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+# API Endpoint Reference
+
+This document summarizes the current FastAPI endpoints implemented in `backend/main.py`. It is intended as a quick reference for frontend contributors, backend contributors, and reviewers.
+
+Default local backend URL:
+
+```text
+http://localhost:8000
+```
+
+Default frontend API base:
+
+```text
+http://localhost:8000/api
+```
+
+## Endpoint Table
+
+| Method | Path | Purpose | Request | Success response |
+| --- | --- | --- | --- | --- |
+| `GET` | `/` | Backend health check and AI mode status. | None | App metadata and offline/Gemini mode. |
+| `POST` | `/api/generate-cards` | Generate study cards from a topic. | JSON `GenerateCardsRequest` | Session id, cards, count, offline flag. |
+| `POST` | `/api/upload-pdf` | Extract PDF text and generate cards. | Multipart PDF file named `file` | Session id, cards, source file, offline flag. |
+| `POST` | `/api/upload-notes` | Generate cards from pasted notes. | JSON `UploadNotesRequest` | Session id, cards, offline flag. |
+| `POST` | `/api/check-answer` | Validate a quiz answer. | JSON `QuizAnswerRequest` | Correct flag, correct answer, explanation. |
+| `POST` | `/api/bookmark/{card_id}` | Save a card id to bookmarks. | Path parameter | Bookmark count and status. |
+| `POST` | `/api/view/{card_id}` | Record a card view/completion. | Path parameter | Updated completed-card count. |
+| `GET` | `/api/progress` | Read aggregate progress stats. | None | Cards completed, quiz stats, accuracy, bookmarks. |
+| `GET` | `/api/sessions` | List generated study sessions. | None | Session summaries. |
+| `GET` | `/api/session/{session_id}` | Read one stored session. | Path parameter | Full session record. |
+
+## Request Models
+
+### GenerateCardsRequest
+
+```json
+{
+  "topic": "Stellar basics",
+  "difficulty": "intermediate",
+  "num_cards": 5
+}
+```
+
+Validation:
+
+- `topic`: 2 to 200 characters.
+- `difficulty`: `beginner`, `intermediate`, or `advanced`.
+- `num_cards`: integer from 3 to 10.
+
+### UploadNotesRequest
+
+```json
+{
+  "text_content": "Paste study notes or article text here.",
+  "title": "Lecture notes"
+}
+```
+
+Validation:
+
+- `text_content`: 10 to 15,000 characters.
+- `title`: 2 to 200 characters.
+
+### QuizAnswerRequest
+
+```json
+{
+  "card_id": "session_0_123456789",
+  "selected_answer": "B"
+}
+```
+
+## Response Examples
+
+### Health Check
+
+```http
+GET /
+```
+
+```json
+{
+  "status": "running",
+  "app": "Study Copilot API",
+  "version": "1.0.0",
+  "ai_provider": "Google Gemini",
+  "mode": "OFFLINE SANDBOX",
+  "model": null,
+  "offline_mode": true
+}
+```
+
+### Generate Cards
+
+```http
+POST /api/generate-cards
+Content-Type: application/json
+```
+
+```json
+{
+  "topic": "Stellar basics",
+  "difficulty": "beginner",
+  "num_cards": 5
+}
+```
+
+```json
+{
+  "success": true,
+  "session_id": "session_1234567890.0",
+  "cards": [],
+  "total_cards": 5,
+  "offline_mode": true
+}
+```
+
+`cards` contains the generated study card objects. The example uses an empty array placeholder to keep the response shape short.
+
+### Upload PDF
+
+```http
+POST /api/upload-pdf
+Content-Type: multipart/form-data
+```
+
+Request field:
+
+```text
+file=<example.pdf>
+```
+
+```json
+{
+  "success": true,
+  "session_id": "pdf_session_1234567890.0",
+  "cards": [],
+  "source_file": "example.pdf",
+  "offline_mode": true
+}
+```
+
+### Upload Notes
+
+```http
+POST /api/upload-notes
+Content-Type: application/json
+```
+
+```json
+{
+  "text_content": "These are notes about accounts, assets, and transactions.",
+  "title": "Stellar lecture"
+}
+```
+
+```json
+{
+  "success": true,
+  "session_id": "notes_session_1234567890.0",
+  "cards": [],
+  "offline_mode": true
+}
+```
+
+### Check Answer
+
+```http
+POST /api/check-answer
+Content-Type: application/json
+```
+
+```json
+{
+  "card_id": "Stellar_basics_2_1234567890.0",
+  "selected_answer": "B"
+}
+```
+
+```json
+{
+  "correct": true,
+  "correct_answer": "B",
+  "explanation": "The correct answer is B: Start with core concepts"
+}
+```
+
+### Progress
+
+```http
+GET /api/progress
+```
+
+```json
+{
+  "cards_completed": 3,
+  "quizzes_attempted": 2,
+  "quizzes_correct": 1,
+  "accuracy": 50.0,
+  "study_streak": 0,
+  "bookmarks": 1
+}
+```
+
+### Sessions
+
+```http
+GET /api/sessions
+```
+
+```json
+{
+  "sessions": [
+    {
+      "session_id": "session_1234567890.0",
+      "topic": "Stellar basics",
+      "num_cards": 5,
+      "created_at": "2026-06-25T12:00:00",
+      "offline_mode": true
+    }
+  ]
+}
+```
+
+## Error Responses
+
+Common errors:
+
+| Status | Endpoint | Cause |
+| --- | --- | --- |
+| `400` | `/api/upload-pdf` | Uploaded file is not a PDF. |
+| `400` | `/api/upload-pdf` | PDF cannot be read. |
+| `400` | `/api/upload-pdf` | PDF contains no readable text. |
+| `400` | `/api/check-answer` | Card is not a quiz card. |
+| `400` | `/api/check-answer` | Quiz card has no correct option. |
+| `404` | `/api/check-answer` | Card id is not found in stored sessions. |
+| `404` | `/api/session/{session_id}` | Session id is not found. |
+| `413` | `/api/upload-pdf` | PDF is larger than 10MB. |
+| `422` | JSON endpoints | Request body fails Pydantic validation. |
+
+Example error shape:
+
+```json
+{
+  "detail": "Only PDF files are supported."
+}
+```
+
+## State Notes
+
+The current backend stores sessions, bookmarks, and progress in memory. Restarting the backend clears that state.
+
+When Gemini is unavailable, endpoints can still return fallback cards with `offline_mode: true`.
+