Cap mcp[cli] < 2.0.0 to prevent startup crash under mcp 2.0 - #85
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thedomato wants to merge 1 commit into
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Cap mcp[cli] < 2.0.0 to prevent startup crash under mcp 2.0#85thedomato wants to merge 1 commit into
thedomato wants to merge 1 commit into
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The MCP SDK's mcp 2.0.0 (2026-07-28) removed the mcp.server.fastmcp
module that app.py imports (`from mcp.server.fastmcp import FastMCP`).
Because mcp[cli] is pinned only as >=1.10.0 with no upper bound, the
`uv run --with mcp[cli] --with-editable . mcp run .../server.py` launch
used by Claude Desktop resolves a fresh env to 2.0.0 and crashes at
import time with:
ModuleNotFoundError: No module named 'mcp.server.fastmcp'
before the server can serve, which Claude Desktop surfaces as the
server disconnecting. Capping at <2.0.0 resolves to the latest 1.x
(which still ships mcp.server.fastmcp) and restores startup. A proper
migration to the mcp 2.0 API can follow separately.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Problem
The server fails to start under Claude Desktop with:
The MCP Python SDK released
mcp 2.0.0on 2026-07-28, which removed themcp.server.fastmcpmodule (app.py,auth.py, andtools/auth.pyall import from it).pyproject.toml/requirements.txtpinmcp[cli]>=1.10.0with no upper bound. The documented Claude Desktop launch —— builds a fresh, unpinned environment and resolves
mcpto the newest match (2.0.0), so the import crashes before the server can serve. Claude Desktop surfaces this as the server disconnecting.Fix
Cap
mcp[cli]>=1.10.0,<2.0.0inpyproject.tomlandrequirements.txt, and refreshuv.lock(resolves tomcp 1.13.1, which still shipsmcp.server.fastmcp).Verification
Reproduced the exact Claude Desktop launch path against this branch:
No
ModuleNotFoundError. A proper migration to the mcp 2.0 API can follow separately; this restores startup in the meantime.