+| `frontend/src/components/sidebar/Sidebar.css` | Sidebar layout, hover indicators, chat item delete, upgrade card, user info |
+| `frontend/src/components/chat-history/ChatHistory.jsx` | Welcome screen with bento suggestions OR message list OR selector flow |
+| `frontend/src/components/chat-history/ChatHistory.css` | Welcome header, bento grid, info panel, message list |
+| `frontend/src/components/message/Message.jsx` | Single message bubble (user or assistant), action buttons, typing dots |
+| `frontend/src/components/message/Message.css` | Bubble styling, avatar, actions, typing animation |
+| `frontend/src/components/chat-input/ChatInput.jsx` | Textarea with auto-expand, mic, send button, attachment |
+| `frontend/src/components/chat-input/ChatInput.css` | Input bar styling, send button states, disclaimer |
+| `backend/src/index.js` | Express app setup, middleware, route registration |
+| `backend/src/middleware/cookies.js` | Cookie parser middleware (cookie-parser) |
+| `backend/src/middleware/supabaseAuth.js` | JWT verification middleware — reads from `padhle.token` httpOnly cookie |
+| `backend/src/routes/auth.js` | POST /api/auth/signup, /api/auth/signin, /api/auth/signout; GET /api/auth/me |
+| `backend/src/routes/chat.js` | POST /api/chat (stream), GET /api/chat/sessions, GET /api/chat/:id (load session+messages); validates metadata and enforces ownership |
+| `backend/src/routes/sessions.js` | GET/POST/DELETE/PATCH /api/sessions; explicit ownership checks |
+| `backend/src/services/db.js` | Supabase database service for profiles, sessions, and messages |
+| `backend/src/routes/chatValidation.js` | Pure chat input validation and session ownership helpers |
+| `backend/src/stores/sessionStore.js` | Legacy in-memory Map-based session + message store; no longer used by chat/session routes |
+| `frontend/src/appState.js` | Central default/reset state for auth transitions |
+| `backend/src/services/ai.js` | AI provider abstraction (OpenAI/Anthropic/Google), streaming, system prompt builder |
+
+---
+
+## Architecture
+
+```
+Frontend (React + Vite) ──SSE──▶ Backend (Express) ──API──▶ AI Provider
+     │                              │
+     ├─ State: activeGrade,        └─ Supabase Postgres service
+        activeSubject,                  └─ Explicit req.user.uid ownership checks
+        activeChapter,
+        messages,
+        selectorStep (grade → subject → chapter → chat)
+     │
+     └─ Auth: httpOnly cookies (padhle.token, padhle.user)
+        └─ Browser API requests use credentials: 'include'
+```
+
+---
+
+## Authentication — How It Works
+
+All authentication is handled server-side via httpOnly cookies. The frontend never sees raw tokens.
+
+### Flow
+
+```
+1. User enters email + password → Frontend POST /api/auth/signin
+2. Backend calls Supabase Auth API → receives JWT
+3. Backend sets httpOnly cookies:
+   - padhle.token = JWT (for API verification)
+   - padhle.user = JSON({ uid, email }) (safe user metadata)
+4. Frontend never sees the JWT — browser stores cookies automatically
+5. All API calls include cookies via credentials: 'include'
+6. Backend middleware reads padhle.token, verifies via /auth/v1/user
+7. If valid, req.user = { uid, email } is attached to request
+```
+
+### Auth Endpoints
+
+| Route | Method | Purpose | Auth Required |
+|---|---|---|---|
+| `/api/auth/signup` | POST | Create account via Supabase Auth | No |
+| `/api/auth/signin` | POST | Sign in via Supabase Auth | No |
+| `/api/auth/signout` | POST | Revoke session + clear cookies | Yes |
+| `/api/auth/me` | GET | Get current user from cookie | Yes |
+
+### Protected Routes
+
+| Route | Middleware | Purpose |
+|---|---|---|
+| `/api/chat` | supabaseAuth | Send message, load conversation |
+| `/api/sessions` | supabaseAuth | CRUD chat sessions |
+
+### Security
+
+- **httpOnly cookies**: JWT is never accessible to JavaScript
+- **sameSite: lax**: CSRF protection
+- **secure flag**: Enabled in production (HTTPS)
+- **JWT verification**: Each request calls Supabase Auth server to validate
+- **No localStorage tokens**: Sensitive data never stored in browser
+- **Session revocation**: Signout calls Supabase `/auth/v1/logout` server-side
+
+---
+
+## Grade Selector — Current State
+
+### Where it lives
+
+| Component | File | Role |
+|---|---|---|
+| `SelectorFlow` | `frontend/src/components/selector-flow/SelectorFlow.jsx` | Generic step-based selector — grade, subject, chapter (grid layout) |
+| `TopNav` grade pill | `frontend/src/components/top-nav/TopNav.jsx` (lines ~46-75) | Always-visible header dropdown |
+| State owner | `frontend/src/App.jsx` | `selectorStep` + `activeGrade/Subject/Chapter` state |
+| `ChatHistory` | `frontend/src/components/chat-history/ChatHistory.jsx` | Renders SelectorFlow when `selectorStep !== "chat"` |
+
+### Sequential selector flow
+
+```
+New Chat → selectorStep = "grade"
+  → Pick Grade → selectorStep = "subject" → fade-in
+    → Pick Subject → selectorStep = "chapter" → fade-in (subject-aware chapters)
+      → Pick Chapter → selectorStep = "chat" → welcome screen / messages
+```
+
+### State values
+
+| State | Type | Default | Stored in DB |
+|---|---|---|---|
+| `activeGrade` | label | `"Choose standard"` | `"Grade 10"` |
+| `activeSubject` | ID | `"choose-subject"` | `"math"` |
+| `activeChapter` | label | `"choose-chapter"` | `"Chapter 1: Rational Numbers"` |
+| `selectorStep` | enum | `"grade"` | — |
+
+| Value | Used in |
+|---|---|
+| `"Choose standard"` | Default for `activeGrade` |
+| `Grade 6` .. `Grade 12` | SelectorFlow + TopNav grade options |
+| `"math"` .. `"geography"` | `activeSubject` IDs (subjectItems) |
+| `"choose-subject"` | Default/placeholder for `activeSubject` |
+| `"choose-chapter"` | Default/placeholder for `activeChapter` |
+
+### Data flow
+
+```
+User clicks grade in SelectorFlow
+  → handleGradeSelect(item) → setActiveGrade(item.label) → pill shows label
+
+User clicks subject in SelectorFlow
+  → handleSubjectSelect(item) → setActiveSubject(item.id) → "math"
+    → getSubjectLabel("math") → "Mathematics" (for pill/placeholder display)
+    → handleChatSelect restores: data.session.subject = "math" → matches subject.id
+
+User clicks subject in Sidebar
+  → onSubjectChange(id) → setActiveSubject(id) → resets selectorStep to "subject"
+
+User clicks subject in TopNav dropdown
+  → selectOption("subject", subject.id) → onSubjectChange(id)
+    → setActiveSubject(id) → subject-aware chapter dropdown updates
+
+User clicks grade/chapter in TopNav
+  → selectOption("grade", grade.label) / selectOption("chapter", chapter.label)
+    → directly updates state
+```
+
+### Subject label resolution (central pattern)
+
+```
+activeSubject (stores ID) = "math"
+  → getSubjectLabel("math") → "Mathematics"
+    → TopNav pill: resolvedSubjectLabel
+    → ChatInput placeholder: `${getSubjectLabel(activeSubject)}`
+    → SelectorFlow subtitle: `Subject – ${getSubjectLabel(activeSubject)}`
+    → TopNav dropdown: activeSubject === subject.id → highlights Mathematics
+    → Sidebar: activeSubject === subject.id → highlights Mathematics
+```
+
+### Chapter label resolution
+
+```
+activeChapter (stores ID/label) = "choose-chapter" → getChapterLabel("choose-chapter") → "Choose Chapter"
+activeChapter (stored label) = "Chapter 1: Rational Numbers" → getChapterLabel(...) → "Chapter 1: Rational Numbers"
+  → TopNav pill: resolvedChapterLabel
+  → ChatInput placeholder: `${getChapterLabel(activeChapter)}`
+```
+
+### TopNav chapter dropdown (subject-aware)
+
+```
+chapterData[activeSubject] → chapterOptions array
+  → ["Choose Chapter", ...chaptersFromSubject]
+  → Changes dynamically when activeSubject changes
+  → Previously: static list mixing chapters from all subjects
+```
+
+### How grade is used
+
+- Sent in `POST /api/chat` body → `grade` field (only when not default)
+- Used in AI system prompt via `buildSystemPrompt()` → `"You are teaching students in grade X"`
+- Displayed in chat input placeholder: `"Ask anything about {grade} {subject} – {chapter}..."`
+- Restored when loading a session from `data.session.grade`
+
+### SelectorFlow component
+- Generic: accepts `title`, `subtitle`, `items`, `selectedValue`, `onSelect`, `step`, `compareField` props
+- `compareField` determines which property to compare against `selectedValue` for active highlighting:
+  - `"label"` → `selectedValue === item.label` (used for grades and chapters)
+  - `"id"` → `selectedValue === item.id` (used for subjects)
+- Renders any item type via 3-column grid with fade-in animation
+- Conditional icon rendering: `item.icon && (<span>icon</span>)`
+- Fade-in animation on step change via `key={step}` — CSS `@keyframes selectorFadeIn` with `opacity` + `translateY(16px→0)` over 0.35s
+
+### Selector data
+- **Grades**: 7 options (Grade 6–12), each with icon (Material Symbols)
+- **Subjects**: 6 options (Math, Science, History, Language, English, Geography) — label only, no icons
+- **Chapters**: 6 per subject, subject-aware via `chapterData` object in App.jsx — label only, no icons
+- **Chapter mapping**: `chapterData[subjectId]` → array of chapters (e.g. math → Rational Numbers, Linear Equations, etc.)
+
+### Current features ✅
+
+- Sequential selector flow: grade → subject → chapter → chat (fade-in transitions)
+- Grade selection via SelectorFlow (new-chat grid)
+- Grade selection via TopNav (header pill dropdown)
+- Subject selection in SelectorFlow (label-only, no icons)
+- Chapter selection in SelectorFlow (subject-aware, label-only, no icons)
+- Grade persists across session restore
+- Grade sent to AI backend for context-aware responses
+- 7 grades available (Grade 6–12)
+- Upgrade-to-Pro card hidden via `showUpgrade={false}` prop
+
+---
+
+## Feature Tracker
+
+### Grade Selector
+- **Status**: Working ✅
+- **Verified**: Yes — hover effect confirmed
+- **Last updated**: 2026-01-18
+
+### Hover effect (✅ verified)
+- Subtle lift: `translateY(-2px)`
+- Box-shadow pop via `--shadow-card-hover`
+- Smooth transition on transform and shadow only
+- Press-down feel on `:active` (`translateY(0)`)
+- File: `frontend/src/components/selector-flow/SelectorFlow.css`
+
+### Sequential selector flow (✅ verified)
+- Steps: grade → subject → chapter → chat (via `selectorStep` state)
+- Fade-in animation on each step change: `opacity 0→1` + `translateY(16px→0)` over 0.35s
+- Triggered by `key={step}` on `.selector-flow` — forces React re-mount
+- Subject-aware chapters: `chapterData[subjectId]` returns relevant chapters
+- File: `frontend/src/App.jsx` (selector data + step logic), `frontend/src/components/chat-history/ChatHistory.jsx` (conditional rendering)
+
+### Subject ID/label resolution pattern (✅ verified in browser)
+- `activeSubject` now stores **IDs** (e.g. `"math"`, `"science"`) instead of labels
+- Default is `"choose-subject"` (a reserved placeholder ID)
+- `getSubjectLabel(id)` resolves IDs to display names: `"math"` → `"Mathematics"`
+- Used in: TopNav pill text, ChatInput placeholder, SelectorFlow subtitle, `handleSend` body
+- `handleSubjectSelect` stores `item.id` (not `item.label`)
+- `handleChatSelect` restoration: backend returns `subject: "math"` which matches `subject.id`
+- **TopNav**: uses `subjectItems` prop; `activeSubject === subject.id` comparison works; pill uses `resolvedSubjectLabel`
+- **TopNav chapter dropdown**: uses `chapterData` prop; generates `chapterOptions[activeSubject]` dynamically (subject-aware)
+- **Sidebar**: `activeSubject === subject.id` comparison now works correctly; sidebar subject change resets selector to `subject` step
+- **SelectorFlow**: `compareField="id"` for subjects, `compareField="label"` for grades/chapters
+- **ChatInput placeholder**: uses `getSubjectLabel(activeSubject)` and `getChapterLabel(activeChapter)` for full label resolution
+- Files: `App.jsx`, `TopNav.jsx`, `Sidebar.jsx`, `SelectorFlow.jsx`, `ChatInput.jsx`
+
+### Chapter pill resolution (✅ verified in browser)
+- `activeChapter` stores chapter labels (e.g. `"Chapter 1: Rational Numbers"`) after selection
+- Default is `"choose-chapter"` (placeholder ID)
+- `getChapterLabel(chapterId)` resolves placeholder: `"choose-chapter"` → `"Choose Chapter"`
+- Used in: TopNav pill, ChatInput placeholder
+- Files: `App.jsx`, `TopNav.jsx`
+
+### TopNav subject-aware chapter dropdown (✅ verified in browser)
+- Replaced static 6-item chapters array in TopNav with dynamic `chapterOptions` generated from `chapterData[activeSubject]`
+- Shows subject-specific chapters based on current selection:
+  - Math → Rational Numbers, Linear Equations, Coordinate Geometry, Algebra, Geometry, Trigonometry
+  - Science → Nutrition in Plants, Photosynthesis, Human Physiology, etc.
+  - (all 6 subjects × 6 chapters)
+- Dropdown placeholder highlights correctly when `activeChapter` is `"choose-chapter"`
+- Files: `TopNav.jsx` (chapterOptions generation, chapterData prop), `App.jsx` (chapterData pass-through)
+
+### Upgrade-to-Pro card hidden (✅ verified)
+- `Sidebar` component accepts `showUpgrade` prop (default `true`)
+- `App.jsx` passes `showUpgrade={false}`
+- Card is wrapped in `{showUpgrade && (...)}` conditional
+- Can be re-enabled by setting `showUpgrade={true}`
+- File: `frontend/src/components/sidebar/Sidebar.jsx`, `frontend/src/App.jsx`
+
+---
+
+## Supabase
+
+### Installation
+
+Supabase CLI (local dev via `npx supabase start`).
+- **Project path**: `supabase/` (CLI-managed)
+- **Start**: `npx supabase start`
+- **Stop**: `npx supabase stop`
+- **Status**: `npx supabase status`
+- **Services**: Postgres, GoTrue (Auth), PostgREST (REST), Storage, Supavisor (pooler), Realtime, Edge Runtime, Studio (dashboard), Kong (API gateway), Mailpit (email testing), Analytics (Logflare), Vector (logging)
+
+### Dashboard (Studio)
+
+| Setting | Value |
+|---|---|
+| **URL** | `http://localhost:54323` |
+| **Mailpit** | `http://localhost:54324` |
+
+### API Keys (local dev)
+
+| Key | Value |
+|---|---|
+| **Publishable Key** (client-side) | `sb_publishable_ACJWlzQHlZjBrEguHvfOxg_3BJgxAaH` |
+| **Secret Key** (server-side) | `sb_secret_N7UND0UgjKTVK-Uodkm0Hg_xSvEMPvz` |
+
+> **Never expose the Secret Key in client code.** Use the publishable key in frontend, secret key in backend.
+> Local dev uses shared default keys — **do not use in production**.
+
+### Database
+
+| Setting | Value |
+|---|---|
+| **Host** | `localhost` |
+| **Port** | `54322` |
+| **Database** | `postgres` |
+| **User** | `postgres` |
+| **Password** | `postgres` |
+
+**Connection string:**
+```bash
+psql 'postgres://postgres:postgres@localhost:54322/postgres'
+```
+
+### API Endpoints (via Kong gateway)
+
+| Service | Endpoint |
+|---|---|
+| **REST** | `http://localhost:54321/rest/v1/` |
+| **Auth** | `http://localhost:54321/auth/v1/` |
+| **Storage** | `http://localhost:54321/storage/v1/` |
+| **Realtime** | `http://localhost:54321/realtime/v1/` |
+| **GraphQL** | `http://localhost:54321/graphql/v1` |
+| **Edge Functions** | `http://localhost:54321/functions/v1/` |
+| **MCP** | `http://localhost:54321/mcp` |
+
+### Supabase MCP
+
+- **Endpoint**: `http://localhost:54321/mcp`
+- **Mode**: Read-only (local dev)
+- **Auth**: Local (no cloud OAuth)
+- **Tools**: 10 (database, debugging, development, docs)
+- **Config**: `~/.pi/agent/supabase.json` (callback port 54326 — default 54324 conflicts with Mailpit)
+- **CLI**: `pi install npm:pi-supabase` → then `supabase_mcp_connect()`
+
+### Database State
+
+- **Custom tables**: `profiles`, `sessions`, and `messages` exist in the connected local Supabase database
+- **Auth tables**: Supabase-managed (`auth.users`, `auth.sessions`, `auth.refresh_tokens`, etc.)
+- **RLS**: Enabled on custom tables and auth tables
+- **Permissions (FIXED)**: API roles lacked table grants, causing PostgreSQL `42501 permission denied`. Fixed via grant to `anon`/`authenticated`/`service_role`; tracked in `supabase/migrations/20260819120000_grant_api_role_table_access.sql`, which also sets `ALTER DEFAULT PRIVILEGES` so future tables inherit the grants.
+- **Profiles (control)**: `profiles` has `role` (default `user`) and `is_banned` (default `false`) columns. A trigger on `auth.users` auto-creates a profile row on any user creation (signup or admin). `authenticated` may update only `display_name`/`avatar_url` — it cannot change `role`/`is_banned`; `service_role` has full control. Migration: `supabase/migrations/20260819130000_profiles_control_and_trigger.sql`.
+- **Signups**: enabled (email verification not yet wired to the app).
+- **User data**: all previous test accounts were deleted (2026-08-19). `auth.users`/`profiles` are empty; no admin user yet. Admin user is added manually in Studio (Auth → Add user, email_confirm on) and then promoted to `role='admin'`.
+- **Verified**: self-signup and admin-created users both auto-create a `profiles` row with `role:'user'`; the role is_banned self-edit is blocked at the column-privilege level.
+
+### Environment Files
+
+- `supabase/config.toml` — CLI project config
+- `supabase/.temp/` — CLI-generated files (do not commit)
+
+---
+
+## Global Design Tokens (global.css)
+
+| Token | Value |
+|---|---|
+| `--color-primary` | `#000000` |
+| `--color-on-primary` | `#ffffff` |
+| `--color-background` | `#fcf8fa` |
+| `--color-surface-lowest` | `#ffffff` |
+| `--color-on-surface` | `#111827` |
+| `--color-on-surface-variant` | `#6b7280` |
+| `--font-heading` | `"Montserrat", sans-serif` |
+| `--font-body` | `"Inter", system-ui, sans-serif` |
+| `--sidebar-width` | `280px` |
+| `--container-max` | `800px` |
+
+---
+
+## Recent Implementation
+
+### Backend Authentication (completed 2026-08-18)
+
+The sign-in/sign-out flow was moved from the frontend (direct Supabase Auth calls) to the backend (cookie-based proxy).
+
+#### What was changed
+
+| File | Change |
+|---|---|
+| `backend/src/routes/auth.js` | New — `POST /api/auth/signup`, `/signin`, `/signout`; `GET /api/auth/me` |
+| `backend/src/middleware/supabaseAuth.js` | Updated — reads JWT from `padhle.token` httpOnly cookie instead of `Authorization` header |
+| `backend/src/index.js` | Updated — added `cookie-parser` middleware; applied `supabaseAuth` to `/api/chat` and `/api/sessions` |
+| `backend/src/stores/sessionStore.js` | Updated — stores `userId` per session; `listSessions(userId)` filters by user |
+| `frontend/src/lib/auth/backendAuth.js` | New — frontend auth client (`signup`, `signin`, `signout`, `me`, `check`) |
+| `frontend/src/lib/auth/SupabaseAuth.jsx` | Updated — removed all `supabase.auth.*` calls; now uses backend client |
+
+#### How it works
+
+```
+1. User enters email + password → Frontend POST /api/auth/signin
+2. Backend calls Supabase Auth API → receives JWT
+3. Backend sets httpOnly cookies:
+   - padhle.token = JWT (for API verification)
+   - padhle.user = JSON({ uid, email }) (safe user metadata)
+4. Frontend never sees the JWT — browser stores cookies automatically
+5. All API calls include cookies via `credentials: 'include'`
+6. Backend middleware reads `padhle.token`, verifies via `/auth/v1/user`
+7. If valid, `req.user = { uid, email }` is attached to request
+```
+
+#### Verification
+- ✅ Build passes (Vite 6.4.3)
+- ✅ Sign-in → user authenticated, sees main app
+- ✅ Sign-out → cookies cleared, back to sign-in modal
+- ✅ Protected routes (`/api/sessions`) properly require auth (401 on missing cookie)
+- ✅ `padhle.token` cookie confirmed as `HttpOnly` in browser DevTools
+- ✅ No `localStorage` tokens used anywhere in frontend
+
+---
+
+## Security Audit — Vulnerabilities & Fixes
+
+An audit was performed on all backend files on 2026-08-18.
+
+### 🔴 Critical — Fixed
+
+#### CVE-2026-011: Cross-user identity leak via truncated token cache key
+**File:** `backend/src/middleware/supabaseAuth.js`
+
+**Problem:** `verifyToken` cached verified users keyed by `token.substring(0, 50)`. Every JWT from the same GoTrue instance shares the header (including `kid`) and the opening claims, so users A and B produced **identical 50-char fingerprints**. B's request then hit A's cached entry and `req.user.uid` resolved to A — so B's `/api/sessions` returned A's sessions/history. Reproduced end-to-end: two fresh users, A created a session, B listed A's session.
+
+**Fix:** Key the cache by the **full token**, which is unique per user:
+```js
+const cached = tokenCache.get(token);            // full token
+...
+tokenCache.set(token, { ...result, expires });  // full token
+```
+Removed the `FINGERPRINT_LEN` constant. `listSessions`/RLS filtering was already correct; the leak was purely identity resolution.
+
+**Verified:** On fixed code, A has 1 session, B has 0; no cross-user leak. (Note: the running backend on :3001 must be restarted to load the fix — it runs `node src/index.js` without `--watch`.)
+
+
+#### CVE-2026-001: IDOR on `GET /api/chat/:chatId`
+**File:** `backend/src/routes/chat.js`
+
+**Problem:** The route checks `supabaseAuth` middleware (so user is authenticated), but never verifies the session belongs to `req.user.uid`. Any authenticated user can read another user's conversation history by guessing a `chatId`.
+
+**Fix:** Add ownership check before returning session data:
+```js
+router.get("/:chatId", (req, res) => {
+  const session = getSession(req.params.chatId);
+  if (!session || session.userId !== req.user.uid) {
+    return res.status(404).json({ error: "Session not found" });
+  }
+  // ... rest of route
+});
+```
+Also apply to `GET /api/chat/sessions` to filter by `req.user.uid`.
+
+---
+
+#### CVE-2026-002: No rate limiting on auth endpoints
+**File:** `backend/src/routes/auth.js`
+
+**Problem:** Zero throttling on signup/signin/signout. Allows unlimited password brute-force, account enumeration via signup attempts, and session flooding.
+
+**Fix:** Apply `express-rate-limit` to auth routes:
+```js
+import rateLimit from 'express-rate-limit';
+const authLimiter = rateLimit({ windowMs: 15 * 60 * 1000, max: 20 });
+router.post('/signup', authLimiter, ...);
+router.post('/signin', authLimiter, ...);
+router.post('/signout', authLimiter, ...);
+```
+
+---
+
+#### CVE-2026-003: SSE stream injection — untrusted data in `data: ` prefix
+**File:** `backend/src/routes/chat.js`
+
+**Problem:** User messages (AI responses built from user input) are written raw into SSE events:
+```js
+res.write(`data: ${JSON.stringify({ type: "chunk", content: chunk })}\n\n`);
+```
+If `chunk` contains `}\n\n` or other control characters, it breaks the SSE stream and can inject fake events.
+
+**Fix:** Sanitize the JSON output before writing:
+```js
+const safe = JSON.stringify({ type: "chunk", content: chunk })
+  .replace(/\n/g, '\\n')
+  .replace(/\u2028/g, '\\u2028')
+  .replace(/\u2029/g, '\\u2029');
+res.write(`data: ${safe}\n\n`);
+```
+
+---
+
+#### CVE-2026-004: Google Gemini API URL path break
+**File:** `backend/src/services/ai.js`
+
+**Problem:** API key concatenated into URL path — if the key contains `/` or `..`, the URL breaks:
+```js
+const url = `${GOOGLE_BASE_URL}${apiKey}/chat/models/...`;
+// = https://generativelanguage.googleapis.com/v1beta/openai/{key}/chat/...  ← wrong
+```
+
+**Fix:** Use `URL` constructor to build safely:
+```js
+const url = new URL(`chat/models/${model}:streamGenerateContent`, GOOGLE_BASE_URL);
+url.searchParams.set('key', apiKey);
+```
+
+---
+
+### 🟡 High — Fixes Applied
+
+#### CVE-2026-005: Supabase error details leaked to client
+**File:** `backend/src/routes/auth.js`
+
+**Problem:** `data.error_description` from Supabase returns detailed error text (e.g., "Email already registered") that reveals system behavior.
+
+**Fix:** Return generic messages from the backend; log raw errors server-side only:
+```js
+// Log the real error
+console.error(`Auth error: ${data.error_description}`);
+// Return generic message
+return res.status(400).json({ error: "Sign up failed" });
+```
+
+---
+
+#### CVE-2026-006: No timeout on Supabase JWT verification HTTP calls
+**File:** `backend/src/middleware/supabaseAuth.js`
+
+**Problem:** Every request to a protected route makes an HTTP call to `Supabase /auth/v1/user` with no timeout. If Supabase is slow or unreachable, the Express server hangs indefinitely.
+
+**Fix:** Add `AbortSignal` timeout:
+```js
+const controller = new AbortController();
+const timeout = setTimeout(() => controller.abort(), 3000);
+const res = await fetch(url, { headers, signal: controller.signal });
+clearTimeout(timeout);
+```
+
+---
+
+#### CVE-2026-007: No token caching — every request hits Supabase
+**File:** `backend/src/middleware/supabaseAuth.js`
+
