FastAPI MCP Library

Runtime Request Lifecycle

sequenceDiagram
    participant AI Agent (MCP Client)
    participant FastAPIMCP Server (at /mcp)
    participant FastAPI App (Internal Router)
    participant API Endpoint Logic (@app.get)

    AI Agent (MCP Client)->>+FastAPIMCP Server (at /mcp): 1. Request: `tools/call` (name: "get_item", args: {"item_id": 1})

    Note over FastAPIMCP Server (at /mcp): Looks up "get_item" in its internal operation map. 
 Finds it maps to GET /items/{item_id}.

    FastAPIMCP Server (at /mcp)->>+FastAPI App (Internal Router): 2. Programmatic ASGI Request (NOT a network call) 
 Simulates `GET /items/1`

    FastAPI App (Internal Router)->>+API Endpoint Logic (@app.get): 3. Routes the request internally to the correct function.

    API Endpoint Logic (@app.get)-->>-FastAPI App (Internal Router): 4. Executes logic and returns data (e.g., a Pydantic model).

    FastAPI App (Internal Router)-->>-FastAPIMCP Server (at /mcp): 5. Internal response is sent back.

    Note over FastAPIMCP Server (at /mcp): Formats the API response into the 
 required MCP `CallToolResult` structure.

    FastAPIMCP Server (at /mcp)-->>-AI Agent (MCP Client): 6. Result: `CallToolResult` with the item data.

Application Startup and Tool Generation

graph TD
    subgraph Development Time
        A["FastAPI App Code 
- @app.get('/items/{item_id}') 
- Pydantic Models 
- Docstrings"]
    end

    subgraph Application Startup
        B("1.mcp = FastApiMCP(app)")
        C["2.Generate OpenAPI Schema"]
        D["3.fastapi-mcp Converter"]
        E["4.Create MCP Tool Definitions"]
        F["5.Configure MCP Server Instance"]
        G("6.mcp.mount_http()")
    end

    subgraph Running Application
        H["Live FastAPI Server 
 with /mcp endpoint"]
    end

    A --> B
    B -- triggers --> C
    C -- schema.json --> D
    D -- "parses & translates" --> E
    E -- "loads into" --> F
    F -- "routes are added by" --> G
    G -- "mounts onto" --> H

    style A fill:#D6EAF8,stroke:#3498DB
    style H fill:#D5F5E3,stroke:#2ECC71

Sources:

• https://github.com/tadata-org/fastapi_mcp