# Fastapi Sse MCP Protocol > Host a hosted MCP (Model Context Protocol) server over FastAPI's StreamingResponse using API-key auth, a session dict, and a tool registry surfaced via the 2025-03-26 Streamable HTTP spec. Use when this capability is needed. - Skill: `tomevault-io/fastapi-sse-mcp-protocol` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/fastapi-sse-mcp-protocol` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/fastapi-sse-mcp-protocol/raw - Safety review: pending (external: skill-scanner PASS, skillspector PASS) - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: AI & ML - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/fastapi-sse-mcp-protocol --- # Pre-hosted MCP Server via FastAPI Streamable HTTP Expose your LLM-tool surface as a pre-hosted MCP server — clients (Claude Desktop, Cursor, custom agents) talk HTTP instead of spawning a local stdio process. Built on FastAPI `APIRouter` + `StreamingResponse`, a lightweight `MCPSession`, API-key auth, and a plain JSON tool registry. ## When to use - You own a backend with rich internal tools (DB queries, user memories, vector search) and want Claude / Cursor to invoke them remotely. - You prefer Bearer API keys per user instead of shipping a client binary. - You want tools discoverable via the standard MCP `tools/list` handshake without an extra daemon. ## Core pattern 1. **Router**: `router = APIRouter()` mounted under `/mcp` in your FastAPI app. 2. **Session object**: `MCPSession(session_id=str, user_id=str)` with `created_at` + `initialized` flag. Keep them in `active_sessions: dict` (or Redis if multi-instance). 3. **Auth**: `authenticate_api_key(Authorization)` accepts `Bearer ` or raw ``, enforces a prefix (`omi_mcp_`), and looks up the user via a dedicated `mcp_api_key` table. 4. **Tool registry**: `MCP_TOOLS = [{name, description, inputSchema}, ...]` — schema is JSON Schema with `type: object`, enums sourced from `MemoryCategory` / `CategoryEnum` Python enums. 5. **Transport**: endpoints return `StreamingResponse(async_gen, media_type="text/event-stream")` for SSE, `JSONResponse` for one-shot ops. ## Steps 1. Define your tool list in code (not YAML): one dict per tool with `name`, `description`, `inputSchema`. Source enum values from your Pydantic models so they can't drift. 2. Implement `authenticate_api_key(authorization)` — return `user_id` or `None`. Use a prefix sentinel to reject stray Bearer tokens. 3. Implement `POST /mcp` — accept JSON body per MCP spec (`initialize`, `tools/list`, `tools/call`). Route by `method` field. 4. For streaming results, yield `data: {json}\n\n` SSE events from an `async def` and wrap with `StreamingResponse(..., media_type="text/event-stream")`. 5. Rate-limit per user with `check_rate_limit_inline(user_id, limit, bucket)` on every call site. 6. Use `uuid.uuid4()` for `session_id`; stash in `active_sessions` only for stateful tools. 7. Serve enum-backed JSON Schema so MCP clients get autocomplete without extra docs. ## Implementation notes - MCP clients expect `{ "jsonrpc": "2.0", "id": , "result": {...} }` envelopes — wrap tool outputs accordingly. - `tools/list` returns the raw `MCP_TOOLS` array — no transformation needed. - Use separate top-level endpoints for `sse` and one-shot `POST` so load balancers with short timeouts don't kill long SSE connections. - API key format `omi_mcp_xxxx` — the prefix doubles as a fast reject for unrelated tokens without a DB hit. - Reuse your existing services (`memories_db`, `conversations_db`, `vector_db`) — MCP is a transport, not a new data layer. ## Evidence in source - `backend/routers/mcp_sse.py` — MCPSession, authenticate_api_key, MCP_TOOLS, StreamingResponse usage - `backend/routers/chat.py` — WebSocket + StreamingResponse reference implementation - `backend/main.py` — FastAPI app composition ## Reusability Any FastAPI product with a meaningful internal tool surface can adopt the same layout. The tool registry is transport-agnostic; you can bolt on stdio MCP later from the same `MCP_TOOLS` list. --- > Source: [kjuhwa/skills-hub](https://github.com/kjuhwa/skills-hub) — distributed by [TomeVault](https://tomevault.io).