+**Problem:** Every single request makes an HTTP call to verify the token. A token is valid for 3600s; it should be cached.
+
+**Fix:** Cache verified tokens in a `Map` with TTL:
+```js
+const tokenCache = new Map(); // tokenFingerprint -> { uid, email, expires }
+const FINGERPRINT_TTL = 5 * 60 * 1000;
+const TOKEN_EXPIRY_TTL = 3600 * 1000;
+
+export async function verifyToken(token) {
+  const fp = token.substring(0, 50);
+  const cached = tokenCache.get(fp);
+  if (cached && Date.now() < cached.expires) {
+    return cached;
+  }
+  // ... verify via Supabase, then:
+  tokenCache.set(fp, { uid, email, expires: Date.now() + FINGERPRINT_TTL });
+  return { uid, email };
+}
+```
+
+---
+
+### 🟢 Medium — Fixes Applied
+
+#### CVE-2026-008: No input length limits on chat messages
+**File:** `backend/src/routes/chat.js`
+
+**Problem:** No max on `text` length. Can fill the in-memory store or blow up the AI context window.
+
+**Fix:** Add length check:
+```js
+if (!text || !text.trim()) return 400;
+if (text.length > 10000) return 400;
+```
+
+---
+
+#### CVE-2026-009: No XSS sanitization on stored messages
+**File:** `backend/src/stores/sessionStore.js`
+
+**Problem:** User messages stored and returned raw. If the frontend renders them as HTML, user input containing `<script>` tags is a stored XSS vector.
+
+**Fix:** Backend should at least flag or truncate dangerous content:
+```js
+const XSS_PATTERN = /[<>"'&]/;
+if (XSS_PATTERN.test(message.text)) {
+  message.text = message.text.replace(/[<>]/g, (c) => c === '<' ? '&lt;' : '&gt;');
+}
+```
+
+---
+
+#### CVE-2026-0010: AI keys exposed in process environment
+**File:** `backend/src/services/ai.js`
+
+**Problem:** API keys read from `process.env` and used in `fetch()` calls. On some systems they appear in `ps aux`.
+
+**Fix:** Not a code fix — ensure `.env` is in `.gitignore`, use secret managers in production, and avoid `console.log` of config values.
+
+---
+
+## Current Implementation Status — 2026-08-19
+
+### Completed and verified
+
+- Backend cookie-based authentication remains implemented; frontend never reads JWTs.
+- Auth, chat, and session rate limiters are installed and mounted.
+- Auth rate-limit boundary verified: requests 1–5 returned validation responses; request 6 returned HTTP 429.
+- Chat validation rejects empty text, text over 10,000 characters, invalid grades, and invalid subjects.
+- Chat/session ownership checks are enforced against `req.user.uid`.
+- `GET /api/chat/sessions` is registered before `GET /api/chat/:chatId`, preventing `sessions` from being interpreted as a chat ID.
+- Frontend auth transitions clear user-scoped conversation state and abort in-flight requests.
+- Frontend API requests include `credentials: "include"` for httpOnly cookies.
+- Added regression tests: `backend/test/chatValidation.test.js` and `frontend/test/appState.test.js`.
+- Backend tests, backend syntax checks, frontend regression test, and Vite production build pass.
+- Chrome MCP verified the sign-in screen renders after fixing a missing `useAuth` import exposed by the browser console.
+
+### Database status
+
+- Supabase API is reachable at `http://127.0.0.1:54321`.
+- The `profiles`, `sessions`, and `messages` tables exist.
+- Table grants were missing, causing `42501 permission denied`. **Fixed and verified** (see Database State above); migration tracked in `supabase/migrations/20260819120000_grant_api_role_table_access.sql`.
+
+---
+
+## What's Next
+
+### 1. Frontend chat-flow design gap
+- Backend does not return the new `session.id` to the client (SSE only sends `chunk`/`done`/`error`).
+- Sidebar refresh (`loadSessions()`) only runs on the successful SSE path, so an AI error leaves the sidebar stale even though the session row is created.
+- Decide: return `session.id` in SSE + refresh sidebar on error path (Option 1), or only persist sessions after a successful AI response (Option 2).
+
+### 2. Frontend authentication handling
+- Handle protected-route HTTP 401 responses by returning to the sign-in modal.
+- Persist selector state per user.
+
+### AI Provider — OpenRouter (connected 2026-08-19)
+
+- Backend uses the OpenAI-compatible path (`AI_PROVIDER=openai`) pointed at OpenRouter.
+- `OPENAI_BASE_URL=https://openrouter.ai/api/v1` (added to `ai.js` as an optional override).
+- Model: `~deepseek/deepseek-v4-flash-latest` (the `~` prefix is REQUIRED — it is OpenRouter's "redirect to latest" alias; the non-tilde `deepseek/deepseek-v4-flash-latest` is an invalid model ID).
+- Verified end-to-end: `/api/chat` streams a real reply and persists both `user` and `assistant` messages (lost messages bug from the no-AI path is gone).
+- Note: this is a reasoning model — it streams thinking in `delta.reasoning` before `delta.content`; the app reads `delta.content`, so there is a brief (sub-second) gap while reasoning, then the answer streams.
+
+**Env (non-secret):** wire via `backend/.env`, and the placeholders are in `backend/.env.example`.
+
+**Secrets:** `backend/.env` contains the real `OPENAI_API_KEY` (OpenRouter) plus `SUPABASE_SERVICE_ROLE_KEY`. Both are git-ignored; never commit. Rotate if exposed.
+
+### Operational hardening
+- Ensure `.env` secrets are never committed or exposed.
+- Rotate any secret key that has been shared outside the local environment.
+- Add monitoring for rate-limit hits and database errors.
diff --git a/IMPLEMENTATION_PLAN.md b/IMPLEMENTATION_PLAN.md
new file mode 100644
index 0000000..2c72351
--- /dev/null
+++ b/IMPLEMENTATION_PLAN.md
@@ -0,0 +1,661 @@
+# padhle — Implementation Plan
+## Rate Limiting + Supabase Integration + Session Bug Fix
+
+**Date:** 2026-08-19  
+**Total Estimated Time:** ~3 hours  
+**Status:** Ready to implement
+
+---
+
+## Issues to Address
+
+### 🔴 Critical: Session Data Leak (User A ↔ User B)
+**Symptoms:**
+- User A logs in → chats with AI → logs out
+- User B logs in → sidebar empty (correct)
+- BUT User B sees User A's conversation on main screen ❌
+
+**Root Cause:**
+- Frontend `activeChat` state not cleared on sign-out
+- In-memory backend store allows session ID guessing
+- Stale `messages` state renders after User A logs out
+
+---
+
+### 🟡 Security: No Rate Limiting
+- Auth endpoints (`/signup`, `/signin`) vulnerable to brute-force
+- No per-IP throttling
+
+---
+
+### 🟠 Architecture: In-Memory Sessions
+- Sessions lost on server restart
+- Not tied to database — only scoped via cookies
+- Need Supabase Postgres for persistence
+
+---
+
+## Implementation Phases
+
+### **PHASE 1: Rate Limiting by IP (30 min)**
+
+**Goal:** Prevent brute-force attacks on auth endpoints
+
+#### 1.1 Install dependency
+```bash
+cd backend
+npm install express-rate-limit
+```
+
+#### 1.2 Create middleware file
+**File:** `backend/src/middleware/rateLimiter.js` (NEW)
+
+```js
+import rateLimit from 'express-rate-limit';
+
+export const authLimiter = rateLimit({
+  windowMs: 15 * 60 * 1000,  // 15 minutes
+  max: 5,                     // 5 requests per window per IP
+  message: { error: 'Too many auth attempts. Try again later.' },
+  standardHeaders: false,
+  skip: (req) => process.env.NODE_ENV !== 'production' && req.ip === '::1',
+});
+
+export const chatLimiter = rateLimit({
+  windowMs: 60 * 1000,  // 1 minute
+  max: 30,              // 30 requests per minute
+  message: { error: 'Rate limit exceeded. Try again later.' },
+  standardHeaders: false,
+});
+
+export const sessionsLimiter = rateLimit({
+  windowMs: 60 * 1000,  // 1 minute
+  max: 20,              // 20 requests per minute
+  message: { error: 'Rate limit exceeded. Try again later.' },
+  standardHeaders: false,
+});
+```
+
+#### 1.3 Apply to auth routes
+**File:** `backend/src/routes/auth.js` (MODIFY top of file)
+
+Add import:
+```js
+import { authLimiter } from '../middleware/rateLimiter.js';
+```
+
+Then apply to each route:
+```js
+router.post('/signup', authLimiter, async (req, res) => { ... });
+router.post('/signin', authLimiter, async (req, res) => { ... });
+router.post('/signout', authLimiter, async (req, res) => { ... });
+```
+
+#### 1.4 Apply to other routes
+**File:** `backend/src/index.js` (MODIFY)
+
+Add imports:
+```js
+import { chatLimiter, sessionsLimiter } from './middleware/rateLimiter.js';
+```
+
+Update middleware order:
+```js
+app.use('/api/chat', chatLimiter, supabaseAuth, chatRoutes);
+app.use('/api/sessions', sessionsLimiter, supabaseAuth, sessionRoutes);
+```
+
+#### 1.5 Test
+```bash
+cd backend && npm start
+
+# Test rate limiting (6th request should fail)
+for i in {1..7}; do
+  curl -X POST http://localhost:3001/api/auth/signup \
+    -H 'Content-Type: application/json' \
+    -d "{\"email\": \"test$i@example.com\", \"password\": \"test\"}"
+  echo ""
+done
+# Expect 429 on 6th attempt
+```
+
+---
+
+### **PHASE 2: Supabase Database Integration (90 min)**
+
+**Goal:** Move sessions & messages from in-memory to Supabase Postgres with RLS
+
+#### 2.1 Create database migration
+**File:** `backend/supabase/migrations/{timestamp}_create_sessions_messages.sql` (NEW)
+
+```sql
+-- sessions table
+CREATE TABLE public.sessions (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  user_id UUID NOT NULL REFERENCES auth.users(id) ON DELETE CASCADE,
+  grade TEXT,
+  subject TEXT,
+  chapter TEXT,
+  preview TEXT,
+  created_at TIMESTAMPTZ DEFAULT now(),
+  updated_at TIMESTAMPTZ DEFAULT now()
+);
+
+-- messages table
+CREATE TABLE public.messages (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  session_id UUID NOT NULL REFERENCES public.sessions(id) ON DELETE CASCADE,
+  role TEXT NOT NULL CHECK (role IN ('user', 'assistant')),
+  text TEXT NOT NULL,
+  created_at TIMESTAMPTZ DEFAULT now()
+);
+
+-- profiles table (for future use)
+CREATE TABLE public.profiles (
+  id UUID PRIMARY KEY REFERENCES auth.users(id) ON DELETE CASCADE,
+  email TEXT NOT NULL,
+  display_name TEXT,
+  avatar_url TEXT,
+  created_at TIMESTAMPTZ DEFAULT now(),
+  updated_at TIMESTAMPTZ DEFAULT now()
+);
+
+-- Indexes
+CREATE INDEX idx_sessions_user_id ON public.sessions(user_id);
+CREATE INDEX idx_messages_session_id ON public.messages(session_id);
+
+-- Enable RLS
+ALTER TABLE public.sessions ENABLE ROW LEVEL SECURITY;
+ALTER TABLE public.messages ENABLE ROW LEVEL SECURITY;
+ALTER TABLE public.profiles ENABLE ROW LEVEL SECURITY;
+
+-- RLS: sessions
+CREATE POLICY "Users view own sessions"
+  ON public.sessions FOR SELECT
+  USING (auth.uid() = user_id);
+
+CREATE POLICY "Users create sessions"
+  ON public.sessions FOR INSERT
+  WITH CHECK (auth.uid() = user_id);
+
+CREATE POLICY "Users update own sessions"
+  ON public.sessions FOR UPDATE
+  USING (auth.uid() = user_id)
+  WITH CHECK (auth.uid() = user_id);
+
+CREATE POLICY "Users delete own sessions"
+  ON public.sessions FOR DELETE
+  USING (auth.uid() = user_id);
+
+-- RLS: messages
+CREATE POLICY "Users view own messages"
+  ON public.messages FOR SELECT
+  USING (session_id IN (SELECT id FROM public.sessions WHERE user_id = auth.uid()));
+
+CREATE POLICY "Users insert own messages"
+  ON public.messages FOR INSERT
+  WITH CHECK (session_id IN (SELECT id FROM public.sessions WHERE user_id = auth.uid()));
+
+CREATE POLICY "Users delete own messages"
+  ON public.messages FOR DELETE
+  USING (session_id IN (SELECT id FROM public.sessions WHERE user_id = auth.uid()));
+
+-- RLS: profiles
+CREATE POLICY "Users view own profile"
+  ON public.profiles FOR SELECT
+  USING (auth.uid() = id);
+
+CREATE POLICY "Users update own profile"
+  ON public.profiles FOR UPDATE
+  USING (auth.uid() = id)
+  WITH CHECK (auth.uid() = id);
+```
+
+#### 2.2 Apply migration
+```bash
+cd /path/to/padhle
+npx supabase db push
+```
+
+Verify in Supabase Studio (http://localhost:54323):
+- Tables exist: `sessions`, `messages`, `profiles`
+- RLS enabled on all three
+- Policies listed under each table
+
+#### 2.3 Create database service
+**File:** `backend/src/services/db.js` (NEW)
+
+```js
+import { createClient } from '@supabase/supabase-js';
+import dotenv from 'dotenv';
+
+dotenv.config();
+
+const SUPABASE_URL = process.env.SUPABASE_URL;
+const SUPABASE_SERVICE_KEY = process.env.SUPABASE_SERVICE_ROLE_KEY;
+
+const supabase = createClient(SUPABASE_URL, SUPABASE_SERVICE_KEY);
+
+export async function createSession(userId, grade, subject, chapter) {
+  const { data, error } = await supabase
+    .from('sessions')
+    .insert({
+      user_id: userId,
+      grade: grade || 'General',
+      subject: subject || 'General',
+      chapter: chapter || 'General',
+      preview: 'New Chat',
+    })
+    .select()
+    .single();
+
+  if (error) throw new Error(`Failed to create session: ${error.message}`);
+  return data;
+}
+
+export async function getSession(sessionId) {
+  const { data, error } = await supabase
+    .from('sessions')
+    .select('*')
+    .eq('id', sessionId)
+    .single();
+
+  if (error && error.code === 'PGRST116') return null;
+  if (error) throw new Error(`Failed to get session: ${error.message}`);
+  return data;
+}
+
+export async function listSessions(userId) {
+  const { data, error } = await supabase
+    .from('sessions')
+    .select('*')
+    .eq('user_id', userId)
+    .order('updated_at', { ascending: false });
+
+  if (error) throw new Error(`Failed to list sessions: ${error.message}`);
+  return data || [];
+}
+
+export async function getMessages(sessionId) {
+  const { data, error } = await supabase
+    .from('messages')
+    .select('*')
+    .eq('session_id', sessionId)
+    .order('created_at', { ascending: true });
+
+  if (error) throw new Error(`Failed to get messages: ${error.message}`);
+  return data || [];
+}
+
+export async function addMessage(sessionId, role, text) {
+  const { data: messageData, error: msgError } = await supabase
+    .from('messages')
+    .insert({ session_id: sessionId, role, text })
+    .select()
+    .single();
+
+  if (msgError) throw new Error(`Failed to add message: ${msgError.message}`);
+
+  const { error: updateError } = await supabase
+    .from('sessions')
+    .update({
+      updated_at: new Date().toISOString(),
+      preview: role === 'user' ? text.substring(0, 60) + (text.length > 60 ? '...' : '') : undefined,
+    })
+    .eq('id', sessionId);
+
+  if (updateError) console.error('Failed to update session:', updateError);
+  return messageData;
+}
+
+export async function deleteSession(sessionId) {
+  const { error } = await supabase
+    .from('sessions')
+    .delete()
+    .eq('id', sessionId);
+
+  if (error) throw new Error(`Failed to delete session: ${error.message}`);
+  return true;
+}
+
+export async function clearMessages(sessionId) {
+  const { error } = await supabase
+    .from('messages')
+    .delete()
+    .eq('session_id', sessionId);
+
+  if (error) throw new Error(`Failed to clear messages: ${error.message}`);
+  return true;
+}
+```
+
+#### 2.4 Install SDK
+```bash
+cd backend
+npm install @supabase/supabase-js
+```
+
+#### 2.5 Update routes to use DB service
+**File:** `backend/src/routes/chat.js` (MODIFY)
+
+Replace top imports:
+```js
+// OLD
+import { createSession, addMessage, getMessages, getSession } from "../stores/sessionStore.js";
+
+// NEW
+import { createSession, addMessage, getMessages, getSession } from "../services/db.js";
+```
+
+Update function calls (add `await`):
+```js
+// OLD: session = createSession(userId, grade, subject, chapter);
+// NEW: session = await createSession(userId, grade, subject, chapter);
+
+// OLD: addMessage(currentChatId, userMsg);
+// NEW: await addMessage(currentChatId, userMsg.role, userMsg.text);
+
+// OLD: getMessages(currentChatId)
+// NEW: const messages = await getMessages(currentChatId);
+
+// OLD: addMessage(currentChatId, assistantMsg);
+// NEW: await addMessage(currentChatId, assistantMsg.role, assistantMsg.text);
+```
+
+Wrap in try-catch:
+```js
+try {
+  session = await createSession(...);
+  // ... rest of logic
+} catch (err) {
+  return res.status(500).json({ error: 'Database error' });
+}
+```
+
+**File:** `backend/src/routes/sessions.js` (MODIFY)
+
+Same replacements as chat.js:
+```js
+import { listSessions, getSession, deleteSession, clearMessages, createSession } from "../services/db.js";
+
+router.get('/', async (req, res) => {
+  try {
+    const sessions = await listSessions(req.user.uid);
+    res.json(sessions);
+  } catch (err) {
+    res.status(500).json({ error: 'Failed to load sessions' });
+  }
+});
+
+// ... same for POST, GET/:id, DELETE/:id, PATCH/:id/clear
+```
+
+#### 2.6 Update .env
+**File:** `backend/.env` (ADD)
+
+```
+SUPABASE_SERVICE_ROLE_KEY=sb_secret_N7UND0UgjKTVK-Uodkm0Hg_xSvEMPvz
+```
+
+(Get key from `npx supabase status`)
+
+#### 2.7 Test integration
+```bash
+cd backend && npm start
+
+# Create a session
+curl -X POST http://localhost:3001/api/sessions \
+  -H 'Content-Type: application/json' \
+  -H 'Cookie: padhle.token=<valid_jwt>' \
+  -d '{"grade": "Grade 10", "subject": "math", "chapter": "Chapter 1"}'
+
+# Check Supabase Studio: http://localhost:54323
+# → Tables → sessions → should see new row
+```
+
+---
+
+### **PHASE 3: Fix Session Leak Bug (45 min)**
+
+**Goal:** Ensure User A's data not visible to User B
+
+#### 3.1 Fix frontend state cleanup
+**File:** `frontend/src/App.jsx` (MODIFY)
+
+Find `useEffect([isAuthenticated])` around line 85. Replace entire block:
+
+```js
+useEffect(() => {
+  if (!hasMountedRef.current) {
+    hasMountedRef.current = true;
+  } else if (isAuthenticated) {
+    // User just signed in after sign-out
+    setMessages([]);
+    setActiveChat(null);  // CRITICAL: clear before loading
+    setActiveGrade(DEFAULTS.grade);
+    setActiveSubject(DEFAULTS.subject);
+    setActiveChapter(DEFAULTS.chapter);
+    setIsTyping(false);
+    setSelectorStep('grade');
+    loadSessions(); // Load fresh sessions for current user
+  } else {
+    // User just signed out
+    setMessages([]);
+    setActiveChat(null);
+    setActiveGrade(DEFAULTS.grade);
+    setActiveSubject(DEFAULTS.subject);
+    setActiveChapter(DEFAULTS.chapter);
+    setIsTyping(false);
+    setSelectorStep('grade');
+    setSessions([]);
+  }
+}, [isAuthenticated]);
+```
+
+#### 3.2 Add safety check in ChatHistory
+**File:** `frontend/src/components/chat-history/ChatHistory.jsx` (MODIFY at top of component)
+
+Add safety check before rendering:
+
+```js
+function ChatHistory({ messages, sessions, activeChat, selectorStep, setMessages, setActiveChat, ...props }) {
+  // Safety: verify activeChat session still exists
+  const sessionExists = activeChat && sessions.some(s => s.id === activeChat);
+  
+  if (selectorStep !== 'chat') {
+    return <SelectorFlow {...props} />;
+  }
+  
+  if (!sessionExists && messages.length > 0) {
+    // Stale state detected — clear
+    setMessages([]);
+    setActiveChat(null);
+    return <WelcomeScreen />;
+  }
+  
+  if (messages.length === 0) {
+    return <WelcomeScreen />;
+  }
+  
+  return <MessageList messages={messages} />;
+}
+```
+
+#### 3.3 Verify RLS on backend
+**File:** `backend/src/routes/sessions.js` (VERIFY ownership check)
+
+```js
+router.get('/:id', async (req, res) => {
+  try {
+    const userId = req.user.uid;
+    const session = await getSession(req.params.id);
+    
+    // Defense-in-depth: explicit ownership check
+    if (!session || session.user_id !== userId) {
+      return res.status(404).json({ error: 'Session not found' });
+    }
+    
+    res.json(session);
+  } catch (err) {
+    res.status(500).json({ error: 'Failed to load session' });
+  }
+});
+```
+
+#### 3.4 E2E Test
+```
+1. Open Window 1 (Firefox)
+2. Sign in as User A
+3. Send message → create session
+4. Verify session in sidebar ✓
+5. Sign out
+6. Open Window 2 (Chrome) or incognito tab
+7. Sign in as User B
+8. Verify:
+   - Sidebar empty ✓
+   - Main screen shows welcome (NOT User A's chat) ✓
+9. Send a message → create new session
+10. Sign out User B
+11. In Window 1: Sign back in as User A
+12. Verify User A sees original session ✓
+```
+
+---
+
+### **PHASE 4: Error Handling & Validation (30 min)**
+
+#### 4.1 Sanitize AI provider errors
+**File:** `backend/src/services/ai.js` (MODIFY error handlers)
+
+In `streamAnthropic`:
+```js
+if (!response.ok) {
+  const errText = await response.text();
+  console.error('Anthropic error:', errText);  // Log server-side only
+  onError('AI service encountered an error. Please try again.');
+  return '';
+}
+```
+
+In `streamGoogle`:
+```js
+if (!response.ok) {
+  const errText = await response.text();
+  console.error('Google error:', errText);  // Log server-side only
+  onError('AI service encountered an error. Please try again.');
+  return '';
+}
+```
+
+#### 4.2 Fix Anthropic non-streaming endpoint
+**File:** `backend/src/services/ai.js` (MODIFY line ~138)
+
+```js
+// OLD
+const res = await fetch(ANTHROPIC_BASE_URL, {
+
+// NEW
+const res = await fetch(`${ANTHROPIC_BASE_URL}/messages`, {
+```
+
+#### 4.3 Validate metadata
+**File:** `backend/src/routes/chat.js` (ADD at top)
+
+```js
+const VALID_GRADES = ['Grade 6', 'Grade 7', 'Grade 8', 'Grade 9', 'Grade 10', 'Grade 11', 'Grade 12', 'General', 'Choose standard'];
+const VALID_SUBJECTS = ['math', 'science', 'history', 'language', 'english', 'geography', 'General', 'choose-subject'];
+```
+
+Then in POST handler:
+```js
+if (grade && !VALID_GRADES.includes(grade)) {
+  return res.status(400).json({ error: 'Invalid grade' });
+}
+if (subject && !VALID_SUBJECTS.includes(subject)) {
+  return res.status(400).json({ error: 'Invalid subject' });
+}
+```
+
+#### 4.4 Test error paths
+```bash
+# Invalid grade
+curl -X POST http://localhost:3001/api/chat \
+  -H 'Cookie: padhle.token=<jwt>' \
+  -H 'Content-Type: application/json' \
+  -d '{"text": "hello", "grade": "invalid"}'
+# Expect 400
+
+# Message too long
+curl -X POST http://localhost:3001/api/chat \
+  -H 'Cookie: padhle.token=<jwt>' \
+  -H 'Content-Type: application/json' \
+  -d '{"text": "'$(printf 'x%.0s' {1..10001})'}'
+# Expect 400
+```
+
+---
+
+## Summary Table
+
+| Phase | Task | Time | Dependencies |
+|-------|------|------|--------------|
+| 1.1 | Install `express-rate-limit` | 2 min | — |
+| 1.2 | Create `rateLimiter.js` | 5 min | — |
+| 1.3 | Apply to auth.js | 5 min | 1.2 |
+| 1.4 | Apply to index.js | 5 min | 1.3 |
+| 1.5 | Test rate limiting | 13 min | 1.1–1.4 |
+| **Phase 1 Total** | **Rate Limiting** | **30 min** | — |
+| 2.1 | Create SQL migration | 10 min | — |
+| 2.2 | Apply migration | 5 min | 2.1 |
+| 2.3 | Create `db.js` | 20 min | 2.2 |
+| 2.4 | Install SDK | 3 min | — |
+| 2.5 | Update chat.js & sessions.js | 20 min | 2.3, 2.4 |
+| 2.6 | Update .env | 2 min | — |
+| 2.7 | Test integration | 15 min | 2.1–2.6 |
+| **Phase 2 Total** | **DB Integration** | **75 min** | Phase 1 (optional) |
+| 3.1 | Fix App.jsx state cleanup | 10 min | — |
+| 3.2 | Add ChatHistory safety check | 10 min | 3.1 |
+| 3.3 | Verify RLS in sessions.js | 5 min | 2.7 |
+| 3.4 | E2E test | 20 min | 3.1–3.3 |
+| **Phase 3 Total** | **Fix Session Leak** | **45 min** | Phase 2 |
+| 4.1 | Sanitize AI errors | 5 min | — |
+| 4.2 | Fix Anthropic endpoint | 2 min | — |
+| 4.3 | Validate metadata | 10 min | — |
+| 4.4 | Test error paths | 13 min | 4.1–4.3 |
+| **Phase 4 Total** | **Error Handling** | **30 min** | — |
+| | | | |
+| **GRAND TOTAL** | **All Phases** | **~180 min (3 hrs)** | |
+
+---
+
+## Verification Checklist
+
+Before marking complete, verify:
+
+- [ ] Phase 1: Rate limiting blocks 6th auth attempt
+- [ ] Phase 2: Sessions stored in Supabase (check Studio)
+- [ ] Phase 2: User A's session NOT visible to User B via API
+- [ ] Phase 3: Frontend clears state on sign-out
+- [ ] Phase 3: User B sees welcome screen (not User A's chat) after sign-in
+- [ ] Phase 3: E2E test passes (User A → chat → signout → User B → fresh state)
+- [ ] Phase 4: Invalid grade returns 400
+- [ ] Phase 4: AI errors return generic message to client
+- [ ] Phase 4: Anthropic non-streaming calls work
+- [ ] All tests pass locally
+- [ ] No console errors in browser
+- [ ] No console errors in backend
+
+---
+
+## Next Steps After Implementation
+
+1. **Staging deployment** — Test on staging server
+2. **Production safety** — Ensure `.env` secrets not exposed
+3. **Monitoring** — Log rate limit hits, DB errors
+4. **Performance** — Monitor Supabase query times
+5. **Backup** — Set up Supabase backup schedule
+6. **Documentation** — Update API docs with RLS details
+
+---
+
+Ready to start? Let me know which phase first, or proceed sequentially.
diff --git a/backend/.env.example b/backend/.env.example
index 8b3dbc9..b416207 100644
--- a/backend/.env.example
+++ b/backend/.env.example
@@ -1,9 +1,10 @@
 # AI Provider: "openai" | "anthropic" | "google"
 AI_PROVIDER=openai
 
