Fresh install of magg 0.10.1 via uv tool install magg pulls fastmcp 3.1.0, which has two breaking incompatibilities that prevent magg from starting.
Environment
- magg: 0.10.1
- fastmcp: 3.1.0 (installed automatically by uv)
- Python: 3.13
- OS: Windows 10
- Transport: stdio via Claude Desktop
Error 1: BearerAuthProvider import removed in fastmcp 3.x
File "magg/auth.py", line 11, in <module>
from fastmcp.server.auth import BearerAuthProvider
ImportError: cannot import name 'BearerAuthProvider' from 'fastmcp.server.auth'
Per the fastmcp upgrade guide (https://gofastmcp.com/development/upgrade-guide), BearerAuthProvider was replaced by JWTVerifier in 3.x.
Workaround: Manually patching auth.py with a try/except fallback gets past this error.
Error 2: ProxyFastMCP._tool_serializer removed in fastmcp 3.x
After patching Error 1, magg crashes on startup with:
File "magg/proxy/server.py", line 87, in _register_proxy_tool
serializer=self._tool_serializer,
^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'ProxyFastMCP' object has no attribute '_tool_serializer'
This is a deeper API change in fastmcp 3.x where _tool_serializer was removed or renamed on the FastMCP base class. Not patchable with a simple import fix.
Full traceback (Error 2)
File "magg/cli.py", line 48, in cmd_serve
runner = MaggRunner(args.config, env=env)
File "magg/server/runner.py", line 37, in __init__
self._server = MaggServer(config_path, env=env)
File "magg/server/server.py", line 40, in __init__
server_manager = ServerManager(config_manager, env=env)
File "magg/server/manager.py", line 55, in __init__
self.mcp = ProxyFastMCP(...)
File "magg/proxy/server.py", line 69, in __init__
super().__init__(*args, **kwargs)
File "magg/proxy/mixin.py", line 41, in __init__
self._register_proxy_tool()
File "magg/proxy/server.py", line 87, in _register_proxy_tool
serializer=self._tool_serializer,
AttributeError: 'ProxyFastMCP' object has no attribute '_tool_serializer'
Suggested Fix
Either:
- Pin fastmcp to
<3.0 in magg's dependencies until proxy compatibility is updated
- Update
magg/proxy/server.py and magg/auth.py for fastmcp 3.x API changes
Notes
We previously hit the BearerAuthProvider incompatibility with fastmcp 2.x back in early March. This is the same fundamental issue — magg's fastmcp dependency isn't version-pinned, so breaking changes cascade into fresh installs.
Love the project — the concept of LLMs managing their own tool ecosystem is exactly what we need. Happy to test fixes.
Fresh install of magg 0.10.1 via
uv tool install maggpulls fastmcp 3.1.0, which has two breaking incompatibilities that prevent magg from starting.Environment
Error 1: BearerAuthProvider import removed in fastmcp 3.x
Per the fastmcp upgrade guide (https://gofastmcp.com/development/upgrade-guide),
BearerAuthProviderwas replaced byJWTVerifierin 3.x.Workaround: Manually patching
auth.pywith a try/except fallback gets past this error.Error 2: ProxyFastMCP._tool_serializer removed in fastmcp 3.x
After patching Error 1, magg crashes on startup with:
This is a deeper API change in fastmcp 3.x where
_tool_serializerwas removed or renamed on the FastMCP base class. Not patchable with a simple import fix.Full traceback (Error 2)
Suggested Fix
Either:
<3.0in magg's dependencies until proxy compatibility is updatedmagg/proxy/server.pyandmagg/auth.pyfor fastmcp 3.x API changesNotes
We previously hit the BearerAuthProvider incompatibility with fastmcp 2.x back in early March. This is the same fundamental issue — magg's fastmcp dependency isn't version-pinned, so breaking changes cascade into fresh installs.
Love the project — the concept of LLMs managing their own tool ecosystem is exactly what we need. Happy to test fixes.