# API Contract: Chat (unchanged endpoints)

**Branch**: `004-rag-retrieval-quality` | **Date**: 2026-04-06

## No endpoint changes

This feature makes no changes to the public API surface. All existing `/api/v1/chat/...` endpoints remain identical in path, method, request body, and response shape.

## Message response — `sources` field addition

The only observable change to callers is that each source entry in `Message.sources` gains an optional `refLabel` field. This is backwards-compatible (additive only).

### Existing contract (unchanged)

```
POST /api/v1/chat/sessions/{sessionId}/messages
Body: { "content": "..." }
Response: Message { id, sessionId, role, content, sources: [...], createdAt }
```

### Source entry schema (additive change)

Before:
```json
{ "type": "TEXT", "bookTitle": "...", "page": 142, "chunkText": "..." }
```

After (new optional field):
```json
{ "type": "TEXT", "refLabel": "S1", "bookTitle": "...", "page": 142, "chunkText": "..." }
```

Frontend consumers that ignore unknown fields are unaffected. `ChatMessage.vue` may optionally use `refLabel` for future inline citation linking.