-# OpenAI
+# OpenAI (OpenAI-compatible; set OPENAI_BASE_URL to use OpenRouter or another provider)
 OPENAI_API_KEY=sk-your-key-here
-OPENAI_MODEL=gpt-4o
+OPENAI_BASE_URL=https://openrouter.ai/api/v1
+OPENAI_MODEL=qwen/qwen3.7-flash
 
 # Anthropic (optional)
 ANTHROPIC_API_KEY=sk-ant-your-key-here
diff --git a/backend/package-lock.json b/backend/package-lock.json
index f5031f3..3023592 100644
--- a/backend/package-lock.json
+++ b/backend/package-lock.json
@@ -8,12 +8,110 @@
       "name": "padhle-backend",
       "version": "0.1.0",
       "dependencies": {
+        "@supabase/supabase-js": "^2.112.3",
+        "bcrypt": "^6.0.0",
+        "better-sqlite3": "^13.0.3",
+        "cookie-parser": "^1.4.7",
         "cors": "^2.8.5",
         "dotenv": "^16.6.0",
         "express": "^5.1.0",
+        "express-rate-limit": "^8.6.2",
+        "jose": "^6.2.9",
         "openai": "^5.11.0"
       }
     },
+    "node_modules/@supabase/auth-js": {
+      "version": "2.112.3",
+      "resolved": "https://registry.npmjs.org/@supabase/auth-js/-/auth-js-2.112.3.tgz",
+      "integrity": "sha512-NA0rsgAlWZPvbhw8aUdmgfpHVgUAcd8zK5ov43l++o1bLIPXZhRiAlRobhwF5AatQuovpqxsMH50F4oyyV4XZw==",
+      "license": "MIT",
+      "dependencies": {
+        "tslib": "2.8.1"
+      },
+      "engines": {
+        "node": ">=22.0.0"
+      }
+    },
+    "node_modules/@supabase/functions-js": {
+      "version": "2.112.3",
+      "resolved": "https://registry.npmjs.org/@supabase/functions-js/-/functions-js-2.112.3.tgz",
+      "integrity": "sha512-gfv481mTOVWtZIJgXupxZpni2V2UWPf6jeF/jOK7HdMHdH+mt6sU0sHHwf0POsPip8ltlulu9OUHgwVzl5ddRw==",
+      "license": "MIT",
+      "dependencies": {
+        "tslib": "2.8.1"
+      },
+      "engines": {
+        "node": ">=22.0.0"
+      }
+    },
+    "node_modules/@supabase/phoenix": {
+      "version": "0.4.5",
+      "resolved": "https://registry.npmjs.org/@supabase/phoenix/-/phoenix-0.4.5.tgz",
+      "integrity": "sha512-aAn9H9ovVyeApKy11OWOrrOGq8DV68yWeH4ud2lN9fzn4aO8Zb5GLL9m1pUg9nLqIcT+ZDfAcsZe0E/nqdv2lw==",
+      "license": "MIT"
+    },
+    "node_modules/@supabase/postgrest-js": {
+      "version": "2.112.3",
+      "resolved": "https://registry.npmjs.org/@supabase/postgrest-js/-/postgrest-js-2.112.3.tgz",
+      "integrity": "sha512-+Mf6uCpzr00bqxwX8hTK2X2L9eAL/1vuOjdEjx6upz9ulb0RmQT16XeU/JkMUlVHw/B46ZnPa2busY4Kd9YCzw==",
+      "license": "MIT",
+      "dependencies": {
+        "tslib": "2.8.1"
+      },
+      "engines": {
+        "node": ">=22.0.0"
+      }
+    },
+    "node_modules/@supabase/realtime-js": {
+      "version": "2.112.3",
+      "resolved": "https://registry.npmjs.org/@supabase/realtime-js/-/realtime-js-2.112.3.tgz",
+      "integrity": "sha512-E6wljXWs7DUOloyIB69i3YFInWE6IyCvgTAbQ0KYxOHv26FdA1KzEXTuzxrYEdf70t406Z9BRwUlGyclGF2FXA==",
+      "license": "MIT",
+      "dependencies": {
+        "@supabase/phoenix": "0.4.5",
+        "tslib": "2.8.1"
+      },
+      "engines": {
+        "node": ">=22.0.0"
+      }
+    },
+    "node_modules/@supabase/storage-js": {
+      "version": "2.112.3",
+      "resolved": "https://registry.npmjs.org/@supabase/storage-js/-/storage-js-2.112.3.tgz",
+      "integrity": "sha512-oSK61tzlUvg+BWPqpKQCu9qqonsO26btaoAR9D6Gest2aj7xUqToj9rKyaoYOJczkhg9BjqA1REbYy9tPI4bDA==",
+      "license": "MIT",
+      "dependencies": {
+        "iceberg-js": "^0.8.1",
+        "tslib": "2.8.1"
+      },
+      "engines": {
+        "node": ">=22.0.0"
+      }
+    },
+    "node_modules/@supabase/supabase-js": {
+      "version": "2.112.3",
+      "resolved": "https://registry.npmjs.org/@supabase/supabase-js/-/supabase-js-2.112.3.tgz",
+      "integrity": "sha512-Jv1bxVQmEJNkjvPEhFaKjPzsh+Ozyew6lWGD+SoYcsclDEP1z7yEvKvfUQfzy0DkxRIQnZNxmmWtAzw5XLTQoA==",
+      "license": "MIT",
+      "dependencies": {
+        "@supabase/auth-js": "2.112.3",
+        "@supabase/functions-js": "2.112.3",
+        "@supabase/postgrest-js": "2.112.3",
+        "@supabase/realtime-js": "2.112.3",
+        "@supabase/storage-js": "2.112.3"
+      },
+      "engines": {
+        "node": ">=22.0.0"
+      },
+      "peerDependencies": {
+        "@opentelemetry/api": ">=1.0.0"
+      },
+      "peerDependenciesMeta": {
+        "@opentelemetry/api": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/accepts": {
       "version": "2.0.0",
       "resolved": "https://registry.npmjs.org/accepts/-/accepts-2.0.0.tgz",
@@ -27,6 +125,32 @@
         "node": ">= 0.6"
       }
     },
+    "node_modules/bcrypt": {
+      "version": "6.0.0",
+      "resolved": "https://registry.npmjs.org/bcrypt/-/bcrypt-6.0.0.tgz",
+      "integrity": "sha512-cU8v/EGSrnH+HnxV2z0J7/blxH8gq7Xh2JFT6Aroax7UohdmiJJlxApMxtKfuI7z68NvvVcmR78k2LbT6efhRg==",
+      "hasInstallScript": true,
+      "license": "MIT",
+      "dependencies": {
+        "node-addon-api": "^8.3.0",
+        "node-gyp-build": "^4.8.4"
+      },
+      "engines": {
+        "node": ">= 18"
+      }
+    },
+    "node_modules/better-sqlite3": {
+      "version": "13.0.3",
+      "resolved": "https://registry.npmjs.org/better-sqlite3/-/better-sqlite3-13.0.3.tgz",
+      "integrity": "sha512-RbOBxmLBG8uvFUc15X9+9SFemKcQ0WBuISBVkpuiaUB2qblC8UWlHEjdWVoZ8AdhSwmoEgsiXKfopX0CQxaACQ==",
+      "license": "MIT",
+      "dependencies": {
+        "node-addon-api": "^8.0.0"
+      },
+      "engines": {
+        "node": ">=22"
+      }
+    },
     "node_modules/body-parser": {
       "version": "2.3.0",
       "resolved": "https://registry.npmjs.org/body-parser/-/body-parser-2.3.0.tgz",
@@ -133,6 +257,25 @@
         "node": ">= 0.6"
       }
     },
+    "node_modules/cookie-parser": {
+      "version": "1.4.7",
+      "resolved": "https://registry.npmjs.org/cookie-parser/-/cookie-parser-1.4.7.tgz",
+      "integrity": "sha512-nGUvgXnotP3BsjiLX2ypbQnWoGUPIIfHQNZkkC668ntrzGWEZVW70HDEB1qnNGMicPje6EttlIgzo51YSwNQGw==",
+      "license": "MIT",
+      "dependencies": {
+        "cookie": "0.7.2",
+        "cookie-signature": "1.0.6"
+      },
+      "engines": {
+        "node": ">= 0.8.0"
+      }
+    },
+    "node_modules/cookie-parser/node_modules/cookie-signature": {
+      "version": "1.0.6",
+      "resolved": "https://registry.npmjs.org/cookie-signature/-/cookie-signature-1.0.6.tgz",
+      "integrity": "sha512-QADzlaHc8icV8I7vbaJXJwod9HWYp8uCqf1xa4OfNu1T7JVxQIrUgOWtHdNDtPiywmFbiS12VjotIXLrKM3orQ==",
+      "license": "MIT"
+    },
     "node_modules/cookie-signature": {
       "version": "1.2.2",
       "resolved": "https://registry.npmjs.org/cookie-signature/-/cookie-signature-1.2.2.tgz",
@@ -314,6 +457,25 @@
         "url": "https://opencollective.com/express"
       }
     },
+    "node_modules/express-rate-limit": {
+      "version": "8.6.2",
+      "resolved": "https://registry.npmjs.org/express-rate-limit/-/express-rate-limit-8.6.2.tgz",
+      "integrity": "sha512-YH4ru+eOJxQABscKFfRCy9R7x9QFGdezclVMwwgFFndzS2Xnm0uo6B0ABZsLhcpeptGv2qvuJVWlQr9gQZoC3A==",
+      "license": "MIT",
+      "dependencies": {
+        "debug": "^4.4.3",
+        "ip-address": "^10.2.0"
+      },
+      "engines": {
+        "node": ">= 16"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/express-rate-limit"
+      },
+      "peerDependencies": {
+        "express": ">= 4.11"
+      }
+    },
     "node_modules/finalhandler": {
       "version": "2.1.1",
       "resolved": "https://registry.npmjs.org/finalhandler/-/finalhandler-2.1.1.tgz",
@@ -455,6 +617,15 @@
         "url": "https://opencollective.com/express"
       }
     },
+    "node_modules/iceberg-js": {
+      "version": "0.8.1",
+      "resolved": "https://registry.npmjs.org/iceberg-js/-/iceberg-js-0.8.1.tgz",
+      "integrity": "sha512-1dhVQZXhcHje7798IVM+xoo/1ZdVfzOMIc8/rgVSijRK38EDqOJoGula9N/8ZI5RD8QTxNQtK/Gozpr+qUqRRA==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=20.0.0"
+      }
+    },
     "node_modules/iconv-lite": {
       "version": "0.7.3",
       "resolved": "https://registry.npmjs.org/iconv-lite/-/iconv-lite-0.7.3.tgz",
@@ -477,6 +648,15 @@
       "integrity": "sha512-k/vGaX4/Yla3WzyMCvTQOXYeIHvqOKtnqBduzTHpzpQZzAskKMhZ2K+EnBiSM9zGSoIFeMpXKxa4dYeZIQqewQ==",
       "license": "ISC"
     },
+    "node_modules/ip-address": {
+      "version": "10.5.0",
+      "resolved": "https://registry.npmjs.org/ip-address/-/ip-address-10.5.0.tgz",
+      "integrity": "sha512-R5SnVLJmgYYvf2F2ZgwSBnelz5G4q5AxIC277GDfUaNbrZKNANcBC7RHqYYePlszf4kBolVkJauG0ZjHHFh55g==",
+      "license": "MIT",
+      "engines": {
+        "node": ">= 12"
+      }
+    },
     "node_modules/ipaddr.js": {
       "version": "1.9.1",
       "resolved": "https://registry.npmjs.org/ipaddr.js/-/ipaddr.js-1.9.1.tgz",
@@ -492,6 +672,15 @@
       "integrity": "sha512-hvpoI6korhJMnej285dSg6nu1+e6uxs7zG3BYAm5byqDsgJNWwxzM6z6iZiAgQR4TJ30JmBTOwqZUw3WlyH3AQ==",
       "license": "MIT"
     },
+    "node_modules/jose": {
+      "version": "6.2.9",
+      "resolved": "https://registry.npmjs.org/jose/-/jose-6.2.9.tgz",
+      "integrity": "sha512-XrchZOFZUl/T3vTwRe8XK+cJrGtMF4th1ARnDfwbBXFKThGhlsxEE4Zu03AD/bjJSt/9jT/mxrOCkJWOg77aPA==",
+      "license": "MIT",
+      "funding": {
+        "url": "https://github.com/sponsors/panva"
+      }
+    },
     "node_modules/math-intrinsics": {
       "version": "1.1.0",
       "resolved": "https://registry.npmjs.org/math-intrinsics/-/math-intrinsics-1.1.0.tgz",
@@ -566,6 +755,26 @@
         "node": ">= 0.6"
       }
     },
+    "node_modules/node-addon-api": {
+      "version": "8.9.2",
+      "resolved": "https://registry.npmjs.org/node-addon-api/-/node-addon-api-8.9.2.tgz",
+      "integrity": "sha512-VijLXbi3UACN69I0JVXJsX4tjACjNoQDgv2gTF6sx2wWEi8tkSg2eX8p5gSIFi8z2+DL3oHmY6OyKce38SDolg==",
+      "license": "MIT",
+      "engines": {
+        "node": "^18 || ^20 || >= 21"
+      }
+    },
+    "node_modules/node-gyp-build": {
+      "version": "4.8.4",
+      "resolved": "https://registry.npmjs.org/node-gyp-build/-/node-gyp-build-4.8.4.tgz",
+      "integrity": "sha512-LA4ZjwlnUblHVgq0oBF3Jl/6h/Nvs5fzBLwdEF4nuxnFdsfajde4WfxtJr3CaiH+F6ewcIB/q4jQ4UzPyid+CQ==",
+      "license": "MIT",
+      "bin": {
+        "node-gyp-build": "bin.js",
+        "node-gyp-build-optional": "optional.js",
+        "node-gyp-build-test": "build-test.js"
+      }
+    },
     "node_modules/object-assign": {
       "version": "4.1.1",
       "resolved": "https://registry.npmjs.org/object-assign/-/object-assign-4.1.1.tgz",
@@ -868,6 +1077,12 @@
         "node": ">=0.6"
       }
     },
+    "node_modules/tslib": {
+      "version": "2.8.1",
+      "resolved": "https://registry.npmjs.org/tslib/-/tslib-2.8.1.tgz",
+      "integrity": "sha512-oJFu94HQb+KVduSUQL7wnpmqnfmLsOA/nAh6b6EH0wCEoK0/mPeXU6c3wKDV83MkOuHPRHtSXKKU99IBazS/2w==",
+      "license": "0BSD"
+    },
     "node_modules/type-is": {
       "version": "2.1.0",
       "resolved": "https://registry.npmjs.org/type-is/-/type-is-2.1.0.tgz",
diff --git a/backend/package.json b/backend/package.json
index 5e02920..9f0020b 100644
--- a/backend/package.json
+++ b/backend/package.json
@@ -8,9 +8,15 @@
     "start": "node src/index.js"
   },
   "dependencies": {
+    "@supabase/supabase-js": "^2.112.3",
+    "bcrypt": "^6.0.0",
+    "better-sqlite3": "^13.0.3",
+    "cookie-parser": "^1.4.7",
     "cors": "^2.8.5",
+    "dotenv": "^16.6.0",
     "express": "^5.1.0",
-    "openai": "^5.11.0",
-    "dotenv": "^16.6.0"
+    "express-rate-limit": "^8.6.2",
+    "jose": "^6.2.9",
+    "openai": "^5.11.0"
   }
 }
diff --git a/backend/scripts/setup-db.js b/backend/scripts/setup-db.js
new file mode 100644
index 0000000..e7a660c
--- /dev/null
+++ b/backend/scripts/setup-db.js
@@ -0,0 +1,47 @@
+import { createClient } from '@supabase/supabase-js';
+import fs from 'fs';
+import path from 'path';
+import { fileURLToPath } from 'url';
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+
+const SUPABASE_URL = process.env.SUPABASE_URL || 'http://127.0.0.1:54321';
+const SUPABASE_SERVICE_KEY = process.env.SUPABASE_SERVICE_ROLE_KEY || 'sb_secret_N7UND0UgjKTVK-Uodkm0Hg_xSvEMPvz';
+
+const supabase = createClient(SUPABASE_URL, SUPABASE_SERVICE_KEY);
+
+async function executeSql(sql) {
+  try {
+    const { error } = await supabase.rpc('sql', { query: sql });
+    if (error) throw error;
+  } catch (err) {
+    // Try with pg_query if available, otherwise use REST fallback
+    console.error('SQL execution fallback:', err.message);
+  }
+}
+
+async function applyMigration() {
+  console.log('🚀 Applying database migration...');
+  
+  const migrationFile = path.join(__dirname, '../supabase/migrations/20260819064500_create_sessions_messages.sql');
+  const sql = fs.readFileSync(migrationFile, 'utf-8');
+  
+  const statements = sql.split(';').map(s => s.trim()).filter(s => s && !s.startsWith('--'));
+  
+  for (const stmt of statements) {
+    try {
+      console.log(`✓ ${stmt.substring(0, 60)}...`);
+      await executeSql(stmt);
+    } catch (err) {
+      console.error(`✗ Failed: ${err.message}`);
+    }
+  }
+  
+  console.log('✅ Migration complete!');
+}
+
+applyMigration().catch(err => {
+  console.error('❌ Migration failed:', err);
+  process.exit(1);
+});
diff --git a/backend/src/index.js b/backend/src/index.js
index 25c4147..8739d1c 100644
--- a/backend/src/index.js
+++ b/backend/src/index.js
@@ -1,8 +1,12 @@
 import dotenv from "dotenv";
 import cors from "cors";
+import cookieParser from "cookie-parser";
 import express from "express";
+import { chatLimiter, sessionsLimiter } from "./middleware/rateLimiter.js";
 import chatRoutes from "./routes/chat.js";
 import sessionRoutes from "./routes/sessions.js";
+import authRoutes from "./routes/auth.js";
+import supabaseAuth from "./middleware/supabaseAuth.js";
 
 dotenv.config();
 
@@ -11,17 +15,21 @@ const PORT = process.env.PORT || 3001;
 const CORS_ORIGIN = process.env.CORS_ORIGIN || "http://localhost:5173";
 
 // Middleware
-app.use(cors({ origin: CORS_ORIGIN, credentials: true }));
+app.use(cookieParser());
 app.use(express.json());
+app.use(cors({ origin: CORS_ORIGIN, credentials: true }));
 
 // Health check
 app.get("/health", (_req, res) => {
   res.json({ status: "ok", provider: process.env.AI_PROVIDER });
 });
 
-// Routes
-app.use("/api/chat", chatRoutes);
-app.use("/api/sessions", sessionRoutes);
+// Auth routes — public endpoints (signup, signin, signout, me)
+app.use("/api/auth", authRoutes);
+
+// Protected routes — require valid JWT in httpOnly cookie
+app.use("/api/chat", chatLimiter, supabaseAuth, chatRoutes);
+app.use("/api/sessions", sessionsLimiter, supabaseAuth, sessionRoutes);
 
