FastMCP: how to get auto-reload / hot reload for the server during development?

Question

I'm developing a FastMCP (PrefectHQ/fastmcp) server and want code changes to be picked up automatically while it's running. Does FastMCP support auto-reload? A feature request for --reload was closed as completed, and comments suggest running Uvicorn directly, but python -m uvicorn main:sse_app --host 0.0.0.0 --port 8000 --reload fails for me with TypeError: 'FastMCP' object is not callable.

Originally asked on GitHub: https://github.com/PrefectHQ/fastmcp/issues/616

AgentArmorHQ team · Accepted Answer

**Status: explained.** This issue asked FastMCP for a `--reload` option and was closed as completed. In the discussion, a maintainer said running Uvicorn is probably best, and that they don't think hot reloading will be reimplemented inside FastMCP itself.

The workaround from the thread is to skip `mcp.run()` and boot Uvicorn directly with the ASGI app, per the FastMCP ASGI docs (see https://gofastmcp.com/deployment/asgi):

```bash
python -m uvicorn main:sse_app --host 0.0.0.0 --port 8000 --reload
```

`sse_app` must be the ASGI app exposed from your module, not the `FastMCP` instance itself. One user tried this exact command and got `TypeError: 'FastMCP' object is not callable`, then asked what `sse_app` should be, so double-check what your module exports.

Passing `uvicorn_config={"reload": True}` to `mcp.run()` was also tried and did not work: the option goes through, but Uvicorn's `Config.should_reload` requires the app to be a string, while FastMCP passes an app instance. The same commenter noted that getting to the app instance inside `anyio.run(run_async(run_http_async(...)))` is tricky, and that rewriting `run_http_async` is possible but they'd prefer a proper hook or FAQ entry.

As general advice, keep your FastMCP app in its own module (e.g. `main.py`) so the `main:sse_app` import string works with Uvicorn's reload mode. Also as general advice: `--host 0.0.0.0` binds all network interfaces; use `127.0.0.1` for local-only development.

AgentArmorHQ team · Answer

**What @Zambonilli tried on GitHub** (Outcome unclear)

Suggested running the server through Uvicorn directly per the FastMCP ASGI docs: `python -m uvicorn main:sse_app --host 0.0.0.0 --port 8000 --reload`.

Source: https://github.com/PrefectHQ/fastmcp/issues/616#issuecomment-2956748514

AgentArmorHQ team · Answer

**What @bojanbg tried on GitHub** (Didn't work)

Ran the suggested Uvicorn command but got `TypeError: 'FastMCP' object is not callable` and asked what `sse_app` refers to.

Source: https://github.com/PrefectHQ/fastmcp/issues/616#issuecomment-3118421726

AgentArmorHQ team · Answer

**What @cpbotha tried on GitHub** (Didn't work)

Tried `mcp.run(transport="http", port=8000)` with `uvicorn_config={"reload": True}`. The option was accepted, but Uvicorn's `Config.should_reload` requires the app to be a string, while FastMCP passes an app instance.

Source: https://github.com/PrefectHQ/fastmcp/issues/616#issuecomment-3410128742

FastMCP: how to get auto-reload / hot reload for the server during development?

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FastMCP: how to get auto-reload / hot reload for the server during development?

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