diff --git a/frontend/CLAUDE.md b/frontend/CLAUDE.md
index 50fd407..25c9fd6 100644
--- a/frontend/CLAUDE.md
+++ b/frontend/CLAUDE.md
@@ -446,8 +446,108 @@ try {
 
 ---
 
+## Backend API Reference
+
+### REST Endpoints
+
+**Auth** (`/api/auth/`)
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| GET | `/auth/google` | Start Google OAuth (redirect) |
+| GET | `/auth/google/callback` | Google OAuth callback |
+| GET | `/auth/discord` | Start Discord OAuth (redirect) |
+| GET | `/auth/discord/callback` | Discord OAuth callback |
+| POST | `/auth/refresh` | Refresh access token |
+| POST | `/auth/logout` | Revoke refresh token |
+| POST | `/auth/logout-all` | Revoke all user tokens |
+| GET | `/auth/link/google` | Link Google account (authed) |
+| GET | `/auth/link/discord` | Link Discord account (authed) |
+
+**Users** (`/api/users/`)
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| GET | `/users/me` | Get current user profile |
+| PATCH | `/users/me` | Update profile (display_name, avatar_url) |
+| GET | `/users/me/linked-accounts` | List linked OAuth accounts |
+| DELETE | `/users/me/link/{provider}` | Unlink OAuth provider |
+| GET | `/users/me/sessions` | Get active session count |
+| GET | `/users/me/starter-status` | Check if starter deck selected |
+| POST | `/users/me/starter-deck` | Select starter deck |
+
+**Collections** (`/api/collections/`)
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| GET | `/collections/me` | Get user's card collection |
+| GET | `/collections/me/cards/{card_id}` | Get specific card quantity |
+
+**Decks** (`/api/decks/`)
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| GET | `/decks` | List user's decks |
+| POST | `/decks` | Create new deck |
+| GET | `/decks/{id}` | Get deck by ID |
+| PUT | `/decks/{id}` | Update deck |
+| DELETE | `/decks/{id}` | Delete deck |
+| POST | `/decks/validate` | Validate deck without saving |
+
+**Games** (`/api/games/`)
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| POST | `/games` | Create new game |
+| GET | `/games/{id}` | Get game info |
+| GET | `/games/me/active` | List user's active games |
+| POST | `/games/{id}/resign` | Resign from game (HTTP fallback) |
+
+### WebSocket Events (Socket.IO)
+
+**Namespace:** `/game`
+
+**Client → Server:**
+| Event | Payload | Description |
+|-------|---------|-------------|
+| `game:join` | `{ game_id }` | Join/rejoin a game room |
+| `game:action` | `{ action_type, ...data }` | Execute game action |
+| `game:resign` | `{ game_id }` | Resign from game |
+| `game:spectate` | `{ game_id }` | Join as spectator |
+| `game:leave_spectate` | `{ game_id }` | Leave spectator mode |
+
+**Server → Client:**
+| Event | Payload | Description |
+|-------|---------|-------------|
+| `game:state` | `GameState` | Full game state (visibility filtered) |
+| `game:action_result` | `ActionResult` | Result of player action |
+| `game:error` | `{ message, code }` | Error notification |
+| `game:turn_timeout_warning` | `{ remaining_seconds }` | Turn timer warning |
+| `game:game_over` | `{ winner, reason }` | Game ended |
+| `game:opponent_connected` | `{ connected }` | Opponent connection status |
+| `game:reconnected` | `{ game_id, state }` | Auto-reconnected to game |
+| `game:spectator_count` | `{ count }` | Spectator count update |
+
+### Key Backend Files
+
+| File | Description |
+|------|-------------|
+| `backend/app/api/auth.py` | Auth endpoints implementation |
+| `backend/app/api/users.py` | User endpoints implementation |
+| `backend/app/api/games.py` | Game REST endpoints |
+| `backend/app/schemas/` | Pydantic request/response schemas |
+| `backend/app/schemas/ws_messages.py` | WebSocket message schemas |
+| `backend/app/socketio/game_namespace.py` | WebSocket event handlers |
+
+### Type Definitions to Mirror
+
+When creating frontend types, reference these backend schemas:
+- `backend/app/schemas/user.py` → `UserResponse`, `UserUpdate`
+- `backend/app/schemas/deck.py` → `DeckResponse`, `DeckCreate`, `DeckUpdate`
+- `backend/app/schemas/game.py` → `GameCreateRequest`, `GameResponse`
+- `backend/app/schemas/ws_messages.py` → All WebSocket message types
+
+---
+
 ## See Also
 
 - `PROJECT_PLAN_FRONTEND.json` - Phase plan and tasks
 - `../backend/CLAUDE.md` - Backend guidelines
+- `../backend/app/schemas/` - API schema definitions
+- `../backend/app/socketio/README.md` - WebSocket documentation
 - `../CLAUDE.md` - Project-wide guidelines