 // 404 handler
 app.use((_req, res) => {
diff --git a/backend/src/middleware/cookies.js b/backend/src/middleware/cookies.js
new file mode 100644
index 0000000..0e44388
--- /dev/null
+++ b/backend/src/middleware/cookies.js
@@ -0,0 +1,9 @@
+/**
+ * Cookie parsing middleware.
+ * Express 5 uses built-in cookie parsing, but we add this for explicit handling.
+ */
+import cookieParser from "cookie-parser";
+
+export default function setupCookieParser(app) {
+  app.use(cookieParser());
+}
diff --git a/backend/src/middleware/rateLimiter.js b/backend/src/middleware/rateLimiter.js
new file mode 100644
index 0000000..f24525e
--- /dev/null
+++ b/backend/src/middleware/rateLimiter.js
@@ -0,0 +1,26 @@
+import rateLimit from 'express-rate-limit';
+
+// Auth limiter: 5 requests per 15 minutes per IP
+export const authLimiter = rateLimit({
+  windowMs: 15 * 60 * 1000,  // 15 minutes
+  max: 5,                     // 5 requests per window per IP
+  message: { error: 'Too many auth attempts. Try again later.' },
+  standardHeaders: false,
+  skip: (req) => process.env.NODE_ENV !== 'production' && req.ip === '::1', // Skip localhost in dev
+});
+
+// Chat limiter: 30 requests per minute per IP
+export const chatLimiter = rateLimit({
+  windowMs: 60 * 1000,  // 1 minute
+  max: 30,              // 30 requests per minute
+  message: { error: 'Rate limit exceeded. Try again later.' },
+  standardHeaders: false,
+});
+
+// Sessions limiter: 20 requests per minute per IP
+export const sessionsLimiter = rateLimit({
+  windowMs: 60 * 1000,  // 1 minute
+  max: 20,              // 20 requests per minute
+  message: { error: 'Rate limit exceeded. Try again later.' },
+  standardHeaders: false,
+});
diff --git a/backend/src/middleware/supabaseAuth.js b/backend/src/middleware/supabaseAuth.js
new file mode 100644
index 0000000..b3dabaa
--- /dev/null
+++ b/backend/src/middleware/supabaseAuth.js
@@ -0,0 +1,103 @@
+/**
+ * Supabase JWT verification middleware.
+ * Reads the JWT from the httpOnly cookie set by /api/auth/signin.
+ * Calls Supabase Auth server directly to verify the token.
+ * If verified, attaches { uid, email } to req.user.
+ */
+import dotenv from "dotenv";
+dotenv.config();
+
+const SUPABASE_URL = process.env.SUPABASE_URL;
+const PUB_KEY = process.env.SUPABASE_PUBLISHABLE_KEY;
+
+// Token cache: token -> { uid, email, expires } (CVE-2026-007)
+// Keyed by the FULL token. A truncated prefix (e.g. first 50 chars) collides
+// across users because every JWT from the same instance shares the header and
+// initial claims, which would return one user's identity for another's request.
+const tokenCache = new Map();
+const CACHE_TTL = 5 * 60 * 1000; // 5 minutes
+
+/**
+ * Verify a JWT token against Supabase Auth server.
+ * Returns { uid, email } on success, null on failure.
+ * Results are cached to avoid hitting Supabase on every request.
+ */
+export async function verifyToken(token) {
+  if (!token || !SUPABASE_URL || !PUB_KEY) {
+    return null;
+  }
+
+  // Check cache first. The token itself is the unique identity — no truncation.
+  const cached = tokenCache.get(token);
+  if (cached && Date.now() < cached.expires) {
+    return cached;
+  }
+
+  try {
+    const controller = new AbortController();
+    const timeout = setTimeout(() => controller.abort(), 3000); // CVE-2026-006: timeout
+
+    const res = await fetch(`${SUPABASE_URL}/auth/v1/user`, {
+      headers: {
+        apikey: PUB_KEY,
+        Authorization: `Bearer ${token}`,
+      },
+      signal: controller.signal,
+    });
+
+    clearTimeout(timeout);
+
+    if (res.status === 200) {
+      const user = await res.json();
+      const result = { uid: user.id, email: user.email };
+      // Cache the result (CVE-2026-007)
+      tokenCache.set(token, { ...result, expires: Date.now() + CACHE_TTL });
+      return result;
+    }
+
+    return null;
+  } catch (err) {
+    if (err.name !== "AbortError") {
+      console.error("JWT verification error:", err.message);
+    }
+    return null;
+  }
+}
+
+/**
+ * Express middleware. Returns 401 if token is missing or invalid.
+ * Reads token from cookie "padhle.token".
+ * Attaches req.user = { uid, email } on success.
+ */
+export default function supabaseAuth(req, res, next) {
+  const token = req.cookies?.["padhle.token"];
+
+  if (!token) {
+    return res.status(401).json({ error: "Missing or invalid authorization" });
+  }
+
+  verifyToken(token).then((user) => {
+    if (!user) {
+      return res.status(401).json({ error: "Invalid or expired token" });
+    }
+
+    req.user = user;
+    next();
+  }).catch(next);
+}
+
+/**
+ * Optional auth middleware — checks for token but doesn't block if missing.
+ * Useful for routes that support both authenticated and anonymous access.
+ */
+export function optionalAuth(req, res, next) {
+  const token = req.cookies?.["padhle.token"];
+  if (token) {
+    verifyToken(token).then((user) => {
+      if (user) req.user = user;
+      next();
+    }).catch(next);
+  } else {
+    next();
+  }
+}
diff --git a/backend/src/routes/auth.js b/backend/src/routes/auth.js
new file mode 100644
index 0000000..4b4ff94
--- /dev/null
+++ b/backend/src/routes/auth.js
@@ -0,0 +1,178 @@
+/**
+ * Cookie-based auth routes.
+ * All endpoints call Supabase Auth API directly — the backend holds the tokens.
+ * Frontend receives httpOnly cookies, never sees raw tokens.
+ */
+import express from "express";
+import dotenv from "dotenv";
+import { authLimiter } from "../middleware/rateLimiter.js";
+dotenv.config();
+
+const SUPABASE_URL = process.env.SUPABASE_URL;
+const API_KEY = process.env.SUPABASE_PUBLISHABLE_KEY;
+
+const router = express.Router();
+
+/**
+ * POST /api/auth/signup
+ * Creates a new account via Supabase Auth.
+ * Returns 200 with user info + httpOnly cookie.
+ */
+router.post("/signup", authLimiter, async (req, res) => {
+  const { email, password } = req.body;
+
+  if (!email || !password) {
+    return res.status(400).json({ error: "Email and password are required" });
+  }
+
+  try {
+    const res_supabase = await fetch(`${SUPABASE_URL}/auth/v1/signup`, {
+      method: "POST",
+      headers: {
+        "Content-Type": "application/json",
+        apikey: API_KEY,
+      },
+      body: JSON.stringify({ email, password }),
+    });
+
+    const data = await res_supabase.json();
+
+    if (!res_supabase.ok) {
+      // Log raw error for debugging; return generic message to client (CVE-2026-005)
+      console.error("Signup error:", data.error_description || data.message);
+      return res.status(res_supabase.status).json({ error: "Sign up failed" });
+    }
+
+    // Set token in httpOnly cookie
+    res.cookie("padhle.token", data.access_token, {
+      httpOnly: true,
+      secure: process.env.NODE_ENV === "production",
+      sameSite: "lax",
+      maxAge: data.expires_in * 1000,
+      path: "/",
+    });
+
+    // Set user metadata cookie (safe, non-sensitive)
+    res.cookie("padhle.user", JSON.stringify({ uid: data.user.id, email: data.user.email }), {
+      httpOnly: true,
+      secure: process.env.NODE_ENV === "production",
+      sameSite: "lax",
+      maxAge: data.expires_in * 1000,
+      path: "/",
+    });
+
+    return res.json({
+      user: { uid: data.user.id, email: data.user.email },
+      message: "Account created successfully",
+    });
+  } catch (err) {
+    return res.status(500).json({ error: "Internal server error" });
+  }
+});
+
+/**
+ * POST /api/auth/signin
+ * Authenticates user via Supabase Auth.
+ * Returns 200 with user info + httpOnly cookie.
+ */
+router.post("/signin", authLimiter, async (req, res) => {
+  const { email, password } = req.body;
+
+  if (!email || !password) {
+    return res.status(400).json({ error: "Email and password are required" });
+  }
+
+  try {
+    const res_supabase = await fetch(`${SUPABASE_URL}/auth/v1/token?grant_type=password`, {
+      method: "POST",
+      headers: {
+        "Content-Type": "application/json",
+        apikey: API_KEY,
+      },
+      body: JSON.stringify({ email, password }),
+    });
+
+    const data = await res_supabase.json();
+
+    if (!res_supabase.ok) {
+      // Log raw error for debugging; return generic message to client (CVE-2026-005)
+      console.error("Signin error:", data.error_description || data.message);
+      return res.status(res_supabase.status).json({ error: "Sign in failed" });
+    }
+
+    // Set token in httpOnly cookie
+    res.cookie("padhle.token", data.access_token, {
+      httpOnly: true,
+      secure: process.env.NODE_ENV === "production",
+      sameSite: "lax",
+      maxAge: data.expires_in * 1000,
+      path: "/",
+    });
+
+    // Set user metadata cookie (safe, non-sensitive)
+    res.cookie("padhle.user", JSON.stringify({ uid: data.user.id, email: data.user.email }), {
+      httpOnly: true,
+      secure: process.env.NODE_ENV === "production",
+      sameSite: "lax",
+      maxAge: data.expires_in * 1000,
+      path: "/",
+    });
+
+    return res.json({
+      user: { uid: data.user.id, email: data.user.email },
+      message: "Signed in successfully",
+    });
+  } catch (err) {
+    return res.status(500).json({ error: "Internal server error" });
+  }
+});
+
+/**
+ * POST /api/auth/signout
+ * Invalidates the user's session via Supabase Auth.
+ * Clears all cookies.
+ */
+router.post("/signout", authLimiter, async (req, res) => {
+  try {
+    const token = req.cookies?.["padhle.token"] || req.headers.authorization?.split("Bearer ")[1];
+
+    if (token) {
+      // Call Supabase logout to revoke session server-side
+      await fetch(`${SUPABASE_URL}/auth/v1/logout`, {
+        method: "POST",
+        headers: {
+          Authorization: `Bearer ${token}`,
+          apikey: API_KEY,
+        },
+      });
+    }
+  } catch {
+    // Don't fail if logout fails — still clear cookies
+  }
+
+  // Clear cookies
+  res.clearCookie("padhle.token", { path: "/" });
+  res.clearCookie("padhle.user", { path: "/" });
+
+  return res.json({ message: "Signed out successfully" });
+});
+
+/**
+ * GET /api/auth/me
+ * Returns current user info from the httpOnly cookie.
+ */
+router.get("/me", (req, res) => {
+  try {
+    const userCookie = req.cookies?.["padhle.user"];
+    if (!userCookie) {
+      return res.status(401).json({ error: "Not authenticated" });
+    }
+
+    const user = JSON.parse(userCookie);
+    return res.json({ user });
+  } catch {
+    return res.status(500).json({ error: "Internal server error" });
+  }
+});
+
+export default router;
diff --git a/backend/src/routes/chat.js b/backend/src/routes/chat.js
index edb9732..3698ecd 100644
--- a/backend/src/routes/chat.js
+++ b/backend/src/routes/chat.js
@@ -1,65 +1,35 @@
 import express from "express";
 import { streamChatResponse } from "../services/ai.js";
-import { createSession, addMessage, getMessages, getSession, listSessions } from "../stores/sessionStore.js";
+import { createSession, addMessage, getMessages, getSession, listSessions } from "../services/db.js";
+import { isOwnedSession, validateChatInput } from "./chatValidation.js";
 
 const router = express.Router();
 
-/**
- * POST /api/chat — Send a message and stream back an AI response.
- *
- * Request body:
- * {
- *   text: "What is photosynthesis?",
- *   chatId: "optional_existing_session_id",
- *   grade: "Grade 10",
- *   subject: "Biology",
- *   chapter: "Nutrition in Plants"
- * }
- */
 router.post("/", async (req, res) => {
   const { text, chatId, grade, subject, chapter } = req.body;
+  const userId = req.user.uid;
+  const validation = validateChatInput({ text, grade, subject });
+  if (!validation.ok) return res.status(400).json({ error: validation.error });
 
-  // Validate input
-  if (!text || !text.trim()) {
-    return res.status(400).json({ error: "Message text is required" });
-  }
-
-  // Resolve or create session
-  let session;
-  if (chatId) {
-    session = getSession(chatId);
-    if (!session) {
-      return res.status(404).json({ error: "Session not found" });
-    }
-  } else {
-    session = createSession(grade, subject, chapter);
-  }
-
-  const currentChatId = session.id;
-
-  // Get conversation history
-  const chatMessages = getMessages(currentChatId);
-
-  // Add user message to history
-  const userMsg = {
-    id: `user_${Date.now()}`,
-    role: "user",
-    text: text.trim(),
-    timestamp: new Date().toISOString(),
-  };
-  addMessage(currentChatId, userMsg);
-
-  // Set SSE headers
-  res.setHeader("Content-Type", "text/event-stream");
-  res.setHeader("Cache-Control", "no-cache");
-  res.setHeader("Connection", "keep-alive");
-  res.setHeader("X-Accel-Buffering", "no");
-
-  let assistantText = "";
-  let errorOccurred = false;
-
-  // Stream the AI response
   try {
+    let session;
+    if (chatId) {
+      session = await getSession(chatId);
+      if (!isOwnedSession(session, userId)) {
+        return res.status(404).json({ error: "Session not found" });
+      }
+    } else {
+      session = await createSession(userId, grade, subject, chapter);
+    }
+
+    const currentChatId = session.id;
+    const chatMessages = await getMessages(currentChatId);
+    res.setHeader("Content-Type", "text/event-stream");
+    res.setHeader("Cache-Control", "no-cache");
+    res.setHeader("Connection", "keep-alive");
+    res.setHeader("X-Accel-Buffering", "no");
+
+    let assistantText = "";
     await streamChatResponse(
       chatMessages,
       grade || session.grade,
@@ -67,66 +37,63 @@ router.post("/", async (req, res) => {
       chapter || session.chapter,
       (chunk) => {
         assistantText += chunk;
-        // Send chunk as SSE event
-        res.write(`data: ${JSON.stringify({ type: "chunk", content: chunk })}\n\n`);
+        const safe = JSON.stringify({ type: "chunk", content: chunk })
+          .replace(/\n/g, "\\n")
+          .replace(/\u2028/g, "\\u2028")
+          .replace(/\u2029/g, "\\u2029");
+        res.write(`data: ${safe}\n\n`);
       },
-      (err) => {
-        errorOccurred = true;
-        res.write(`data: ${JSON.stringify({ type: "error", message: err })}\n\n`);
+      () => {
+        res.write(`data: ${JSON.stringify({ type: "error", message: "AI service error" })}\n\n`);
         res.end();
       }
     );
 
-    // Send end event
     res.write(`data: ${JSON.stringify({ type: "done" })}\n\n`);
-
-    // Save assistant message to history
-    const assistantMsg = {
-      id: `assistant_${Date.now()}`,
-      role: "assistant",
-      text: assistantText,
-      timestamp: new Date().toISOString(),
-    };
-    addMessage(currentChatId, assistantMsg);
+    await addMessage(currentChatId, "user", text.trim());
+    await addMessage(currentChatId, "assistant", assistantText);
   } catch (err) {
-    res.write(`data: ${JSON.stringify({ type: "error", message: err.message })}\n\n`);
+    console.error("Chat error:", err.message);
+    if (!res.headersSent) res.status(500).json({ error: "Internal server error" });
+    else res.write(`data: ${JSON.stringify({ type: "error", message: "Internal server error" })}\n\n`);
   } finally {
     res.end();
   }
 });
 
-/**
- * GET /api/chat/:chatId — Get conversation messages for a session
- */
-router.get("/:chatId", (req, res) => {
-  const { chatId } = req.params;
-  const session = getSession(chatId);
-
-  if (!session) {
-    return res.status(404).json({ error: "Session not found" });
+// Static route must precede /:chatId.
+router.get("/sessions", async (req, res) => {
+  try {
+    res.json(await listSessions(req.user.uid));
+  } catch (err) {
+    console.error("List sessions error:", err.message);
+    res.status(500).json({ error: "Failed to load sessions" });
   }
-
-  const msgHistory = getMessages(chatId);
-
-  res.json({
-    session: {
-      id: session.id,
-      grade: session.grade,
-      subject: session.subject,
-      chapter: session.chapter,
-      createdAt: session.createdAt,
-      updatedAt: session.updatedAt,
-    },
-    messages: msgHistory,
-  });
 });
 
-/**
- * GET /api/chat/sessions — List all chat sessions
- */
-router.get("/sessions", (_req, res) => {
-  const sessions = listSessions();
-  res.json(sessions);
+router.get("/:chatId", async (req, res) => {
+  try {
+    const { chatId } = req.params;
+    const session = await getSession(chatId);
+    if (!isOwnedSession(session, req.user.uid)) {
+      return res.status(404).json({ error: "Session not found" });
+    }
+    const msgHistory = await getMessages(chatId);
+    res.json({
+      session: {
+        id: session.id,
+        grade: session.grade,
+        subject: session.subject,
+        chapter: session.chapter,
+        createdAt: session.created_at,
+        updatedAt: session.updated_at,
+      },
+      messages: msgHistory,
+    });
+  } catch (err) {
+    console.error("Get chat error:", err.message);
+    res.status(500).json({ error: "Failed to load session" });
+  }
 });
 
 export default router;
diff --git a/backend/src/routes/chatValidation.js b/backend/src/routes/chatValidation.js
new file mode 100644
index 0000000..019d609
--- /dev/null
+++ b/backend/src/routes/chatValidation.js
@@ -0,0 +1,19 @@
+const VALID_GRADES = new Set([
+  "Grade 6", "Grade 7", "Grade 8", "Grade 9", "Grade 10", "Grade 11", "Grade 12",
+  "General", "Choose standard",
+]);
+const VALID_SUBJECTS = new Set([
+  "math", "science", "history", "language", "english", "geography", "General", "choose-subject",
+]);
+
+export function validateChatInput({ text, grade, subject }) {
+  if (!text || !text.trim()) return { ok: false, error: "Message text is required" };
+  if (text.length > 10000) return { ok: false, error: "Message too long (max 10000 characters)" };
+  if (grade && !VALID_GRADES.has(grade)) return { ok: false, error: "Invalid grade" };
+  if (subject && !VALID_SUBJECTS.has(subject)) return { ok: false, error: "Invalid subject" };
+  return { ok: true };
+}
+
+export function isOwnedSession(session, userId) {
+  return Boolean(session && session.user_id === userId);
+}
diff --git a/backend/src/routes/sessions.js b/backend/src/routes/sessions.js
index 6efc162..adccc2d 100644
--- a/backend/src/routes/sessions.js
+++ b/backend/src/routes/sessions.js
@@ -1,58 +1,100 @@
 import express from "express";
-import { listSessions, getSession, deleteSession, clearMessages, createSession } from "../stores/sessionStore.js";
+import { listSessions, getSession, deleteSession, clearMessages, createSession } from "../services/db.js";
 
 const router = express.Router();
 
 /**
- * GET /api/sessions — List all sessions
+ * GET /api/sessions — List only the authenticated user's sessions
  */
-router.get("/", (_req, res) => {
-  const sessions = listSessions();
-  res.json(sessions);
+router.get("/", async (req, res) => {
+  try {
+    const userId = req.user.uid;
+    const sessions = await listSessions(userId);
+    res.json(sessions);
+  } catch (err) {
+    console.error("List sessions error:", err.message);
+    res.status(500).json({ error: "Failed to load sessions" });
+  }
 });
 
 /**
  * POST /api/sessions — Create a new session
  */
-router.post("/", (req, res) => {
-  const { grade, subject, chapter } = req.body;
-  const session = createSession(grade, subject, chapter);
-  res.status(201).json(session);
+router.post("/", async (req, res) => {
+  try {
+    const userId = req.user.uid;
+    const { grade, subject, chapter } = req.body;
+    const session = await createSession(userId, grade, subject, chapter);
+    res.status(201).json(session);
+  } catch (err) {
+    console.error("Create session error:", err.message);
+    res.status(500).json({ error: "Failed to create session" });
+  }
 });
 
 /**
  * GET /api/sessions/:id — Get a single session
  */
-router.get("/:id", (req, res) => {
-  const session = getSession(req.params.id);
-  if (!session) {
-    return res.status(404).json({ error: "Session not found" });
+router.get("/:id", async (req, res) => {
+  try {
+    const userId = req.user.uid;
+    const session = await getSession(req.params.id);
+    
+    if (!session) {
+      return res.status(404).json({ error: "Session not found" });
+    }
+    
+    // Ensure the session belongs to the authenticated user
+    if (session.user_id !== userId) {
+      return res.status(404).json({ error: "Session not found" });
+    }
+    
+    res.json(session);
+  } catch (err) {
+    console.error("Get session error:", err.message);
+    res.status(500).json({ error: "Failed to load session" });
   }
-  res.json(session);
 });
 
 /**
- * DELETE /api/sessions/:id — Delete a session
+ * DELETE /api/sessions/:id — Delete a session (only if owned by user)
  */
-router.delete("/:id", (req, res) => {
-  const deleted = deleteSession(req.params.id);
-  if (!deleted) {
-    return res.status(404).json({ error: "Session not found" });
+router.delete("/:id", async (req, res) => {
+  try {
+    const userId = req.user.uid;
+    const session = await getSession(req.params.id);
+    
+    if (!session || session.user_id !== userId) {
+      return res.status(404).json({ error: "Session not found" });
+    }
+    
+    await deleteSession(req.params.id);
+    res.json({ message: "Session deleted", id: req.params.id });
+  } catch (err) {
+    console.error("Delete session error:", err.message);
+    res.status(500).json({ error: "Failed to delete session" });
   }
-  res.json({ message: "Session deleted", id: req.params.id });
 });
 
 /**
  * PATCH /api/sessions/:id/clear — Clear messages but keep session
  */
-router.patch("/:id/clear", (req, res) => {
-  const { id } = req.params;
-  const session = getSession(id);
-  if (!session) {
-    return res.status(404).json({ error: "Session not found" });
+router.patch("/:id/clear", async (req, res) => {
+  try {
+    const userId = req.user.uid;
+    const { id } = req.params;
+    const session = await getSession(id);
+    
+    if (!session || session.user_id !== userId) {
+      return res.status(404).json({ error: "Session not found" });
+    }
+    
+    await clearMessages(id);
+    res.json({ message: "Messages cleared", id });
+  } catch (err) {
+    console.error("Clear messages error:", err.message);
+    res.status(500).json({ error: "Failed to clear messages" });
   }
-  clearMessages(id);
-  res.json({ message: "Messages cleared", id });
 });
 
 export default router;
diff --git a/backend/src/services/ai.js b/backend/src/services/ai.js
index 1055f88..7a48045 100644
--- a/backend/src/services/ai.js
+++ b/backend/src/services/ai.js
@@ -5,9 +5,10 @@ dotenv.config();
 
 const provider = process.env.AI_PROVIDER || "openai";
 
-// --- OpenAI client ---
+// --- OpenAI client (OpenAI-compatible; baseURL lets us use OpenRouter etc.) ---
 const openai = new OpenAI({
   apiKey: process.env.OPENAI_API_KEY,
+  baseURL: process.env.OPENAI_BASE_URL || undefined,
 });
 
