Server fails to start: ModuleNotFoundError: No module named 'mcp.server.fastmcp'

[yuryu]

As of early August 2026, the server fails to start with ModuleNotFoundError: No module named 'mcp.server.fastmcp'. The MCP Python SDK v2 renamed FastMCP to MCPServer and moved mcp.server.fastmcp.* to mcp.server.mcpserver.* (see the v1→v2 migration guide). Because the recommended launch command uses an unpinned --with 'mcp[cli]', uv now resolves to SDK v2 and the import in app.py fails at startup.

Steps to reproduce

1. Configure the server per the README, e.g.:

   uv run --with 'mcp[cli]' --with-editable /path/to/monarch-mcp-server mcp run /path/to/monarch-mcp-server/src/monarch_mcp_server/server.py

2. Let uv resolve mcp[cli] to the latest version (now ≥2.0).
3. Start the server (e.g., via Claude Desktop).

Expected behavior

Server starts and connects.

Actual behavior

Failed to run server
Traceback (most recent call last):
  File ".../site-packages/mcp/cli/cli.py", line 341, in run
    server = _import_server(file, server_object)
  File ".../site-packages/mcp/cli/cli.py", line 141, in _import_server
    spec.loader.exec_module(module)
  File ".../monarch-mcp-server/src/monarch_mcp_server/server.py", line 10, in <module>
    from monarch_mcp_server.app import mcp, main, app  # noqa: F401
  File ".../monarch-mcp-server/src/monarch_mcp_server/app.py", line 5, in <module>
    from mcp.server.fastmcp import FastMCP
ModuleNotFoundError: No module named 'mcp.server.fastmcp'

The MCP client (Claude Desktop) then reports "Server disconnected."

Environment

• macOS (Apple Silicon), Python 3.13 via uv
• mcp[cli] unpinned → resolves to SDK v2
• Client: Claude Desktop

Workaround

Pin the SDK to v1 in the launch command:

--with 'mcp[cli]<2'

(or pin mcp<2 in pyproject.toml). This restores the previous behavior.

[jkmaxwell]

+1, workaround works; thank you.