FastMCP Injector

Dependency injection integration for FastMCP using injector.

Inspired by fastapi-injector.

Installation

pip install fastmcp-injector

Usage

from fastmcp import FastMCP
from injector import Injector, inject, singleton

from fastmcp_injector import Injected, attach_injector


# Define your services
class Database:
    @inject
    def __init__(self):
        self._data = {"answer": 42}

    def query(self, q: str) -> str:
        return str(self._data)


# Set up DI and MCP
injector = Injector()
mcp = FastMCP()
attach_injector(mcp, injector)


# Use Injected[T] to declare dependencies — they won't appear in the tool schema
@mcp.tool()
def ask_database(question: str, db: Injected[Database]) -> str:
    return db.query(question)


if __name__ == "__main__":
    mcp.run()

Injected[T] parameters are resolved automatically from the injector container at call time. Only regular parameters (like question: str) are exposed in the MCP tool schema.

How it works

1. attach_injector(mcp, injector) patches mcp.tool() to intercept tool registration.
2. Parameters annotated with Injected[T] are stripped from the function signature before FastMCP generates the tool schema.
3. At invocation time, a wrapper resolves those dependencies via injector.get(T) and injects them into the original function.

License

MIT