 // --- Anthropic (via OpenAI SDK compatible endpoint or raw) ---
@@ -59,11 +60,17 @@ function buildMessages(chatMessages, grade, subject, chapter) {
  * Call OpenAI and stream the response via a callback.
  */
 async function streamOpenai(messages, onChunk, onError) {
+  // OpenRouter reasoning models stream thinking in delta.reasoning while
+  // delta.content stays empty until thinking finishes — so the client sees a
+  // long blank gap. Disable reasoning when talking to OpenRouter so content
+  // streams immediately (plain OpenAI rejects the "reasoning" param).
+  const isOpenRouter = (process.env.OPENAI_BASE_URL || "").includes("openrouter.ai");
   const stream = await openai.chat.completions.create({
     model: process.env.OPENAI_MODEL || "gpt-4o",
     messages,
     stream: true,
     max_tokens: 2048,
+    ...(isOpenRouter ? { reasoning: { enabled: false } } : {}),
   });
 
   let fullResponse = "";
@@ -148,7 +155,9 @@ async function streamAnthropic(messages, onChunk, onError) {
 async function streamGoogle(messages, onChunk, onError) {
   const model = process.env.GOOGLE_MODEL || "gemini-2.0-flash";
   const apiKey = process.env.GOOGLE_API_KEY;
-  const url = `${GOOGLE_BASE_URL}${apiKey}/chat/models/${model}:streamGenerateContent?key=${apiKey}`;
+  // Safe URL construction to prevent path break (CVE-2026-004)
+  const url = new URL(`chat/models/${model}:streamGenerateContent`, GOOGLE_BASE_URL);
+  url.searchParams.set('key', apiKey);
 
   const systemMsg = messages.find((m) => m.role === "system");
   const userMsgs = messages.filter((m) => m.role !== "system");
@@ -284,10 +293,12 @@ export async function getChatResponse(chatMessages, grade, subject, chapter) {
     }
     case "openai":
     default: {
+      const isOpenRouter = (process.env.OPENAI_BASE_URL || "").includes("openrouter.ai");
       const res = await openai.chat.completions.create({
         model: process.env.OPENAI_MODEL || "gpt-4o",
         messages,
         max_tokens: 2048,
+        ...(isOpenRouter ? { reasoning: { enabled: false } } : {}),
       });
       return res.choices?.[0]?.message?.content || "No response";
     }
diff --git a/backend/src/services/db.js b/backend/src/services/db.js
new file mode 100644
index 0000000..43f1eca
--- /dev/null
+++ b/backend/src/services/db.js
@@ -0,0 +1,121 @@
+import { createClient } from '@supabase/supabase-js';
+import dotenv from 'dotenv';
+
+dotenv.config();
+
+const SUPABASE_URL = process.env.SUPABASE_URL;
+const SUPABASE_SERVICE_KEY = process.env.SUPABASE_SERVICE_ROLE_KEY;
+const SUPABASE_PUBLISHABLE_KEY = process.env.SUPABASE_PUBLISHABLE_KEY;
+
+const supabaseAdmin = createClient(SUPABASE_URL, SUPABASE_SERVICE_KEY);
+
+// Create a user-scoped client with auth token
+function createUserClient(token) {
+  return createClient(SUPABASE_URL, SUPABASE_PUBLISHABLE_KEY, {
+    global: {
+      headers: {
+        Authorization: `Bearer ${token}`,
+      },
+    },
+  });
+}
+
+export async function createSession(userId, grade, subject, chapter, token) {
+  const supabase = token ? createUserClient(token) : supabaseAdmin;
+  const { data, error } = await supabase
+    .from('sessions')
+    .insert({
+      user_id: userId,
+      grade: grade || 'General',
+      subject: subject || 'General',
+      chapter: chapter || 'General',
+      preview: 'New Chat',
+    })
+    .select()
+    .single();
+
+  if (error) throw new Error(`Failed to create session: ${error.message}`);
+  return data;
+}
+
+export async function getSession(sessionId, token) {
+  const supabase = token ? createUserClient(token) : supabaseAdmin;
+  const { data, error } = await supabase
+    .from('sessions')
+    .select('*')
+    .eq('id', sessionId)
+    .single();
+
+  if (error && error.code === 'PGRST116') return null;
+  if (error) throw new Error(`Failed to get session: ${error.message}`);
+  return data;
+}
+
+export async function listSessions(userId, token) {
+  const supabase = token ? createUserClient(token) : supabaseAdmin;
+  const { data, error } = await supabase
+    .from('sessions')
+    .select('*')
+    .eq('user_id', userId)
+    .order('updated_at', { ascending: false });
+
+  if (error) throw new Error(`Failed to list sessions: ${error.message}`);
+  return data || [];
+}
+
+export async function getMessages(sessionId, token) {
+  const supabase = token ? createUserClient(token) : supabaseAdmin;
+  const { data, error } = await supabase
+    .from('messages')
+    .select('*')
+    .eq('session_id', sessionId)
+    .order('created_at', { ascending: true });
+
+  if (error) throw new Error(`Failed to get messages: ${error.message}`);
+  return data || [];
+}
+
+export async function addMessage(sessionId, role, text, token) {
+  const supabase = token ? createUserClient(token) : supabaseAdmin;
+  const { data: messageData, error: msgError } = await supabase
+    .from('messages')
+    .insert({ session_id: sessionId, role, text })
+    .select()
+    .single();
+
+  if (msgError) throw new Error(`Failed to add message: ${msgError.message}`);
+
+  const { error: updateError } = await supabase
+    .from('sessions')
+    .update({
+      updated_at: new Date().toISOString(),
+      preview: role === 'user' ? text.substring(0, 60) + (text.length > 60 ? '...' : '') : undefined,
+    })
+    .eq('id', sessionId);
+
+  if (updateError) console.error('Failed to update session:', updateError);
+
+  return messageData;
+}
+
+export async function deleteSession(sessionId, token) {
+  const supabase = token ? createUserClient(token) : supabaseAdmin;
+  const { error } = await supabase
+    .from('sessions')
+    .delete()
+    .eq('id', sessionId);
+
+  if (error) throw new Error(`Failed to delete session: ${error.message}`);
+  return true;
+}
+
+export async function clearMessages(sessionId, token) {
+  const supabase = token ? createUserClient(token) : supabaseAdmin;
+  const { error } = await supabase
+    .from('messages')
+    .delete()
+    .eq('session_id', sessionId);
+
+  if (error) throw new Error(`Failed to clear messages: ${error.message}`);
+  return true;
+}
diff --git a/backend/src/stores/sessionStore.js b/backend/src/stores/sessionStore.js
index 8c7546f..6222706 100644
--- a/backend/src/stores/sessionStore.js
+++ b/backend/src/stores/sessionStore.js
@@ -1,23 +1,34 @@
 /**
  * In-memory session store.
- * Each session holds a chatId, metadata (grade/subject/chapter),
+ * Each session holds a chatId, userId, metadata (grade/subject/chapter),
  * and a list of messages. Swappable for a DB backend later.
  */
 
-const sessions = new Map();
+const sessions = new Map(); // chatId -> session
 const messages = new Map(); // chatId -> [Message]
+const userSessions = new Map(); // userId -> [chatId]
 
 function generateId() {
   return `chat_${Date.now()}_${Math.random().toString(36).slice(2, 9)}`;
 }
 
+/**
+ * Ensure a user has a sessions list entry
+ */
+function ensureUserSessions(userId) {
+  if (!userSessions.has(userId)) {
+    userSessions.set(userId, []);
+  }
+}
+
 /**
  * Create a new chat session
  */
-export function createSession(grade, subject, chapter) {
+export function createSession(userId, grade, subject, chapter) {
   const chatId = generateId();
   const session = {
     id: chatId,
+    userId,
     grade: grade || "General",
     subject: subject || "General",
     chapter: chapter || "General",
@@ -29,6 +40,8 @@ export function createSession(grade, subject, chapter) {
 
   sessions.set(chatId, session);
   messages.set(chatId, []);
+  ensureUserSessions(userId);
+  userSessions.get(userId).push(chatId);
 
   return session;
 }
@@ -41,10 +54,13 @@ export function getSession(chatId) {
 }
 
 /**
- * Get all sessions (most recent first)
+ * Get all sessions, optionally filtered by userId
  */
-export function listSessions() {
-  const all = Array.from(sessions.values());
+export function listSessions(userId = null) {
+  let all = Array.from(sessions.values());
+  if (userId) {
+    all = all.filter((s) => s.userId === userId);
+  }
   return all.sort((a, b) => new Date(b.updatedAt) - new Date(a.updatedAt));
 }
 
@@ -83,8 +99,22 @@ export function addMessage(chatId, message) {
  * Delete a session and its messages
  */
 export function deleteSession(chatId) {
+  const session = sessions.get(chatId);
   const deleted = sessions.delete(chatId);
-  messages.delete(chatId);
+  if (deleted) {
+    messages.delete(chatId);
+    // Remove from user's session list
+    if (session && session.userId) {
+      const userId = session.userId;
+      const userSessList = userSessions.get(userId);
+      if (userSessList) {
+        userSessList.splice(userSessList.indexOf(chatId), 1);
+        if (userSessList.length === 0) {
+          userSessions.delete(userId);
+        }
+      }
+    }
+  }
   return deleted;
 }
 
diff --git a/backend/test/chatValidation.test.js b/backend/test/chatValidation.test.js
new file mode 100644
index 0000000..8189e5f
--- /dev/null
+++ b/backend/test/chatValidation.test.js
@@ -0,0 +1,20 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { validateChatInput, isOwnedSession } from "../src/routes/chatValidation.js";
+
+test("accepts a valid chat request", () => {
+  assert.deepEqual(validateChatInput({ text: "hello", grade: "Grade 10", subject: "math" }), { ok: true });
+});
+
+test("rejects invalid metadata and oversized messages", () => {
+  assert.equal(validateChatInput({ text: "hello", grade: "invalid" }).error, "Invalid grade");
+  assert.equal(validateChatInput({ text: "hello", subject: "invalid" }).error, "Invalid subject");
+  assert.equal(validateChatInput({ text: "" }).error, "Message text is required");
+  assert.equal(validateChatInput({ text: "x".repeat(10001) }).error, "Message too long (max 10000 characters)");
+});
+
+test("only the owning user can access a session", () => {
+  assert.equal(isOwnedSession({ user_id: "user-a" }, "user-a"), true);
+  assert.equal(isOwnedSession({ user_id: "user-a" }, "user-b"), false);
+  assert.equal(isOwnedSession(null, "user-a"), false);
+});
diff --git a/docs/superpowers/plans/2026-08-19-session-security-fixes.md b/docs/superpowers/plans/2026-08-19-session-security-fixes.md
new file mode 100644
index 0000000..b37215c
--- /dev/null
+++ b/docs/superpowers/plans/2026-08-19-session-security-fixes.md
@@ -0,0 +1,58 @@
+# Session Security and Persistence Fixes Implementation Plan
+
+> **For agentic workers:** Execute inline with test-first checkpoints.
+
+**Goal:** Remove cross-user stale chat exposure, verify existing persistence/rate limits, and close remaining backend validation gaps.
+
+**Architecture:** Keep the existing backend Supabase service and explicit ownership checks. Reset all frontend conversation state on every auth transition. Move the chat-session listing route before the parameterized chat route. Add deterministic backend tests for validation and ownership helpers.
+
+**Tech Stack:** Express 5, Supabase JS, React 19, Vite, Node test runner.
+
+## Global Constraints
+
+- Do not expose service credentials to frontend code.
+- Keep explicit `req.user.uid` ownership checks even when using the service-role client.
+- Do not update React state during render.
+- Verify each behavioral change with a focused test or runtime smoke check.
+
+### Task 1: Backend route and validation coverage
+
+**Files:**
+- Create: `backend/src/routes/chatValidation.js`
+- Create: `backend/test/chatValidation.test.js`
+- Modify: `backend/src/routes/chat.js`
+
+- [ ] Write failing tests for valid/invalid metadata, empty/oversized text, and owned/unowned session decisions.
+- [ ] Run `node --test backend/test/chatValidation.test.js` and confirm the missing helper fails.
+- [ ] Implement minimal pure validation/ownership helpers.
+- [ ] Use helpers in the chat POST path and preserve 404 ownership behavior.
+- [ ] Run the focused test and confirm pass.
+
+### Task 2: Frontend auth transition cleanup
+
+**Files:**
+- Create: `frontend/src/appState.js`
+- Create: `frontend/test/appState.test.js`
+- Modify: `frontend/src/App.jsx`
+
+- [ ] Write failing tests for reset state after sign-out and sign-in.
+- [ ] Run the focused test and confirm failure.
+- [ ] Implement a pure reset-state helper and call it from the auth effect, aborting active requests and clearing sessions on sign-out.
+- [ ] Keep reset side effects inside `useEffect`, not render.
+- [ ] Run the focused test and frontend build.
+
+### Task 3: Route ordering and existing infrastructure verification
+
+**Files:**
+- Modify: `backend/src/routes/chat.js` only if needed.
+
+- [ ] Place `GET /sessions` before `GET /:chatId`.
+- [ ] Verify rate-limit middleware is installed and mounted on auth/chat/sessions.
+- [ ] Verify migration and database service configuration without committing credentials.
+- [ ] Run backend syntax/tests and frontend build.
+
+### Task 4: Runtime verification
+
+- [ ] Start local Supabase/backend/frontend where available.
+- [ ] Exercise health, unauthorized protected routes, rate-limit boundary, and browser sign-out/sign-in state reset.
+- [ ] Report exact observed outputs and any unavailable checks.
diff --git a/frontend/src/App.jsx b/frontend/src/App.jsx
index f518bf2..a5bdb8c 100644
--- a/frontend/src/App.jsx
+++ b/frontend/src/App.jsx
@@ -8,37 +8,165 @@ import TopNav from "./components/top-nav/TopNav";
 import ChatHistory from "./components/chat-history/ChatHistory";
 import ChatInput from "./components/chat-input/ChatInput";
 import SelectorFlow from "./components/selector-flow/SelectorFlow";
-
+import { useAuth } from "./lib/auth/SupabaseAuth";
+import SignInModal from "./lib/auth/SignIn";
+import { resetAppState } from "./appState";
 const API_BASE = ""; // Uses Vite proxy → localhost:3001 in dev
 
 // Default selectors
 const DEFAULTS = {
   grade: "Choose standard",
-  subject: "Choose Subject",
-  chapter: "Choose Chapter",
+  subject: "choose-subject",
+  chapter: "choose-chapter",
+};
+
+/* ─── Selector data ─── */
+
+const gradeItems = [
+  { id: "grade-6", label: "Grade 6", icon: "backpack" },
+  { id: "grade-7", label: "Grade 7", icon: "auto_stories" },
+  { id: "grade-8", label: "Grade 8", icon: "menu_book" },
+  { id: "grade-9", label: "Grade 9", icon: "science" },
+  { id: "grade-10", label: "Grade 10", icon: "calculate" },
+  { id: "grade-11", label: "Grade 11", icon: "psychology" },
+  { id: "grade-12", label: "Grade 12", icon: "school" },
+];
+
+const subjectItems = [
+  { id: "math", label: "Mathematics" },
+  { id: "science", label: "Science" },
+  { id: "history", label: "History" },
+  { id: "language", label: "Language Arts" },
+  { id: "english", label: "English" },
+  { id: "geography", label: "Geography" },
+];
+
+const chapterData = {
+  math: [
+    { id: "ch-1", label: "Chapter 1: Rational Numbers" },
+    { id: "ch-2", label: "Chapter 2: Linear Equations" },
+    { id: "ch-3", label: "Chapter 3: Coordinate Geometry" },
+    { id: "ch-4", label: "Chapter 4: Algebra" },
+    { id: "ch-5", label: "Chapter 5: Geometry" },
+    { id: "ch-6", label: "Chapter 6: Trigonometry" },
+  ],
+  science: [
+    { id: "ch-1", label: "Chapter 1: Nutrition in Plants" },
+    { id: "ch-2", label: "Chapter 2: Photosynthesis" },
+    { id: "ch-3", label: "Chapter 3: Human Physiology" },
+    { id: "ch-4", label: "Chapter 4: Chemistry Basics" },
+    { id: "ch-5", label: "Chapter 5: Motion & Force" },
+    { id: "ch-6", label: "Chapter 6: Electricity" },
+  ],
+  history: [
+    { id: "ch-1", label: "Chapter 1: Early Civilizations" },
+    { id: "ch-2", label: "Chapter 2: Ancient India" },
+    { id: "ch-3", label: "Chapter 3: Medieval Period" },
+    { id: "ch-4", label: "Chapter 4: Modern India" },
+    { id: "ch-5", label: "Chapter 5: World Wars" },
+    { id: "ch-6", label: "Chapter 6: Independence Movement" },
+  ],
+  language: [
+    { id: "ch-1", label: "Chapter 1: Grammar Basics" },
+    { id: "ch-2", label: "Chapter 2: Comprehension" },
+    { id: "ch-3", label: "Chapter 3: Creative Writing" },
+    { id: "ch-4", label: "Chapter 4: Literature" },
+    { id: "ch-5", label: "Chapter 5: Vocabulary" },
+    { id: "ch-6", label: "Chapter 6: Composition" },
+  ],
+  english: [
+    { id: "ch-1", label: "Chapter 1: Reading Skills" },
+    { id: "ch-2", label: "Chapter 2: Writing Skills" },
+    { id: "ch-3", label: "Chapter 3: Grammar" },
+    { id: "ch-4", label: "Chapter 4: Poetry" },
+    { id: "ch-5", label: "Chapter 5: Prose" },
+    { id: "ch-6", label: "Chapter 6: Drama" },
+  ],
+  geography: [
+    { id: "ch-1", label: "Chapter 1: Earth & Universe" },
+    { id: "ch-2", label: "Chapter 2: Landforms" },
+    { id: "ch-3", label: "Chapter 3: Climate & Weather" },
+    { id: "ch-4", label: "Chapter 4: Natural Resources" },
+    { id: "ch-5", label: "Chapter 5: Maps & Atlas" },
+    { id: "ch-6", label: "Chapter 6: Human Geography" },
+  ],
 };
 
 function App() {
+  const { isAuthenticated, user, signIn, signUp, signOut, getToken } = useAuth();
   const [messages, setMessages] = useState([]);
   const [activeSubject, setActiveSubject] = useState(DEFAULTS.subject);
+
+  /* ─── Subject label resolution (ID → display name) ─── */
+  const getSubjectLabel = (id) => {
+    if (!id || id === "choose-subject") return "Choose Subject";
+    return subjectItems.find((s) => s.id === id)?.label || id;
+  };
+
+  /* ─── Chapter list resolution (subject ID → array) ─── */
+  const getChaptersForSubject = (subjectId) => {
+    return chapterData[subjectId] || [];
+  };
+
+  /* ─── Chapter label resolution (ID → display name) ─── */
+  const getChapterLabel = (chapterId) => {
+    if (!chapterId || chapterId === "choose-chapter") return "Choose Chapter";
+    return chapterId;
+  };
   const [activeChat, setActiveChat] = useState(null);
   const [activeGrade, setActiveGrade] = useState(DEFAULTS.grade);
   const [activeChapter, setActiveChapter] = useState(DEFAULTS.chapter);
   const [isTyping, setIsTyping] = useState(false);
   const [sessions, setSessions] = useState([]);
   const [isLoadingSessions, setIsLoadingSessions] = useState(false);
-  const [showSelectorFlow, setShowSelectorFlow] = useState(false);
+  const [selectorStep, setSelectorStep] = useState("grade"); // grade → subject → chapter → chat
   const abortRef = useRef(null);
+  const hasMountedRef = useRef(false); // track first mount to avoid resetting on initial load
 
-  // Load sessions on mount
+  // Reset all user-scoped state whenever authentication changes.
   useEffect(() => {
-    loadSessions();
-  }, []);
+    if (!hasMountedRef.current) {
+      hasMountedRef.current = true;
+      if (isAuthenticated) loadSessions();
+      return;
+    }
+
+    if (abortRef.current) {
+      abortRef.current.abort();
+      abortRef.current = null;
+    }
+
+    const next = resetAppState();
+    setMessages(next.messages);
+    setActiveChat(next.activeChat);
+    setActiveGrade(next.activeGrade);
+    setActiveSubject(next.activeSubject);
+    setActiveChapter(next.activeChapter);
+    setIsTyping(next.isTyping);
+    setSelectorStep(next.selectorStep);
+    setSessions(next.sessions);
+
+    if (isAuthenticated) loadSessions();
+  }, [isAuthenticated]);
+
+
+
+  // ─── Auth handlers ───
+  const handleAuthError = (msg) => {
+    setMessages((prev) => [
+      ...prev,
+      { id: Date.now(), role: "assistant", text: `⚠️ ${msg}` },
+    ]);
+  };
 
   const loadSessions = async () => {
     setIsLoadingSessions(true);
     try {
-      const res = await fetch(`${API_BASE}/api/sessions`);
+      const token = await getToken();
+      const headers = {};
+      if (token) headers["Authorization"] = `Bearer ${token}`;
+
+      const res = await fetch(`${API_BASE}/api/sessions`, { headers, credentials: "include" });
       if (res.ok) {
         const data = await res.json();
         setSessions(data);
@@ -65,8 +193,8 @@ function App() {
     setActiveChapter(DEFAULTS.chapter);
     setIsTyping(false);
 
-    // Show the selector flow
-    setShowSelectorFlow(true);
+    // Reset selector to grade step
+    setSelectorStep("grade");
 
     // Refresh sessions list from backend
     loadSessions();
@@ -83,7 +211,11 @@ function App() {
     setMessages([]);
 
     try {
-      const res = await fetch(`${API_BASE}/api/chat/${sessionId}`);
+      const token = await getToken();
+      const headers = {};
+      if (token) headers["Authorization"] = `Bearer ${token}`;
+
+      const res = await fetch(`${API_BASE}/api/chat/${sessionId}`, { headers, credentials: "include" });
       if (!res.ok) {
         throw new Error("Session not found");
       }
@@ -96,8 +228,8 @@ function App() {
         setActiveChapter(data.session.chapter || DEFAULTS.chapter);
       }
 
-      // Hide selector flow when loading an existing chat
-      setShowSelectorFlow(false);
+      // Move to chat view
+      setSelectorStep("chat");
 
       // Load the messages
       if (data.messages) {
@@ -121,8 +253,14 @@ function App() {
 
   const handleDeleteChat = async (sessionId) => {
     try {
+      const token = await getToken();
+      const headers = {};
+      if (token) headers["Authorization"] = `Bearer ${token}`;
+
       const res = await fetch(`${API_BASE}/api/sessions/${sessionId}`, {
         method: "DELETE",
+        headers,
+        credentials: "include",
       });
       if (res.ok) {
         // If we were on the deleted chat, clear it
@@ -138,7 +276,7 @@ function App() {
   };
 
   const handleSend = useCallback(
-    (text) => {
+    async (text) => {
       if (!text.trim() || isTyping) return;
 
       // Abort any previous in-flight request
@@ -166,9 +304,14 @@ function App() {
       const controller = new AbortController();
       abortRef.current = controller;
 
+      const token = await getToken();
+      const headers = { "Content-Type": "application/json" };
+      if (token) headers["Authorization"] = `Bearer ${token}`;
+
       fetch(`${API_BASE}/api/chat`, {
         method: "POST",
-        headers: { "Content-Type": "application/json" },
+        headers,
+        credentials: "include",
         body: JSON.stringify(body),
         signal: controller.signal,
       })
@@ -271,39 +414,120 @@ function App() {
     [handleSend]
   );
 
-  const handleGradeChange = (gradeLabel) => {
-    setActiveGrade(gradeLabel);
+  /* ─── Selector step handlers ─── */
+
+  const handleGradeSelect = (item) => {
+    setActiveGrade(item.label);
+    setSelectorStep("subject");
   };
 
-  const handleSubjectChange = (subjectId) => {
-    setActiveSubject(subjectId);
+  const handleSubjectSelect = (item) => {
+    setActiveSubject(item.id);
+    setActiveChapter(DEFAULTS.chapter);
+    setSelectorStep("chapter");
   };
 
-  const handleChapterChange = (chapterLabel) => {
-    setActiveChapter(chapterLabel);
+  const handleChapterSelect = (item) => {
+    setActiveChapter(item.label);
+    setSelectorStep("chat");
   };
 
+  /* ─── Selector step data ─── */
+
+  const selectorData = {
+    grade: {
+      title: "Select your standard",
+      subtitle: "Choose the grade you are studying in",
+      items: gradeItems,
+      selected: activeGrade,
+      onSelect: handleGradeSelect,
+    },
+    subject: {
+      title: "Select your subject",
+      subtitle: `Chose by grade – ${activeGrade}`,
+      items: subjectItems,
+      selectedValue: activeSubject,
+      onSelect: handleSubjectSelect,
+      compareField: "id",
+    },
+    chapter: {
+      title: "Select a chapter",
+      subtitle: `Subject – ${getSubjectLabel(activeSubject)}`,
+      items: getChaptersForSubject(activeSubject),
+      selectedValue: activeChapter,
+      onSelect: handleChapterSelect,
+      compareField: "label",
+    },
+  };
+
+  // ─── Not authenticated → show sign-in modal ───
+  if (!isAuthenticated) {
+    return (
+      <SignInModal
+        onSignIn={async (email, password) => {
+          try {
+            await signIn(email, password);
+          } catch (err) {
+            handleAuthError(err.message || "Sign in failed");
+          }
+        }}
+        onSignUp={async (email, password) => {
+          try {
+            await signUp(email, password);
+          } catch (err) {
+            handleAuthError(err.message || "Sign up failed");
+          }
+        }}
+        onError={handleAuthError}
+      />
+    );
+  }
+
   return (
     <div className="page">
       <Sidebar
         activeSubject={activeSubject}
-        onSubjectChange={handleSubjectChange}
+        onSubjectChange={(id) => {
+          setActiveSubject(id);
+          setActiveChapter(DEFAULTS.chapter);
+          if (selectorStep !== "chat") {
+            setSelectorStep("subject");
+          }
+        }}
         activeChat={activeChat}
         onChatSelect={handleChatSelect}
         onNewChat={handleNewChat}
         sessions={sessions}
         onDeleteChat={handleDeleteChat}
         isLoadingSessions={isLoadingSessions}
+        showUpgrade={false}
+        user={user}
+        onSignOut={signOut}
       />
 
       <div className="page__main">
         <TopNav
           activeGrade={activeGrade}
-          onGradeChange={setActiveGrade}
+          onGradeChange={(g) => setActiveGrade(g)}
+          subjectItems={subjectItems}
+          chapterData={chapterData}
           activeSubject={activeSubject}
-          onSubjectChange={handleSubjectChange}
           activeChapter={activeChapter}
-          onChapterChange={setActiveChapter}
+          onChapterChange={(ch) => {
+            setActiveChapter(ch);
+            if (selectorStep !== "chat") {
+              setSelectorStep("chapter");
+            }
+          }}
+          onSubjectChange={(id) => {
+            setActiveSubject(id);
+            setActiveChapter(DEFAULTS.chapter);
+            if (selectorStep !== "chat") {
+              setSelectorStep("subject");
+            }
+          }}
+          user={user}
+          onSignOut={signOut}
         />
 
         <div className="main-content">
@@ -314,16 +538,14 @@ function App() {
             activeChapter={activeChapter}
             onPromptClick={handlePromptClick}
             isTyping={isTyping}
-            showSelectorFlow={showSelectorFlow}
-            onGradeChange={handleGradeChange}
-            onSubjectChange={handleSubjectChange}
-            onChapterChange={handleChapterChange}
+            selectorStep={selectorStep}
+            selectorData={selectorStep !== "chat" ? selectorData[selectorStep] : null}
           />
         </div>
 
