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FastAPI MCP Server + LangChain Client Example

@SDCalvo

Repository

Basic information

Category

AI & Agents

License

Apache-2.0 license

Transports

stdio

Author

SDCalvo

About FastAPI MCP Server + LangChain Client Example

Example project demonstrating how to expose FastAPI endpoints as Model Context Protocol (MCP) tools using `fastapi-mcp`. Includes a basic LangChain agent (`langchain_client.py`) that connects to the local FastAPI server via HTTP/SSE using `langchain-mcp-adapters` to discover and

FastAPI MCP Server + LangChain Client Example config

Add this server to your MCP-compatible client using the configuration below.

{
  "mcpServers": {
    "sse-mcp-and-langchain-client-example": {
      "command": "uv",
      "args": [
        "init",
        "#",
        "If",
        "pyproject.toml",
        "doesnt exist"
      ]
    }
  }
}

FastAPI MCP Server + LangChain Client Example tools

Tool list not available yet

This listing does not currently have a tool list to display. This does not mean the server has no tools; check its documentation for supported capabilities.

We auto-extract tools from the README. The maintainer can list them under a ## Tools heading to populate this section.

Overview

What is FastAPI MCP Server + LangChain Client Example?

This example project demonstrates how to expose FastAPI endpoints as Model Context Protocol (MCP) tools using the fastapi-mcp library. It includes a LangChain agent client (langchain_client.py) that connects to the local FastAPI server via HTTP/SSE using langchain-mcp-adapters to discover and use the exposed tools. It is intended for learning and testing, runnable locally and connectable from MCP clients like Cursor’s agent.

How to use FastAPI MCP Server + LangChain Client Example?

Clone the repository, install uv, create a virtual environment, and install dependencies (fastapi, uvicorn, fastapi-mcp, langchain-mcp-adapters, langgraph, langchain-openai, python-dotenv). Create a .env file with your OpenAI API key. In one terminal, run uvicorn main:app --reload --port 8000. In a second terminal, run uv run python langchain_client.py. Optionally, test with the MCP Inspector via npx @modelcontextprotocol/inspector or configure Cursor using a .cursor/mcp.json with the server URL http://127.0.0.1:8000/mcp.

Key features of FastAPI MCP Server + LangChain Client Example

  • Exposes FastAPI endpoints as MCP tools automatically via fastapi-mcp.
  • LangChain agent discovers and calls tools through SSE transport.
  • Supports Cursor editor integration via HTTP-based MCP configuration.
  • Includes a ready‑to‑run example with sample endpoints and queries.
  • Demonstrates optional MCP Inspector for interactive tool testing.

Use cases of FastAPI MCP Server + LangChain Client Example

  • Learning how to turn a FastAPI application into an MCP server.
  • Rapid prototyping of MCP tools using familiar FastAPI route definitions.
  • Integrating FastAPI‑backed tools into AI agents like LangChain or Cursor.
  • Testing and debugging MCP server features locally before deployment.

Frequently asked questions

1

What prerequisites are needed?

Python 3.10 or higher, the `uv` package manager, Node.js and npm (for the optional MCP Inspector), and an OpenAI API key (for the LangChain client). Git is required for cloning the repository.

2

How are MCP tool names determined?

MCP tool names are derived from the FastAPI route’s `operation_id`. If not set explicitly, FastAPI generates an automatic name (e.g., `read_root__get`). Setting explicit `operation_id` values on routes is recommended for clearer tool names.

3

What if a port is already in use?

Use `netstat -ano | findstr "<PORT>"` (Windows) to find the process ID, then `taskkill /F /PID <PID>` to terminate it. Sometimes a short wait or restarting the IDE is needed for the OS to fully release the port.

4

How do I add new routes after the server is mounted?

Routes defined after calling `mcp.mount()` are not automatically included. Call `mcp.setup_server()` again after defining the new routes to refresh the tool list.

5

Does this server support authentication?

Yes. `fastapi-mcp` supports FastAPI dependencies for authorization and includes OAuth 2 support. Basic token passthrough can be configured via `mcp-remote` with headers, or server‑side dependencies can be added using `AuthConfig`.

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