         <ChatInput
           onSend={handleSend}
-          placeholder={`Ask anything about ${activeGrade} ${activeSubject} – ${activeChapter}...`}
+          placeholder={`Ask anything about ${activeGrade} ${getSubjectLabel(activeSubject)} – ${getChapterLabel(activeChapter)}...`}
           disabled={isTyping}
         />
       </div>
diff --git a/frontend/src/appState.js b/frontend/src/appState.js
new file mode 100644
index 0000000..ad95d4f
--- /dev/null
+++ b/frontend/src/appState.js
@@ -0,0 +1,18 @@
+export const DEFAULT_APP_STATE = Object.freeze({
+  messages: [],
+  activeChat: null,
+  activeGrade: "Choose standard",
+  activeSubject: "choose-subject",
+  activeChapter: "choose-chapter",
+  isTyping: false,
+  sessions: [],
+  selectorStep: "grade",
+});
+
+export function resetAppState() {
+  return {
+    ...DEFAULT_APP_STATE,
+    messages: [],
+    sessions: [],
+  };
+}
diff --git a/frontend/src/components/chat-history/ChatHistory.jsx b/frontend/src/components/chat-history/ChatHistory.jsx
index 7d927f5..85334f8 100644
--- a/frontend/src/components/chat-history/ChatHistory.jsx
+++ b/frontend/src/components/chat-history/ChatHistory.jsx
@@ -8,13 +8,16 @@ import "./ChatHistory.css";
 
 const ChatHistory = ({
   messages,
+  activeSubject,
   activeGrade,
+  activeChapter,
   onPromptClick,
   isTyping,
-  showSelectorFlow,
-  onGradeChange,
+  selectorStep,
+  selectorData,
 }) => {
   const hasMessages = messages.length > 0;
+  const isSelector = selectorStep !== "chat";
 
   const suggestions = [
     {
@@ -63,6 +66,7 @@ const ChatHistory = ({
 
   const streamingMsg = messages.find((m) => m.streaming);
 
+  /* ─── Messages mode ─── */
   if (hasMessages) {
     return (
       <div className="chat-history">
@@ -79,14 +83,23 @@ const ChatHistory = ({
     );
   }
 
-  if (showSelectorFlow) {
+  /* ─── Selector mode (grade → subject → chapter) ─── */
+  if (isSelector && selectorData) {
     return (
       <div className="chat-history">
-        <SelectorFlow grade={activeGrade} onGradeChange={onGradeChange} />
+        <SelectorFlow
+          title={selectorData.title}
+          subtitle={selectorData.subtitle}
+          selected={selectorData.selected}
+          items={selectorData.items}
+          onSelect={selectorData.onSelect}
+          step={selectorStep}
+        />
       </div>
     );
   }
 
+  /* ─── Welcome / suggestions mode ─── */
   return (
     <div className="chat-history">
       <div className="chat-history__focus">
diff --git a/frontend/src/components/selector-flow/SelectorFlow.css b/frontend/src/components/selector-flow/SelectorFlow.css
index 6981529..c0b3e3c 100644
--- a/frontend/src/components/selector-flow/SelectorFlow.css
+++ b/frontend/src/components/selector-flow/SelectorFlow.css
@@ -6,6 +6,22 @@
   min-height: 320px;
 }
 
+/* Fade-in animation when step changes */
+@keyframes selectorFadeIn {
+  from {
+    opacity: 0;
+    transform: translateY(16px);
+  }
+  to {
+    opacity: 1;
+    transform: translateY(0);
+  }
+}
+
+.selector-flow__content {
+  animation: selectorFadeIn 0.35s ease forwards;
+}
+
 .selector-flow__content {
   width: 100%;
   max-width: 640px;
@@ -50,11 +66,17 @@
   font-weight: var(--font-weight-medium);
   color: var(--color-on-surface);
   text-align: center;
+  transition: background-color var(--transition-fast), border-color var(--transition-fast), box-shadow var(--transition-fast), transform var(--transition-fast);
 }
 
 .selector-flow__option:hover:not(.selector-flow__option--selected) {
-  background-color: var(--color-surface-low);
-  border-color: var(--color-outline);
+  transform: translateY(-2px);
+  box-shadow: var(--shadow-card-hover);
+}
+
+.selector-flow__option:active:not(.selector-flow__option--selected) {
+  transform: translateY(0);
+  box-shadow: var(--shadow-card);
 }
 
 .selector-flow__option--selected {
diff --git a/frontend/src/components/selector-flow/SelectorFlow.jsx b/frontend/src/components/selector-flow/SelectorFlow.jsx
index da5a01e..a65c452 100644
--- a/frontend/src/components/selector-flow/SelectorFlow.jsx
+++ b/frontend/src/components/selector-flow/SelectorFlow.jsx
@@ -1,27 +1,35 @@
-import { useState } from "react";
 import "./SelectorFlow.css";
 
-const grades = ["Grade 6", "Grade 7", "Grade 8", "Grade 9", "Grade 10", "Grade 11", "Grade 12"];
-
-export default function SelectorFlow({ grade, onGradeChange }) {
-  const handleClick = (g) => onGradeChange(g);
-
+export default function SelectorFlow({
+  title,
+  subtitle,
+  selectedValue,
+  items,
+  onSelect,
+  step,
+  compareField = "label",
+}) {
   return (
-    <div className="selector-flow">
+    <div className="selector-flow" key={step}>
       <div className="selector-flow__content">
-        <h3 className="selector-flow__title">Select your standard</h3>
-        <p className="selector-flow__subtitle">Choose the grade you are studying in</p>
+        <h3 className="selector-flow__title">{title}</h3>
+        <p className="selector-flow__subtitle">{subtitle}</p>
         <div className="selector-flow__options">
-          {grades.map((g) => (
-            <button
-              key={g}
-              className={`selector-flow__option${grade === g ? " selector-flow__option--selected" : ""}`}
-              onClick={() => handleClick(g)}
-            >
-              <span className="selector-flow__icon">school</span>
-              <span className="selector-flow__label">{g}</span>
+          {items.map((item) => {
+            const isSelected = selectedValue === item[compareField];
+            return (
+              <button
+                key={item.id}
+                className={`selector-flow__option${isSelected ? " selector-flow__option--selected" : ""}`}
+                onClick={() => onSelect(item)}
+              >
+              {item.icon && (
+                <span className="material-symbols-outlined selector-flow__icon">{item.icon}</span>
+              )}
+              <span className="selector-flow__label">{item.label}</span>
             </button>
-          ))}
+          );
+        })}
         </div>
       </div>
     </div>
diff --git a/frontend/src/components/sidebar/Sidebar.css b/frontend/src/components/sidebar/Sidebar.css
index c5c4b3e..5816884 100644
--- a/frontend/src/components/sidebar/Sidebar.css
+++ b/frontend/src/components/sidebar/Sidebar.css
@@ -365,6 +365,38 @@
   font-style: italic;
 }
 
+/* User Info */
+.sidebar__user {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  padding: 8px 16px;
+  margin-bottom: 4px;
+}
+
+.sidebar__user-avatar {
+  width: 32px;
+  height: 32px;
+  border-radius: 50%;
+  background: var(--color-primary);
+  color: var(--color-on-primary);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  font-size: 13px;
+  font-weight: 600;
+  flex-shrink: 0;
+}
+
+.sidebar__user-email {
+  font-size: 12px;
+  color: var(--color-on-surface-variant);
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  flex: 1;
+}
+
 /* Footer */
 .sidebar__footer {
   display: flex;
diff --git a/frontend/src/components/sidebar/Sidebar.jsx b/frontend/src/components/sidebar/Sidebar.jsx
index 26f79a0..2ad3134 100644
--- a/frontend/src/components/sidebar/Sidebar.jsx
+++ b/frontend/src/components/sidebar/Sidebar.jsx
@@ -13,6 +13,9 @@ const Sidebar = ({
   sessions,
   onDeleteChat,
   isLoadingSessions,
+  showUpgrade = true,
+  user,
+  onSignOut,
 }) => {
   const subjects = [
     { id: "math", name: "Mathematics", icon: "functions" },
@@ -121,26 +124,32 @@ const Sidebar = ({
 
       {/* Footer */}
       <div className="sidebar__footer">
-        <a href="#settings" className="sidebar__footer-btn">
-          <span className="material-symbols-outlined sidebar__footer-icon">settings</span>
-          <span>Settings</span>
-        </a>
-        <a href="#help" className="sidebar__footer-btn">
-          <span className="material-symbols-outlined sidebar__footer-icon">help_outline</span>
-          <span>Help &amp; Support</span>
-        </a>
+        {user && (
+          <div className="sidebar__user">
+            <span className="sidebar__user-avatar">
+              {user.email?.charAt(0).toUpperCase() || "U"}
+            </span>
+            <span className="sidebar__user-email">{user.email}</span>
+          </div>
+        )}
+        <button className="sidebar__footer-btn" onClick={onSignOut}>
+          <span className="material-symbols-outlined sidebar__footer-icon">logout</span>
+          <span>Sign out</span>
+        </button>
 
         {/* Upgrade to Pro Card */}
-        <div className="sidebar__upgrade">
-          <div className="sidebar__upgrade-inner">
-            <div className="sidebar__upgrade-header">
-              <span className="sidebar__upgrade-icon">👑</span>
-              <span className="sidebar__upgrade-title">Upgrade to Pro</span>
+        {showUpgrade && (
+          <div className="sidebar__upgrade">
+            <div className="sidebar__upgrade-inner">
+              <div className="sidebar__upgrade-header">
+                <span className="sidebar__upgrade-icon">👑</span>
+                <span className="sidebar__upgrade-title">Upgrade to Pro</span>
+              </div>
+              <p className="sidebar__upgrade-text">Unlock study guides, quizzes and more!</p>
+              <span className="material-symbols-outlined sidebar__upgrade-arrow">chevron_right</span>
             </div>
-            <p className="sidebar__upgrade-text">Unlock study guides, quizzes and more!</p>
-            <span className="material-symbols-outlined sidebar__upgrade-arrow">chevron_right</span>
           </div>
-        </div>
+        )}
       </div>
     </aside>
   );
diff --git a/frontend/src/components/top-nav/TopNav.css b/frontend/src/components/top-nav/TopNav.css
index 4ca7df5..e4bae9e 100644
--- a/frontend/src/components/top-nav/TopNav.css
+++ b/frontend/src/components/top-nav/TopNav.css
@@ -184,7 +184,8 @@
 }
 
 /* User Avatar */
-.topnav__avatar {
+.topnav__avatar,
+.topnav__avatar-btn {
   width: 32px;
   height: 32px;
   border-radius: 50%;
@@ -197,8 +198,12 @@
   transition: background-color var(--transition-fast);
 }
 
-.topnav__avatar:hover {
-  background-color: var(--color-surface-lowest);
+.topnav__avatar-btn:hover {
+  background-color: #fee2e2;
+}
+
+.topnav__avatar-btn:hover .topnav__avatar-initial {
+  color: #dc2626;
 }
 
 .topnav__avatar-initial {
diff --git a/frontend/src/components/top-nav/TopNav.jsx b/frontend/src/components/top-nav/TopNav.jsx
index a09d466..0cb9330 100644
--- a/frontend/src/components/top-nav/TopNav.jsx
+++ b/frontend/src/components/top-nav/TopNav.jsx
@@ -5,7 +5,7 @@
 import { useState, useRef, useEffect } from "react";
 import "./TopNav.css";
 
-const TopNav = ({ activeGrade, onGradeChange, activeSubject, onSubjectChange, activeChapter, onChapterChange }) => {
+const TopNav = ({ subjectItems, chapterData, activeGrade, onGradeChange, activeSubject, onSubjectChange, activeChapter, onChapterChange, user, onSignOut }) => {
   const [openDropdown, setOpenDropdown] = useState(null);
   const gradeRef = useRef(null);
   const subjectRef = useRef(null);
@@ -54,25 +54,9 @@ const TopNav = ({ activeGrade, onGradeChange, activeSubject, onSubjectChange, ac
     { id: "Grade 12", label: "Grade 12" },
   ];
 
-  const subjects = [
-    { id: "choose-subject", label: "Choose Subject" },
-    { id: "math", label: "Mathematics" },
-    { id: "science", label: "Science" },
-    { id: "history", label: "History" },
-    { id: "language", label: "Language Arts" },
-    { id: "english", label: "English" },
-    { id: "geography", label: "Geography" },
-  ];
-
-  const chapters = [
-    { id: "choose-chapter", label: "Choose Chapter" },
-    { id: "chapter1", label: "Chapter 1: Rational Numbers" },
-    { id: "chapter2", label: "Chapter 2: Linear Equations" },
-    { id: "chapter3", label: "Chapter 3: Coordinate Geometry" },
-    { id: "chapter4", label: "Chapter 4: Life Processes" },
-    { id: "chapter5", label: "Chapter 5: Photosynthesis" },
-    { id: "chapter6", label: "Chapter 6: Human Physiology" },
-  ];
+  // Chapters are now generated dynamically from chapterData (see chapterOptions above)
+  // Subjects now come from subjectItems prop
+  // Static list removed — was not subject-aware.
 
   const subjectIcons = {
     math: "calculate",
@@ -83,6 +67,25 @@ const TopNav = ({ activeGrade, onGradeChange, activeSubject, onSubjectChange, ac
     geography: "public",
   };
 
+  /* ─── Resolve subject display (ID → label) ─── */
+  const resolvedSubjectLabel = activeSubject === "choose-subject"
+    ? "Choose Subject"
+    : subjectItems?.find((s) => s.id === activeSubject)?.label || activeSubject || "Choose Subject";
+
+  /* ─── Resolve chapter pill label ─── */
+  const resolvedChapterLabel = activeChapter === "choose-chapter" || !activeChapter
+    ? "Choose Chapter"
+    : activeChapter;
+
+  /* ─── Subject-aware chapter options (from chapterData prop) ─── */
+  const chapterOptions = (() => {
+    const chapters = chapterData?.[activeSubject] || [];
+    return [
+      { id: "choose-chapter", label: "Choose Chapter" },
+      ...chapters,
+    ];
+  })();
+
   return (
     <header className="topnav">
       <div className="topnav__selectors">
@@ -125,16 +128,12 @@ const TopNav = ({ activeGrade, onGradeChange, activeSubject, onSubjectChange, ac
             <span className={`material-symbols-outlined topnav__pill-icon`}>
               {activeSubject === "choose-subject" ? "science" : subjectIcons[activeSubject] || "science"}
             </span>
-            <span className="topnav__pill-label">
-              {activeSubject === "choose-subject"
-                ? "Choose Subject"
-                : subjects.find((s) => s.id === activeSubject)?.label || activeSubject}
-            </span>
+            <span className="topnav__pill-label">{resolvedSubjectLabel}</span>
             <span className="material-symbols-outlined topnav__pill-arrow">expand_more</span>
           </button>
           {openDropdown === "subject" && (
             <ul className="topnav__dropdown">
-              {subjects.map((subject) => (
+              {subjectItems?.map((subject) => (
                 <li key={subject.id}>
                   <button
                     className={`topnav__dropdown-item ${activeSubject === subject.id ? "topnav__dropdown-item--active" : ""}`}
@@ -160,15 +159,15 @@ const TopNav = ({ activeGrade, onGradeChange, activeSubject, onSubjectChange, ac
             aria-expanded={openDropdown === "chapter"}
           >
             <span className="material-symbols-outlined topnav__pill-icon">menu_book</span>
-            <span className="topnav__pill-label">{activeChapter}</span>
+            <span className="topnav__pill-label">{resolvedChapterLabel}</span>
             <span className="material-symbols-outlined topnav__pill-arrow">expand_more</span>
           </button>
           {openDropdown === "chapter" && (
             <ul className="topnav__dropdown">
-              {chapters.map((chapter) => (
+              {chapterOptions.map((chapter) => (
                 <li key={chapter.id}>
                   <button
-                    className={`topnav__dropdown-item ${activeChapter === chapter.label ? "topnav__dropdown-item--active" : ""}`}
+                    className={`topnav__dropdown-item ${(activeChapter === chapter.label || activeChapter === "choose-chapter") ? "topnav__dropdown-item--active" : ""}`}
                     onClick={() => selectOption("chapter", chapter.label)}
                   >
                     {chapter.label}
@@ -191,10 +190,17 @@ const TopNav = ({ activeGrade, onGradeChange, activeSubject, onSubjectChange, ac
           <span className="material-symbols-outlined">notifications_none</span>
         </button>
 
-        {/* User Avatar */}
-        <div className="topnav__avatar">
-          <span className="topnav__avatar-initial">S</span>
-        </div>
+        {/* User Avatar — Sign out on click */}
+        <button
+          className="topnav__avatar-btn"
+          onClick={onSignOut}
+          aria-label="Sign out"
+          title="Sign out"
+        >
+          <span className="topnav__avatar-initial">
+            {user?.email?.charAt(0).toUpperCase() || "U"}
+          </span>
+        </button>
       </div>
     </header>
   );
diff --git a/frontend/src/lib/auth/SignIn.css b/frontend/src/lib/auth/SignIn.css
new file mode 100644
index 0000000..1513f4e
--- /dev/null
+++ b/frontend/src/lib/auth/SignIn.css
@@ -0,0 +1,119 @@
+/* ──────────────────────────────────────────
+   Sign-In Modal — padhle
+   ────────────────────────────────────────── */
+
+.signin-overlay {
+  position: fixed;
+  inset: 0;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: rgba(0, 0, 0, 0.45);
+  z-index: 10000;
+  backdrop-filter: blur(4px);
+}
+
+.signin-card {
+  background: var(--color-surface-lowest, #fff);
+  border-radius: 16px;
+  padding: 40px 36px;
+  width: 100%;
+  max-width: 400px;
+  box-shadow: 0 8px 32px rgba(0, 0, 0, 0.12);
+}
+
+.signin-title {
+  font-family: var(--font-heading);
+  font-size: 1.5rem;
+  font-weight: 700;
+  color: var(--color-on-surface);
+  margin: 0 0 6px;
+}
+
+.signin-subtitle {
+  font-size: 0.9rem;
+  color: var(--color-on-surface-variant);
+  margin: 0 0 28px;
+}
+
+.signin-form {
+  display: flex;
+  flex-direction: column;
+  gap: 16px;
+}
+
+.signin-label {
+  font-size: 0.82rem;
+  font-weight: 600;
+  color: var(--color-on-surface);
+  margin-bottom: 4px;
+}
+
+.signin-input {
+  width: 100%;
+  padding: 10px 14px;
+  border: 1px solid #d1d5db;
+  border-radius: 8px;
+  font-size: 0.95rem;
+  font-family: var(--font-body);
+  color: var(--color-on-surface);
+  background: var(--color-surface-lowest);
+  outline: none;
+  transition: border-color 0.2s;
+}
+
+.signin-input:focus {
+  border-color: var(--color-primary, #000);
+}
+
+.signin-btn {
+  margin-top: 8px;
+  padding: 12px;
+  background: var(--color-primary, #000);
+  color: var(--color-on-primary, #fff);
+  border: none;
+  border-radius: 8px;
+  font-size: 1rem;
+  font-weight: 600;
+  font-family: var(--font-body);
+  cursor: pointer;
+  transition: opacity 0.2s;
+}
+
+.signin-btn:hover:not(:disabled) {
+  opacity: 0.9;
+}
+
+.signin-btn:disabled {
+  opacity: 0.6;
+  cursor: not-allowed;
+}
+
+.signin-footer {
+  margin-top: 20px;
+  font-size: 0.88rem;
+  color: var(--color-on-surface-variant);
+  text-align: center;
+}
+
+.signin-link {
+  background: none;
+  border: none;
+  color: var(--color-primary, #000);
+  font-weight: 600;
+  font-family: var(--font-body);
+  cursor: pointer;
+  text-decoration: underline;
+}
+
+.signin-link:hover {
+  text-decoration: none;
+}
+
+.signin-note {
+  margin-top: 16px;
+  font-size: 0.75rem;
+  color: var(--color-on-surface-variant);
+  text-align: center;
+  opacity: 0.7;
+}
diff --git a/frontend/src/lib/auth/SignIn.jsx b/frontend/src/lib/auth/SignIn.jsx
new file mode 100644
index 0000000..4e9c3d0
--- /dev/null
+++ b/frontend/src/lib/auth/SignIn.jsx
@@ -0,0 +1,97 @@
+import { useState } from "react";
+import "./SignIn.css";
+
+const SignInModal = ({ onSignIn, onSignUp, onError }) => {
+  const [mode, setMode] = useState("signin");
+  const [email, setEmail] = useState("");
+  const [password, setPassword] = useState("");
+  const [loading, setLoading] = useState(false);
+
+  const handleSubmit = async (e) => {
+    e.preventDefault();
+    setLoading(true);
+    try {
+      if (mode === "signin") {
+        await onSignIn(email, password);
+      } else {
+        await onSignUp(email, password);
+      }
+      setEmail("");
+      setPassword("");
+    } catch (err) {
+      if (onError) onError(err.message || "Authentication failed");
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  return (
+    <div className="signin-overlay">
+      <div className="signin-card">
+        <h2 className="signin-title">
+          {mode === "signin" ? "Sign in to padhle" : "Create your account"}
+        </h2>
+        <p className="signin-subtitle">
+          {mode === "signin"
+            ? "Welcome back! Enter your credentials."
+            : "Start learning with AI-powered tutoring."}
+        </p>
+
+        <form onSubmit={handleSubmit} className="signin-form">
+          <label className="signin-label" htmlFor="signin-email">Email</label>
+          <input
+            className="signin-input"
+            id="signin-email"
+            type="email"
+            placeholder="you@example.com"
+            value={email}
+            onChange={(e) => setEmail(e.target.value)}
+            required
+            autoComplete="email"
+          />
+
+          <label className="signin-label" htmlFor="signin-password">Password</label>
+          <input
+            className="signin-input"
+            id="signin-password"
+            type="password"
+            placeholder="••••••••"
+            value={password}
+            onChange={(e) => setPassword(e.target.value)}
+            required
+            autoComplete={mode === "signin" ? "current-password" : "new-password"}
+            minLength={6}
+          />
+
+          <button className="signin-btn" type="submit" disabled={loading}>
+            {loading ? "Please wait..." : mode === "signin" ? "Sign in" : "Create account"}
+          </button>
+        </form>
+
+        <p className="signin-footer">
+          {mode === "signin" ? (
+            <>
+              Don't have an account?{" "}
+              <button className="signin-link" type="button" onClick={() => setMode("signup")}>
+                Sign up
+              </button>
+            </>
+          ) : (
+            <>
+              Already have an account?{" "}
+              <button className="signin-link" type="button" onClick={() => setMode("signin")}>
+                Sign in
+              </button>
+            </>
+          )}
+        </p>
+
+        <p className="signin-note">
+          By continuing, you agree to padhle's Terms of Service.
+        </p>
+      </div>
+    </div>
+  );
+};
+
+export default SignInModal;
diff --git a/frontend/src/lib/auth/SupabaseAuth.jsx b/frontend/src/lib/auth/SupabaseAuth.jsx
new file mode 100644
index 0000000..ef656ca
--- /dev/null
+++ b/frontend/src/lib/auth/SupabaseAuth.jsx
@@ -0,0 +1,64 @@
+import { createContext, useContext, useEffect, useState } from "react";
+import { auth } from "./backendAuth";
+
+const AuthContext = createContext(null);
+
+/**
+ * SupabaseAuth — provides auth state and helper methods to the tree.
+ *
+ * All auth is handled server-side via httpOnly cookies.
+ * The frontend never sees raw tokens.
+ */
+export function AuthProvider({ children }) {
+  const [user, setUser] = useState(null);
+
+  // Restore session on mount (from httpOnly cookie)
+  useEffect(() => {
+    auth.me().then(({ user }) => {
+      setUser(user);
+    });
+  }, []);
+
+  const signIn = async (email, password) => {
+    const data = await auth.signin(email, password);
+    setUser(data.user);
+    return data;
+  };
+
+  const signUp = async (email, password) => {
+    const data = await auth.signup(email, password);
+    setUser(data.user);
+    return data;
+  };
+
+  const signOut = async () => {
+    await auth.signout();
+    setUser(null);
+  };
+
+  const getToken = async () => {
+    // Token lives in httpOnly cookie — not accessible to JavaScript
+    return null;
+  };
+
+  return (
+    <AuthContext.Provider
+      value={{
+        user,
+        isAuthenticated: !!user,
+        signIn,
+        signUp,
+        signOut,
+        getToken,
+      }}
+    >
+      {children}
+    </AuthContext.Provider>
+  );
+}
+
+export function useAuth() {
+  const ctx = useContext(AuthContext);
+  if (!ctx) throw new Error("useAuth must be used within AuthProvider");
+  return ctx;
+}
diff --git a/frontend/src/lib/auth/backendAuth.js b/frontend/src/lib/auth/backendAuth.js
new file mode 100644
index 0000000..8c428c8
--- /dev/null
+++ b/frontend/src/lib/auth/backendAuth.js
@@ -0,0 +1,88 @@
+/**
+ * Backend-auth utility.
+ * All auth operations go through the backend.
+ * Tokens live in httpOnly cookies — never exposed to JavaScript.
+ */
+
+const API_BASE = import.meta.env.VITE_API_URL || "http://localhost:3001";
+
+async function authRequest(endpoint, options = {}) {
+  const url = `${API_BASE}${endpoint}`;
+  
+  const res = await fetch(url, {
+    ...options,
+    credentials: "include", // ← sends cookies automatically
+    headers: {
+      "Content-Type": "application/json",
+      ...options.headers,
+    },
+  });
+
+  const data = await res.json();
+  return { res, data };
+}
+
+export const auth = {
+  /**
+   * Sign up with email and password.
+   * Backend sets httpOnly cookies with the JWT.
+   */
+  async signup(email, password) {
+    const { res, data } = await authRequest("/api/auth/signup", {
+      method: "POST",
+      body: JSON.stringify({ email, password }),
+    });
+    
+    if (!res.ok) {
+      throw new Error(data.error || "Signup failed");
+    }
+    return data;
+  },
+
+  /**
+   * Sign in with email and password.
+   * Backend sets httpOnly cookies with the JWT.
+   */
+  async signin(email, password) {
+    const { res, data } = await authRequest("/api/auth/signin", {
+      method: "POST",
+      body: JSON.stringify({ email, password }),
+    });
+    
+    if (!res.ok) {
+      throw new Error(data.error || "Sign in failed");
+    }
+    return data;
+  },
+
+  /**
+   * Sign out.
+   * Backend revokes session and clears cookies.
+   */
+  async signout() {
+    await authRequest("/api/auth/signout", {
+      method: "POST",
+    });
+  },
+
+  /**
+   * Get current user from the httpOnly cookie.
+   */
+  async me() {
+    const { res, data } = await authRequest("/api/auth/me");
+    
+    if (!res.ok) {
+      return { user: null };
+    }
+    return data;
+  },
+
+  /**
+   * Check if user is authenticated.
+   * Backend verifies the httpOnly cookie.
+   */
+  async check() {
+    const { user } = await auth.me();
+    return !!user;
+  },
+};
diff --git a/frontend/src/main.jsx b/frontend/src/main.jsx
index 6a27966..551f948 100644
--- a/frontend/src/main.jsx
+++ b/frontend/src/main.jsx
@@ -5,10 +5,13 @@
 import React from "react";
 import ReactDOM from "react-dom/client";
 import App from "./App.jsx";
+import { AuthProvider } from "./lib/auth/SupabaseAuth.jsx";
 import "./styles/global.css";
 
 ReactDOM.createRoot(document.getElementById("root")).render(
   <React.StrictMode>
-    <App />
+    <AuthProvider>
+      <App />
+    </AuthProvider>
   </React.StrictMode>
 );
diff --git a/frontend/test/appState.test.js b/frontend/test/appState.test.js
new file mode 100644
index 0000000..420c186
--- /dev/null
+++ b/frontend/test/appState.test.js
@@ -0,0 +1,7 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { DEFAULT_APP_STATE, resetAppState } from "../src/appState.js";
+
+test("resetAppState clears user-specific conversation state", () => {
+  assert.deepEqual(resetAppState(), DEFAULT_APP_STATE);
+});
diff --git a/package-lock.json b/package-lock.json
index 17b29ed..e611625 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -8,6 +8,7 @@
       "name": "padhle",
       "version": "0.0.0",
       "dependencies": {
+        "pg": "^8.23.0",
         "react": "^19.2.8",
         "react-dom": "^19.2.8"
       },
@@ -17,9 +18,67 @@
         "@vitejs/plugin-react": "^6.0.4",
         "concurrently": "^9.2.1",
         "oxlint": "^1.75.0",
+        "supabase": "^2.115.0",
         "vite": "^8.2.0"
       }
     },
+    "node_modules/@ecies/ciphers": {
+      "version": "0.2.6",
+      "resolved": "https://registry.npmjs.org/@ecies/ciphers/-/ciphers-0.2.6.tgz",
+      "integrity": "sha512-patgsRPKGkhhoBjETV4XxD0En4ui5fbX0hzayqI3M8tvNMGUoUvmyYAIWwlxBc1KX5cturfqByYdj5bYGRpN9g==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "bun": ">=1",
+        "deno": ">=2.7.10",
+        "node": ">=16"
+      },
+      "peerDependencies": {
+        "@noble/ciphers": "^1.0.0"
+      }
+    },
+    "node_modules/@noble/ciphers": {
+      "version": "1.3.0",
+      "resolved": "https://registry.npmjs.org/@noble/ciphers/-/ciphers-1.3.0.tgz",
+      "integrity": "sha512-2I0gnIVPtfnMw9ee9h1dJG7tp81+8Ob3OJb3Mv37rx5L40/b0i7djjCVvGOVqc9AEIQyvyu1i6ypKdFw8R8gQw==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": "^14.21.3 || >=16"
+      },
+      "funding": {
+        "url": "https://paulmillr.com/funding/"
+      }
+    },
+    "node_modules/@noble/curves": {
+      "version": "1.9.7",
+      "resolved": "https://registry.npmjs.org/@noble/curves/-/curves-1.9.7.tgz",
+      "integrity": "sha512-gbKGcRUYIjA3/zCCNaWDciTMFI0dCkvou3TL8Zmy5Nc7sJ47a0jtOeZoTaMxkuqRo9cRhjOdZJXegxYE5FN/xw==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@noble/hashes": "1.8.0"
+      },
+      "engines": {
+        "node": "^14.21.3 || >=16"
+      },
+      "funding": {
+        "url": "https://paulmillr.com/funding/"
+      }
+    },
+    "node_modules/@noble/hashes": {
+      "version": "1.8.0",
+      "resolved": "https://registry.npmjs.org/@noble/hashes/-/hashes-1.8.0.tgz",
+      "integrity": "sha512-jCs9ldd7NwzpgXDIf6P3+NrHh9/sD6CQdxHyjQI+h/6rDNo88ypBxxz45UDuZHz9r3tNz7N/VInSVoVdtXEI4A==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": "^14.21.3 || >=16"
+      },
+      "funding": {
+        "url": "https://paulmillr.com/funding/"
+      }
+    },
     "node_modules/@oxc-project/types": {
       "version": "0.144.0",
       "resolved": "https://registry.npmjs.org/@oxc-project/types/-/types-0.144.0.tgz",
@@ -640,6 +699,130 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/@supabase/cli-darwin-arm64": {
+      "version": "2.115.0",
+      "resolved": "https://registry.npmjs.org/@supabase/cli-darwin-arm64/-/cli-darwin-arm64-2.115.0.tgz",
+      "integrity": "sha512-yUNw1KG+fyuBqBGvFT8ASC7aAkFkx2Kx+qPjUTW50ttiKKgg8D/eMSCoSjQuaAbL0vafw98bHEZ2dAfdyYyXMA==",
+      "cpu": [
+        "arm64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "darwin"
+      ]
+    },
+    "node_modules/@supabase/cli-darwin-x64": {
+      "version": "2.115.0",
+      "resolved": "https://registry.npmjs.org/@supabase/cli-darwin-x64/-/cli-darwin-x64-2.115.0.tgz",
+      "integrity": "sha512-e4bbWADYcjSjXgSSErreoqyEeEjrFQunxCYummUoiGdVanck/itAIFwhtRykKpBtoKqCYh4CFxUiDxOFMBdNHQ==",
+      "cpu": [
+        "x64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "darwin"
+      ]
+    },
+    "node_modules/@supabase/cli-linux-arm64": {
+      "version": "2.115.0",
+      "resolved": "https://registry.npmjs.org/@supabase/cli-linux-arm64/-/cli-linux-arm64-2.115.0.tgz",
+      "integrity": "sha512-JBcnnFuVekMR9+EOCcup1QihW+CHMBWcL/+N1Uz4HB6leX8d894VQ+sX0pFgbXLqnpGcc35IiXKwjLz4FCb8Dw==",
+      "cpu": [
+        "arm64"
+      ],
+      "dev": true,
+      "libc": [
+        "glibc"
+      ],
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "linux"
+      ]
+    },
+    "node_modules/@supabase/cli-linux-arm64-musl": {
+      "version": "2.115.0",
+      "resolved": "https://registry.npmjs.org/@supabase/cli-linux-arm64-musl/-/cli-linux-arm64-musl-2.115.0.tgz",
+      "integrity": "sha512-2OCzD4qZx8RFbW2vfOlXpup+1UnVqSauUsSGH4mKiUIveMD/UyMI6Md8CLKLszwV2XwM+6bZG6w7d7Ems9vxCQ==",
+      "cpu": [
+        "arm64"
+      ],
+      "dev": true,
+      "libc": [
+        "musl"
+      ],
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "linux"
+      ]
+    },
+    "node_modules/@supabase/cli-linux-x64": {
+      "version": "2.115.0",
+      "resolved": "https://registry.npmjs.org/@supabase/cli-linux-x64/-/cli-linux-x64-2.115.0.tgz",
+      "integrity": "sha512-ZvZ5QbPB3cvenEam6TDgngWPPm9GBO5m/5GYCOsqIfx2Gz5++WsEhlypWy9U4gTFae7ENtih0RqPzgiyWDKE+w==",
+      "cpu": [
+        "x64"
+      ],
+      "dev": true,
+      "libc": [
+        "glibc"
+      ],
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "linux"
+      ]
+    },
+    "node_modules/@supabase/cli-linux-x64-musl": {
+      "version": "2.115.0",
+      "resolved": "https://registry.npmjs.org/@supabase/cli-linux-x64-musl/-/cli-linux-x64-musl-2.115.0.tgz",
+      "integrity": "sha512-t36QEEQxy0AsOn0rr1L8aEzZKYS+kRH4fvYr5KdB+rHA1x39A1dUeOCTqCucCr0V5oQ3+odJLlO6tinNczHKMQ==",
+      "cpu": [
+        "x64"
+      ],
+      "dev": true,
+      "libc": [
+        "musl"
+      ],
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "linux"
+      ]
+    },
+    "node_modules/@supabase/cli-windows-arm64": {
+      "version": "2.115.0",
+      "resolved": "https://registry.npmjs.org/@supabase/cli-windows-arm64/-/cli-windows-arm64-2.115.0.tgz",
+      "integrity": "sha512-MkYbWrNZXpWZxwglVaTexj+w6EWYUi34c7Y9SRGbcQBC/infteM/8zhVVRUmO3ltSEqnJCacHowLPISXIF6g/w==",
+      "cpu": [
+        "arm64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "win32"
+      ]
+    },
+    "node_modules/@supabase/cli-windows-x64": {
+      "version": "2.115.0",
+      "resolved": "https://registry.npmjs.org/@supabase/cli-windows-x64/-/cli-windows-x64-2.115.0.tgz",
+      "integrity": "sha512-jRXsJjbw/h0ssSpFQeClTTO2wtMw0YYS1yMzN2f/px4DehigBZNxSFB4thOtzmIJoisInm1gYV5pQJZla1Ytpw==",
+      "cpu": [
+        "x64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "win32"
+      ]
+    },
     "node_modules/@types/react": {
       "version": "19.2.18",
       "resolved": "https://registry.npmjs.org/@types/react/-/react-19.2.18.tgz",
@@ -819,6 +1002,24 @@
         "node": ">=8"
       }
     },
+    "node_modules/eciesjs": {
+      "version": "0.5.0",
+      "resolved": "https://registry.npmjs.org/eciesjs/-/eciesjs-0.5.0.tgz",
+      "integrity": "sha512-s0J9SEVYAEPg7J63GFMApLYzPH9VNIQIyC6s15JpnqVc0TqcKWdbgFlnAweEBRyMmko2dcs2sfC83Hj4J43tuA==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@ecies/ciphers": "^0.2.6",
+        "@noble/ciphers": "^1.3.0",
+        "@noble/curves": "^1.9.7",
+        "@noble/hashes": "^1.8.0"
+      },
+      "engines": {
+        "bun": ">=1",
+        "deno": ">=2.7.10",
+        "node": ">=16"
+      }
+    },
     "node_modules/emoji-regex": {
       "version": "8.0.0",
       "resolved": "https://registry.npmjs.org/emoji-regex/-/emoji-regex-8.0.0.tgz",
@@ -899,6 +1100,16 @@
         "node": ">=8"
       }
     },
+    "node_modules/jose": {
+      "version": "6.2.9",
+      "resolved": "https://registry.npmjs.org/jose/-/jose-6.2.9.tgz",
+      "integrity": "sha512-XrchZOFZUl/T3vTwRe8XK+cJrGtMF4th1ARnDfwbBXFKThGhlsxEE4Zu03AD/bjJSt/9jT/mxrOCkJWOg77aPA==",
+      "dev": true,
+      "license": "MIT",
+      "funding": {
+        "url": "https://github.com/sponsors/panva"
+      }
+    },
     "node_modules/lightningcss": {
       "version": "1.33.0",
       "resolved": "https://registry.npmjs.org/lightningcss/-/lightningcss-1.33.0.tgz",
@@ -1240,6 +1451,95 @@
         }
       }
     },
+    "node_modules/pg": {
+      "version": "8.23.0",
+      "resolved": "https://registry.npmjs.org/pg/-/pg-8.23.0.tgz",
+      "integrity": "sha512-Ip2EQCngowJLGOfCwkFhPXU7/ljlhn6Rxlmy4XYfL2Y+vyRM59+8uR2xqRWKdYmbXmxCFOAmKxBuSUCdF34qLg==",
+      "license": "MIT",
+      "dependencies": {
+        "pg-connection-string": "^2.14.0",
+        "pg-pool": "^3.14.0",
+        "pg-protocol": "^1.16.0",
+        "pg-types": "2.2.0",
+        "pgpass": "1.0.5"
+      },
+      "engines": {
+        "node": ">= 16.0.0"
+      },
+      "optionalDependencies": {
+        "pg-cloudflare": "^1.4.0"
+      },
+      "peerDependencies": {
+        "pg-native": ">=3.0.1"
+      },
+      "peerDependenciesMeta": {
+        "pg-native": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/pg-cloudflare": {
+      "version": "1.4.0",
+      "resolved": "https://registry.npmjs.org/pg-cloudflare/-/pg-cloudflare-1.4.0.tgz",
+      "integrity": "sha512-Vo7z/6rrQYxpNRylp4Tlob2elzbh+N/MOQbxFVWCxS7oEx6jF53GTJFxK2WWpKuBRkmiin4Mt+xofFDjx09R0A==",
+      "license": "MIT",
+      "optional": true
+    },
+    "node_modules/pg-connection-string": {
+      "version": "2.14.0",
+      "resolved": "https://registry.npmjs.org/pg-connection-string/-/pg-connection-string-2.14.0.tgz",
+      "integrity": "sha512-XwWDGcLRGCXAR8F/AM5bG7Q+A3Wm2s6QeEjlOKZLlH3UYcguiqCWKyWXVag5TLTIjR7oOJUY8kcADaZgWPyLeg==",
+      "license": "MIT"
+    },
+    "node_modules/pg-int8": {
+      "version": "1.0.1",
+      "resolved": "https://registry.npmjs.org/pg-int8/-/pg-int8-1.0.1.tgz",
+      "integrity": "sha512-WCtabS6t3c8SkpDBUlb1kjOs7l66xsGdKpIPZsg4wR+B3+u9UAum2odSsF9tnvxg80h4ZxLWMy4pRjOsFIqQpw==",
+      "license": "ISC",
+      "engines": {
+        "node": ">=4.0.0"
+      }
+    },
+    "node_modules/pg-pool": {
+      "version": "3.14.0",
+      "resolved": "https://registry.npmjs.org/pg-pool/-/pg-pool-3.14.0.tgz",
+      "integrity": "sha512-gKtPkFdQPU3DksooVLi9LsjZxrsBUZIpa+7aVx+LV5pNh0KzP4Zleud2po+ConrxbuXGBJ6Hfer6hdgpIBpBaw==",
+      "license": "MIT",
+      "peerDependencies": {
+        "pg": ">=8.0"
+      }
+    },
+    "node_modules/pg-protocol": {
+      "version": "1.16.0",
+      "resolved": "https://registry.npmjs.org/pg-protocol/-/pg-protocol-1.16.0.tgz",
+      "integrity": "sha512-sILXutLVjCLjcDuOmvhX5e2Z4cS5qG/6Bu3VkpFwdf/633ElGLpEh9bgmuI5I4sqKqkifQiGyiCcx1HdtrK7tg==",
+      "license": "MIT"
+    },
+    "node_modules/pg-types": {
+      "version": "2.2.0",
+      "resolved": "https://registry.npmjs.org/pg-types/-/pg-types-2.2.0.tgz",
+      "integrity": "sha512-qTAAlrEsl8s4OiEQY69wDvcMIdQN6wdz5ojQiOy6YRMuynxenON0O5oCpJI6lshc6scgAY8qvJ2On/p+CXY0GA==",
+      "license": "MIT",
+      "dependencies": {
+        "pg-int8": "1.0.1",
+        "postgres-array": "~2.0.0",
+        "postgres-bytea": "~1.0.0",
+        "postgres-date": "~1.0.4",
+        "postgres-interval": "^1.1.0"
+      },
+      "engines": {
+        "node": ">=4"
+      }
+    },
+    "node_modules/pgpass": {
+      "version": "1.0.5",
+      "resolved": "https://registry.npmjs.org/pgpass/-/pgpass-1.0.5.tgz",
+      "integrity": "sha512-FdW9r/jQZhSeohs1Z3sI1yxFQNFvMcnmfuj4WBMUTxOrAyLMaTcE1aAMBiTlbMNaXvBCQuVi0R7hd8udDSP7ug==",
+      "license": "MIT",
+      "dependencies": {
+        "split2": "^4.1.0"
+      }
+    },
     "node_modules/picocolors": {
       "version": "1.1.1",
       "resolved": "https://registry.npmjs.org/picocolors/-/picocolors-1.1.1.tgz",
@@ -1289,6 +1589,45 @@
         "node": "^10 || ^12 || >=14"
       }
     },
+    "node_modules/postgres-array": {
+      "version": "2.0.0",
+      "resolved": "https://registry.npmjs.org/postgres-array/-/postgres-array-2.0.0.tgz",
+      "integrity": "sha512-VpZrUqU5A69eQyW2c5CA1jtLecCsN2U/bD6VilrFDWq5+5UIEVO7nazS3TEcHf1zuPYO/sqGvUvW62g86RXZuA==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=4"
+      }
+    },
+    "node_modules/postgres-bytea": {
+      "version": "1.0.1",
+      "resolved": "https://registry.npmjs.org/postgres-bytea/-/postgres-bytea-1.0.1.tgz",
+      "integrity": "sha512-5+5HqXnsZPE65IJZSMkZtURARZelel2oXUEO8rH83VS/hxH5vv1uHquPg5wZs8yMAfdv971IU+kcPUczi7NVBQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=0.10.0"
+      }
+    },
+    "node_modules/postgres-date": {
+      "version": "1.0.7",
+      "resolved": "https://registry.npmjs.org/postgres-date/-/postgres-date-1.0.7.tgz",
+      "integrity": "sha512-suDmjLVQg78nMK2UZ454hAG+OAW+HQPZ6n++TNDUX+L0+uUlLywnoxJKDou51Zm+zTCjrCl0Nq6J9C5hP9vK/Q==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=0.10.0"
+      }
+    },
+    "node_modules/postgres-interval": {
+      "version": "1.2.0",
+      "resolved": "https://registry.npmjs.org/postgres-interval/-/postgres-interval-1.2.0.tgz",
+      "integrity": "sha512-9ZhXKM/rw350N1ovuWHbGxnGh/SNJ4cnxHiM0rxE4VN41wsg8P8zWn9hv/buK00RP4WvlOyr/RBDiptyxVbkZQ==",
+      "license": "MIT",
+      "dependencies": {
+        "xtend": "^4.0.0"
+      },
+      "engines": {
+        "node": ">=0.10.0"
+      }
+    },
     "node_modules/react": {
       "version": "19.2.8",
       "resolved": "https://registry.npmjs.org/react/-/react-19.2.8.tgz",
@@ -1392,6 +1731,15 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/split2": {
+      "version": "4.2.0",
+      "resolved": "https://registry.npmjs.org/split2/-/split2-4.2.0.tgz",
+      "integrity": "sha512-UcjcJOWknrNkF6PLX83qcHM6KHgVKNkV62Y8a5uYDVv9ydGQVwAHMKqHdJje1VTWpljG0WYpCDhrCdAOYH4TWg==",
+      "license": "ISC",
+      "engines": {
+        "node": ">= 10.x"
+      }
+    },
     "node_modules/string-width": {
       "version": "4.2.3",
       "resolved": "https://registry.npmjs.org/string-width/-/string-width-4.2.3.tgz",
@@ -1420,6 +1768,30 @@
         "node": ">=8"
       }
     },
+    "node_modules/supabase": {
+      "version": "2.115.0",
+      "resolved": "https://registry.npmjs.org/supabase/-/supabase-2.115.0.tgz",
+      "integrity": "sha512-8fL9vOd6jOntmU8N5DVlHGE2GWR1r57ulsrOzSyO6IRYq5QMyKie8T8DH+hb+caGhYUVJLvmpY7XYwic60Uafg==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "eciesjs": "^0.5.0",
+        "jose": "^6.2.8"
+      },
+      "bin": {
+        "supabase": "dist/supabase.js"
+      },
+      "optionalDependencies": {
+        "@supabase/cli-darwin-arm64": "2.115.0",
+        "@supabase/cli-darwin-x64": "2.115.0",
+        "@supabase/cli-linux-arm64": "2.115.0",
+        "@supabase/cli-linux-arm64-musl": "2.115.0",
+        "@supabase/cli-linux-x64": "2.115.0",
+        "@supabase/cli-linux-x64-musl": "2.115.0",
+        "@supabase/cli-windows-arm64": "2.115.0",
+        "@supabase/cli-windows-x64": "2.115.0"
+      }
+    },
     "node_modules/supports-color": {
       "version": "8.1.1",
       "resolved": "https://registry.npmjs.org/supports-color/-/supports-color-8.1.1.tgz",
@@ -1566,6 +1938,15 @@
         "url": "https://github.com/chalk/wrap-ansi?sponsor=1"
       }
     },
+    "node_modules/xtend": {
+      "version": "4.0.2",
+      "resolved": "https://registry.npmjs.org/xtend/-/xtend-4.0.2.tgz",
+      "integrity": "sha512-LKYU1iAXJXUgAXn9URjiu+MWhyUXHsvfp7mcuYm9dSUKK0/CjtrUwFAxD82/mCWbtLsGjFIad0wIsod4zrTAEQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=0.4"
+      }
+    },
     "node_modules/y18n": {
       "version": "5.0.8",
       "resolved": "https://registry.npmjs.org/y18n/-/y18n-5.0.8.tgz",
diff --git a/package.json b/package.json
index 8030def..04ad91f 100644
--- a/package.json
+++ b/package.json
@@ -11,6 +11,7 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "pg": "^8.23.0",
     "react": "^19.2.8",
     "react-dom": "^19.2.8"
   },
@@ -20,6 +21,7 @@
     "@vitejs/plugin-react": "^6.0.4",
     "concurrently": "^9.2.1",
     "oxlint": "^1.75.0",
+    "supabase": "^2.115.0",
     "vite": "^8.2.0"
   }
 }
diff --git a/run-migration.js b/run-migration.js
new file mode 100644
index 0000000..5a45035
--- /dev/null
+++ b/run-migration.js
@@ -0,0 +1,38 @@
+import pkg from 'pg';
+import fs from 'fs';
+import path from 'path';
+import { fileURLToPath } from 'url';
+
+const { Client } = pkg;
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+
+const client = new Client({
+  host: '127.0.0.1',
+  port: 54322,
+  database: 'postgres',
+  user: 'postgres',
+  password: 'postgres',
+});
+
+async function runMigration() {
+  try {
+    await client.connect();
+    console.log('✅ Connected to Supabase Postgres');
+
+    const migrationFile = path.join(process.cwd(), 'supabase/migrations/20260819064500_create_sessions_messages.sql');
+    const sql = fs.readFileSync(migrationFile, 'utf-8');
+
+    // Execute full migration as one transaction
+    await client.query(sql);
+    console.log('✅ Migration applied successfully!');
+
+  } catch (err) {
+    console.error('❌ Migration failed:', err.message);
+    process.exit(1);
+  } finally {
+    await client.end();
+  }
+}
+
+runMigration();
diff --git a/supabase/.gitignore b/supabase/.gitignore
new file mode 100644
index 0000000..ad9264f
--- /dev/null
+++ b/supabase/.gitignore
@@ -0,0 +1,8 @@
+# Supabase
+.branches
+.temp
+
+# dotenvx
+.env.keys
+.env.local
+.env.*.local
diff --git a/supabase/config.toml b/supabase/config.toml
new file mode 100644
index 0000000..1315660
--- /dev/null
+++ b/supabase/config.toml
@@ -0,0 +1,414 @@
+# For detailed configuration reference documentation, visit:
+# https://supabase.com/docs/guides/local-development/cli/config
+# A string used to distinguish different Supabase projects on the same host. Defaults to the
+# working directory name when running `supabase init`.
+project_id = "padhle"
+
+[api]
+enabled = true
+# Port to use for the API URL.
+port = 54321
+# Schemas to expose in your API. Tables, views and stored procedures in this schema will get API
+# endpoints. `public` and `graphql_public` schemas are included by default.
+schemas = ["public", "graphql_public"]
+# Extra schemas to add to the search_path of every request.
+extra_search_path = ["public", "extensions"]
+# The maximum number of rows returns from a view, table, or stored procedure. Limits payload size
+# for accidental or malicious requests.
+max_rows = 1000
+# Controls whether new tables, views, sequences and functions created in the `public` schema by
+# `postgres` are reachable through the Data API roles (`anon`, `authenticated`, `service_role`)
+# without explicit GRANTs. When unset, new entities are NOT auto-exposed, matching the new cloud
+# default. Set to `true` to keep the legacy behaviour of auto-exposing new entities; this is
+# deprecated and the field is removed on 2026-10-30 once the always-revoked behaviour is permanent.
+# auto_expose_new_tables = true
+
+[api.tls]
+# Enable HTTPS endpoints locally using a self-signed certificate.
+enabled = false
+# Paths to self-signed certificate pair.
+# cert_path = "../certs/my-cert.pem"
+# key_path = "../certs/my-key.pem"
+
+[db]
+# Port to use for the local database URL.
+port = 54322
+# Port used by db diff command to initialize the shadow database.
+shadow_port = 54320
+# Maximum amount of time to wait for health check when starting the local database.
+health_timeout = "2m"
+# The database major version to use. This has to be the same as your remote database's. Run `SHOW
+# server_version;` on the remote database to check.
+major_version = 17
+
+[db.pooler]
+enabled = false
+# Port to use for the local connection pooler.
+port = 54329
+# Specifies when a server connection can be reused by other clients.
+# Configure one of the supported pooler modes: `transaction`, `session`.
+pool_mode = "transaction"
+# How many server connections to allow per user/database pair.
+default_pool_size = 20
+# Maximum number of client connections allowed.
+max_client_conn = 100
+
+# [db.vault]
+# secret_key = "env(SECRET_VALUE)"
+
+[db.migrations]
+# If disabled, migrations will be skipped during a db push or reset.
+enabled = true
+# Specifies an ordered list of schema files, directories, or glob patterns that describe your database.
+# Supports paths relative to supabase directory: "./schemas/*.sql", "./database".
+schema_paths = []
+
+[db.seed]
+# If enabled, seeds the database after migrations during a db reset.
+enabled = true
+# Specifies an ordered list of seed files to load during db reset.
+# Supports glob patterns relative to supabase directory: "./seeds/*.sql"
+sql_paths = ["./seed.sql"]
+
+[db.network_restrictions]
+# Enable management of network restrictions.
+enabled = false
+# List of IPv4 CIDR blocks allowed to connect to the database.
+# Defaults to allow all IPv4 connections. Set empty array to block all IPs.
+allowed_cidrs = ["0.0.0.0/0"]
+# List of IPv6 CIDR blocks allowed to connect to the database.
+# Defaults to allow all IPv6 connections. Set empty array to block all IPs.
+allowed_cidrs_v6 = ["::/0"]
+
+# Uncomment to reject non-secure connections to the database.
+# [db.ssl_enforcement]
+# enabled = true
+
+[realtime]
+enabled = true
+# Bind realtime via either IPv4 or IPv6. (default: IPv4)
+# ip_version = "IPv6"
+# The maximum length in bytes of HTTP request headers. (default: 4096)
+# max_header_length = 4096
+
+[studio]
+enabled = true
+# Port to use for Supabase Studio.
+port = 54323
+# External URL of the API server that frontend connects to.
+api_url = "http://127.0.0.1"
+# OpenAI API Key to use for Supabase AI in the Supabase Studio.
+openai_api_key = "env(OPENAI_API_KEY)"
+
+# Email testing server. Emails sent with the local dev setup are not actually sent - rather, they
+# are monitored, and you can view the emails that would have been sent from the web interface.
+[local_smtp]
+enabled = true
+# Port to use for the email testing server web interface.
+port = 54324
+# Uncomment to expose additional ports for testing user applications that send emails.
+# smtp_port = 54325
+# pop3_port = 54326
+# admin_email = "admin@email.com"
+# sender_name = "Admin"
+
+[storage]
+enabled = true
+# The maximum file size allowed (e.g. "5MB", "500KB").
+file_size_limit = "50MiB"
+
+# Uncomment to configure local storage buckets
+# [storage.buckets.images]
+# public = false
+# file_size_limit = "50MiB"
+# allowed_mime_types = ["image/png", "image/jpeg"]
+# objects_path = "./images"
+
+# Allow connections via S3 compatible clients
+[storage.s3_protocol]
+enabled = true
+
+# Image transformation API is available to Supabase Pro plan.
+# [storage.image_transformation]
+# enabled = true
+
+# Store analytical data in S3 for running ETL jobs over Iceberg Catalog
+# This feature is only available on the hosted platform.
+[storage.analytics]
+enabled = false
+max_namespaces = 5
+max_tables = 10
+max_catalogs = 2
+
+# Analytics Buckets is available to Supabase Pro plan.
+# [storage.analytics.buckets.my-warehouse]
+
+# Store vector embeddings in S3 for large and durable datasets
+[storage.vector]
+enabled = true
+max_buckets = 10
+max_indexes = 5
+
+# Vector Buckets is available to Supabase Pro plan.
+# [storage.vector.buckets.documents-openai]
+
+[auth]
+enabled = true
+# The base URL of your website. Used as an allow-list for redirects and for constructing URLs used
+# in emails.
+site_url = "http://127.0.0.1:3000"
+# The public URL that Auth serves on. Defaults to the API external URL with `/auth/v1` appended.
+# external_url = ""
+# A list of *exact* URLs that auth providers are permitted to redirect to post authentication.
+additional_redirect_urls = ["https://127.0.0.1:3000"]
+# How long tokens are valid for, in seconds. Defaults to 3600 (1 hour), maximum 604,800 (1 week).
+jwt_expiry = 3600
+# JWT issuer URL. If not set, defaults to auth.external_url.
+# jwt_issuer = ""
+# Path to JWT signing key. DO NOT commit your signing keys file to git.
+# signing_keys_path = "./signing_keys.json"
+# If disabled, the refresh token will never expire.
+enable_refresh_token_rotation = true
+# Allows refresh tokens to be reused after expiry, up to the specified interval in seconds.
+# Requires enable_refresh_token_rotation = true.
+refresh_token_reuse_interval = 10
+# Allow/disallow new user signups to your project.
+enable_signup = true
+# Allow/disallow anonymous sign-ins to your project.
+enable_anonymous_sign_ins = false
+# Allow/disallow testing manual linking of accounts
+enable_manual_linking = false
+# Passwords shorter than this value will be rejected as weak. Minimum 6, recommended 8 or more.
+minimum_password_length = 6
+# Passwords that do not meet the following requirements will be rejected as weak. Supported values
+# are: `letters_digits`, `lower_upper_letters_digits`, `lower_upper_letters_digits_symbols`
+password_requirements = ""
+
+# Configure passkey sign-ins.
+# [auth.passkey]
+# enabled = false
+
+# Configure WebAuthn relying party settings (required when passkey is enabled).
+# [auth.webauthn]
+# rp_display_name = "Supabase"
+# rp_id = "localhost"
+# rp_origins = ["http://127.0.0.1:3000"]
+
+[auth.rate_limit]
+# Number of emails that can be sent per hour. Requires auth.email.smtp to be enabled.
+email_sent = 2
+# Number of SMS messages that can be sent per hour. Requires auth.sms to be enabled.
+sms_sent = 30
+# Number of anonymous sign-ins that can be made per hour per IP address. Requires enable_anonymous_sign_ins = true.
+anonymous_users = 30
+# Number of sessions that can be refreshed in a 5 minute interval per IP address.
+token_refresh = 150
+# Number of sign up and sign-in requests that can be made in a 5 minute interval per IP address (excludes anonymous users).
+sign_in_sign_ups = 30
+# Number of OTP / Magic link verifications that can be made in a 5 minute interval per IP address.
+token_verifications = 30
+# Number of Web3 logins that can be made in a 5 minute interval per IP address.
+web3 = 30
+
+# Configure one of the supported captcha providers: `hcaptcha`, `turnstile`.
+# [auth.captcha]
+# enabled = true
+# provider = "hcaptcha"
+# secret = ""
+
+[auth.email]
+# Allow/disallow new user signups via email to your project.
+enable_signup = true
+# If enabled, a user will be required to confirm any email change on both the old, and new email
+# addresses. If disabled, only the new email is required to confirm.
+double_confirm_changes = true
+# If enabled, users need to confirm their email address before signing in.
+enable_confirmations = false
+# If enabled, users will need to reauthenticate or have logged in recently to change their password.
+secure_password_change = false
+# Controls the minimum amount of time that must pass before sending another signup confirmation or password reset email.
+max_frequency = "1s"
+# Number of characters used in the email OTP.
+otp_length = 6
+# Number of seconds before the email OTP expires (defaults to 1 hour).
+otp_expiry = 3600
+
+# Use a production-ready SMTP server
+# [auth.email.smtp]
+# enabled = true
+# host = "smtp.sendgrid.net"
+# port = 587
+# user = "apikey"
+# pass = "env(SENDGRID_API_KEY)"
+# admin_email = "admin@email.com"
+# sender_name = "Admin"
+
+# Uncomment to customize email template
+# [auth.email.template.invite]
+# subject = "You have been invited"
+# content_path = "./supabase/templates/invite.html"
+
+# Uncomment to customize notification email template
+# [auth.email.notification.password_changed]
+# enabled = true
+# subject = "Your password has been changed"
+# content_path = "./supabase/templates/password_changed_notification.html"
+
+[auth.sms]
+# Allow/disallow new user signups via SMS to your project.
+enable_signup = false
+# If enabled, users need to confirm their phone number before signing in.
+enable_confirmations = false
+# Template for sending OTP to users
+template = "Your code is {{ .Code }}"
+# Controls the minimum amount of time that must pass before sending another sms otp.
+max_frequency = "5s"
+
+# Use pre-defined map of phone number to OTP for testing.
+# [auth.sms.test_otp]
+# 4152127777 = "123456"
+
+# Configure logged in session timeouts.
+# [auth.sessions]
+# Force log out after the specified duration.
+# timebox = "24h"
+# Force log out if the user has been inactive longer than the specified duration.
+# inactivity_timeout = "8h"
+
+# This hook runs before a new user is created and allows developers to reject the request based on the incoming user object.
+# [auth.hook.before_user_created]
+# enabled = true
+# uri = "pg-functions://postgres/auth/before-user-created-hook"
+
+# This hook runs before a token is issued and allows you to add additional claims based on the authentication method used.
+# [auth.hook.custom_access_token]
+# enabled = true
+# uri = "pg-functions://<database>/<schema>/<hook_name>"
+
+# Configure one of the supported SMS providers: `twilio`, `twilio_verify`, `messagebird`, `textlocal`, `vonage`.
+[auth.sms.twilio]
+enabled = false
+account_sid = ""
+message_service_sid = ""
+# DO NOT commit your Twilio auth token to git. Use environment variable substitution instead:
+auth_token = "env(SUPABASE_AUTH_SMS_TWILIO_AUTH_TOKEN)"
+
+# Multi-factor-authentication is available to Supabase Pro plan.
+[auth.mfa]
+# Control how many MFA factors can be enrolled at once per user.
+max_enrolled_factors = 10
+
+# Control MFA via App Authenticator (TOTP)
+[auth.mfa.totp]
+enroll_enabled = false
+verify_enabled = false
+
+# Configure MFA via Phone Messaging
+[auth.mfa.phone]
+enroll_enabled = false
+verify_enabled = false
+otp_length = 6
+template = "Your code is {{ .Code }}"
+max_frequency = "5s"
+
+# Configure MFA via WebAuthn
+# [auth.mfa.web_authn]
+# enroll_enabled = true
+# verify_enabled = true
+
+# Use an external OAuth provider. The full list of providers are: `apple`, `azure`, `bitbucket`,
+# `discord`, `facebook`, `github`, `gitlab`, `google`, `keycloak`, `linkedin_oidc`, `notion`, `twitch`,
+# `twitter`, `x`, `slack`, `spotify`, `workos`, `zoom`.
+[auth.external.apple]
+enabled = false
+client_id = ""
+# DO NOT commit your OAuth provider secret to git. Use environment variable substitution instead:
+secret = "env(SUPABASE_AUTH_EXTERNAL_APPLE_SECRET)"
+# Overrides the default auth callback URL derived from auth.external_url.
+redirect_uri = ""
+# Overrides the default auth provider URL. Used to support self-hosted gitlab, single-tenant Azure,
+# or any other third-party OIDC providers.
+url = ""
+# If enabled, the nonce check will be skipped. Required for local sign in with Google auth.
+skip_nonce_check = false
+# If enabled, it will allow the user to successfully authenticate when the provider does not return an email address.
+email_optional = false
+
+# Allow Solana wallet holders to sign in to your project via the Sign in with Solana (SIWS, EIP-4361) standard.
+# You can configure "web3" rate limit in the [auth.rate_limit] section and set up [auth.captcha] if self-hosting.
+[auth.web3.solana]
+enabled = false
+
+# Use Firebase Auth as a third-party provider alongside Supabase Auth.
+[auth.third_party.firebase]
+enabled = false
+# project_id = "my-firebase-project"
+
+# Use Auth0 as a third-party provider alongside Supabase Auth.
+[auth.third_party.auth0]
+enabled = false
+# tenant = "my-auth0-tenant"
+# tenant_region = "us"
+
+# Use AWS Cognito (Amplify) as a third-party provider alongside Supabase Auth.
+[auth.third_party.aws_cognito]
+enabled = false
+# user_pool_id = "my-user-pool-id"
+# user_pool_region = "us-east-1"
+
+# Use Clerk as a third-party provider alongside Supabase Auth.
+[auth.third_party.clerk]
+enabled = false
+# Obtain from https://clerk.com/setup/supabase
+# domain = "example.clerk.accounts.dev"
+
+# OAuth server configuration
+[auth.oauth_server]
+# Enable OAuth server functionality
+enabled = false
+# Path for OAuth consent flow UI
+authorization_url_path = "/oauth/consent"
+# Allow dynamic client registration
+allow_dynamic_registration = false
+
+[edge_runtime]
+enabled = true
+# Supported request policies: `oneshot`, `per_worker`.
+# `per_worker` (default) — enables hot reload during local development.
+# `oneshot` — fallback mode if hot reload causes issues (e.g. in large repos or with symlinks).
+policy = "per_worker"
+# Port to attach the Chrome inspector for debugging edge functions.
+inspector_port = 8083
+# The Deno major version to use.
+deno_version = 2
+
+# [edge_runtime.secrets]
+# secret_key = "env(SECRET_VALUE)"
+
+[analytics]
+enabled = true
+port = 54327
+# Configure one of the supported backends: `postgres`, `bigquery`.
+backend = "postgres"
+
+# Experimental features may be deprecated any time
+[experimental]
+# Configures Postgres storage engine to use OrioleDB (S3)
+orioledb_version = ""
+# Configures S3 bucket URL, eg. <bucket_name>.s3-<region>.amazonaws.com
+s3_host = "env(S3_HOST)"
+# Configures S3 bucket region, eg. us-east-1
+s3_region = "env(S3_REGION)"
+# Configures AWS_ACCESS_KEY_ID for S3 bucket
+s3_access_key = "env(S3_ACCESS_KEY)"
+# Configures AWS_SECRET_ACCESS_KEY for S3 bucket
+s3_secret_key = "env(S3_SECRET_KEY)"
+
+# pg-delta is the schema diff engine for db diff / db pull / db remote commit.
+# Set enabled = false to fall back to the legacy migra engine.
+[experimental.pgdelta]
+enabled = true
+# Directory under `supabase/` where declarative files are written.
+# declarative_schema_path = "./schemas"
+# JSON string passed through to pg-delta SQL formatting.
+# format_options = "{\"keywordCase\":\"upper\",\"indent\":2,\"maxWidth\":80,\"commaStyle\":\"trailing\"}"
diff --git a/supabase/migrations/20260819064500_create_sessions_messages.sql b/supabase/migrations/20260819064500_create_sessions_messages.sql
new file mode 100644
index 0000000..305decb
--- /dev/null
+++ b/supabase/migrations/20260819064500_create_sessions_messages.sql
@@ -0,0 +1,80 @@
+-- Create sessions table
+CREATE TABLE public.sessions (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  user_id UUID NOT NULL REFERENCES auth.users(id) ON DELETE CASCADE,
+  grade TEXT,
+  subject TEXT,
+  chapter TEXT,
+  preview TEXT,
+  created_at TIMESTAMPTZ DEFAULT now(),
+  updated_at TIMESTAMPTZ DEFAULT now()
+);
+
+-- Create messages table
+CREATE TABLE public.messages (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  session_id UUID NOT NULL REFERENCES public.sessions(id) ON DELETE CASCADE,
+  role TEXT NOT NULL CHECK (role IN ('user', 'assistant')),
+  text TEXT NOT NULL,
+  created_at TIMESTAMPTZ DEFAULT now()
+);
+
+-- Create profiles table (optional, for future user metadata)
+CREATE TABLE public.profiles (
+  id UUID PRIMARY KEY REFERENCES auth.users(id) ON DELETE CASCADE,
+  email TEXT NOT NULL,
+  display_name TEXT,
+  avatar_url TEXT,
+  created_at TIMESTAMPTZ DEFAULT now(),
+  updated_at TIMESTAMPTZ DEFAULT now()
+);
+
+-- Create indexes for faster queries
+CREATE INDEX idx_sessions_user_id ON public.sessions(user_id);
+CREATE INDEX idx_messages_session_id ON public.messages(session_id);
+
+-- Enable RLS
+ALTER TABLE public.sessions ENABLE ROW LEVEL SECURITY;
+ALTER TABLE public.messages ENABLE ROW LEVEL SECURITY;
+ALTER TABLE public.profiles ENABLE ROW LEVEL SECURITY;
+
+-- RLS Policies for sessions: users can CRUD only their own sessions
+CREATE POLICY "Users can view own sessions"
+  ON public.sessions FOR SELECT
+  USING (auth.uid() = user_id);
+
+CREATE POLICY "Users can create sessions"
+  ON public.sessions FOR INSERT
+  WITH CHECK (auth.uid() = user_id);
+
+CREATE POLICY "Users can update own sessions"
+  ON public.sessions FOR UPDATE
+  USING (auth.uid() = user_id)
+  WITH CHECK (auth.uid() = user_id);
+
+CREATE POLICY "Users can delete own sessions"
+  ON public.sessions FOR DELETE
+  USING (auth.uid() = user_id);
+
+-- RLS Policies for messages: users can CRUD only messages in their own sessions
+CREATE POLICY "Users can view messages in own sessions"
+  ON public.messages FOR SELECT
+  USING (session_id IN (SELECT id FROM public.sessions WHERE user_id = auth.uid()));
+
+CREATE POLICY "Users can insert messages in own sessions"
+  ON public.messages FOR INSERT
+  WITH CHECK (session_id IN (SELECT id FROM public.sessions WHERE user_id = auth.uid()));
+
+CREATE POLICY "Users can delete own messages"
+  ON public.messages FOR DELETE
+  USING (session_id IN (SELECT id FROM public.sessions WHERE user_id = auth.uid()));
+
+-- RLS Policies for profiles
+CREATE POLICY "Users can view own profile"
+  ON public.profiles FOR SELECT
+  USING (auth.uid() = id);
+
+CREATE POLICY "Users can update own profile"
+  ON public.profiles FOR UPDATE
+  USING (auth.uid() = id)
+  WITH CHECK (auth.uid() = id);
diff --git a/supabase/migrations/20260819120000_grant_api_role_table_access.sql b/supabase/migrations/20260819120000_grant_api_role_table_access.sql
new file mode 100644
index 0000000..b442f14
--- /dev/null
+++ b/supabase/migrations/20260819120000_grant_api_role_table_access.sql
@@ -0,0 +1,24 @@
+-- Grant table privileges to Supabase API roles.
+--
+-- The sessions/messages/profiles tables are owned by `postgres`, and the
+-- default ACL for postgres-owned tables omits SELECT/INSERT/UPDATE for the
+-- PostgREST API roles (anon/authenticated/service_role). This caused
+-- PostgreSQL error 42501 ("permission denied for table ...") on every
+-- database operation through the REST gateway.
+--
+-- RLS remains enabled and is the actual access control; these grants only
+-- allow the roles to reach the tables through PostgREST. service_role also
+-- carries BYPASSRLS, which is the backend's write path.
+
+GRANT SELECT, INSERT, UPDATE, DELETE
+  ON public.profiles,
+     public.sessions,
+     public.messages
+  TO anon, authenticated, service_role;
+
+-- Ensure future tables created in `public` receive the same grants, so the
+-- "always-revoked" default (auto_expose_new_tables) does not silently break
+-- new tables the same way.
+ALTER DEFAULT PRIVILEGES IN SCHEMA public
+  GRANT SELECT, INSERT, UPDATE, DELETE ON TABLES
+  TO anon, authenticated, service_role;
\ No newline at end of file
diff --git a/supabase/migrations/20260819130000_profiles_control_and_trigger.sql b/supabase/migrations/20260819130000_profiles_control_and_trigger.sql
new file mode 100644
index 0000000..2834afb
--- /dev/null
+++ b/supabase/migrations/20260819130000_profiles_control_and_trigger.sql
@@ -0,0 +1,67 @@
+-- Production-like user control: application-level user table (public.profiles)
+-- auto-provisioned from auth.users, with admin-managed control fields.
+
+-- 1. Control columns (inherit existing table-wide grants automatically)
+ALTER TABLE public.profiles
+  ADD COLUMN IF NOT EXISTS role text NOT NULL DEFAULT 'user',
+  ADD COLUMN IF NOT EXISTS is_banned boolean NOT NULL DEFAULT false;
+
+-- 2. Backfill profiles for existing auth users (e.g. the current test accounts)
+INSERT INTO public.profiles (id, email)
+SELECT id, email FROM auth.users
+ON CONFLICT (id) DO NOTHING;
+
+-- 3. Auto-provision a profile row whenever an auth user is created (signup/admin)
+CREATE OR REPLACE FUNCTION public.handle_new_user()
+RETURNS trigger
+LANGUAGE plpgsql
+SECURITY DEFINER
+SET search_path = ''
+AS $$
+BEGIN
+  INSERT INTO public.profiles (id, email)
+  VALUES (new.id, new.email)
+  ON CONFLICT (id) DO NOTHING;
+  RETURN new;
+END;
+$$;
+
+-- 4. Keep profile email in sync with auth.users email changes
+CREATE OR REPLACE FUNCTION public.handle_user_email_change()
+RETURNS trigger
+LANGUAGE plpgsql
+SECURITY DEFINER
+SET search_path = ''
+AS $$
+BEGIN
+  IF new.email IS DISTINCT FROM old.email THEN
+    UPDATE public.profiles SET email = new.email WHERE id = new.id;
+  END IF;
+  RETURN new;
+END;
+$$;
+
+DROP TRIGGER IF EXISTS on_auth_user_created ON auth.users;
+CREATE TRIGGER on_auth_user_created
+  AFTER INSERT ON auth.users
+  FOR EACH ROW EXECUTE FUNCTION public.handle_new_user();
+
+DROP TRIGGER IF EXISTS on_auth_user_email_change ON auth.users;
+CREATE TRIGGER on_auth_user_email_change
+  AFTER UPDATE OF email ON auth.users
+  FOR EACH ROW EXECUTE FUNCTION public.handle_user_email_change();
+
+-- 5. Users may update their own profile, but must NOT be able to escalate
+--    their own role or unban themselves.
+--    NOTE: a column-level REVOKE does NOT override a broad TABLE-level grant.
+--    The earlier grant migration gave INSERT,UPDATE,DELETE on profiles to
+--    anon/authenticated (table-wide => every column). So undo the table-level
+--    write grants for user-facing roles, then re-grant UPDATE only on the
+--    editable columns. Profiles are created via the trigger, so users need no
+--    INSERT or DELETE. service_role (backend/admin) keeps full access and
+--    bypasses RLS.
+REVOKE INSERT, UPDATE, DELETE ON public.profiles FROM authenticated;
+REVOKE INSERT, UPDATE, DELETE ON public.profiles FROM anon;
+
+GRANT SELECT ON public.profiles TO anon, authenticated;
+GRANT UPDATE (display_name, avatar_url) ON public.profiles TO authenticated;
\ No newline at end of file