unity-mcp/CursorHelp.md

Cursor/VSCode/Windsurf: UV path issue on Windows (diagnosis and fix)

The issue

• Some Windows machines have multiple uv.exe locations. Our auto-config sometimes picked a less stable path, causing the MCP client to fail to launch the MCP for Unity Server or for the path to be auto-rewritten on repaint/restart.

Typical symptoms

• Cursor shows the MCP for Unity server but never connects or reports it “can’t start.”
• Your %USERPROFILE%\\.cursor\\mcp.json flips back to a different command path when Unity or the MCP for Unity window refreshes.

Real-world example

• Wrong/fragile path (auto-picked):
  • C:\Users\mrken.local\bin\uv.exe (malformed, not standard)
  • C:\Users\mrken\AppData\Local\Microsoft\WinGet\Packages\astral-sh.uv_Microsoft.Winget.Source_8wekyb3d8bbwe\uv.exe
• Correct/stable path (works with Cursor):
  • C:\Users\mrken\AppData\Local\Microsoft\WinGet\Links\uv.exe

Quick fix (recommended)

1. In MCP for Unity: Window > MCP for Unity → select your MCP client (Cursor or Windsurf)
2. If you see “uv Not Found,” click “Choose uv Install Location” and browse to:
   • C:\Users\<YOU>\AppData\Local\Microsoft\WinGet\Links\uv.exe
3. If uv is already found but wrong, still click “Choose uv Install Location” and select the Links\uv.exe path above. This saves a persistent override.
4. Click “Auto Configure” (or re-open the client) and restart Cursor.

This sets an override stored in the Editor (key: MCPForUnity.UvPath) so MCP for Unity won’t auto-rewrite the config back to a different uv.exe later.

Verify the fix

• Confirm global Cursor config is at: %USERPROFILE%\\.cursor\\mcp.json
• You should see something like:

{
  "mcpServers": {
    "unityMCP": {
      "command": "C:\\Users\\YOU\\AppData\\Local\\Microsoft\\WinGet\\Links\\uv.exe",
      "args": [
        "--directory",
        "C:\\Users\\YOU\\AppData\\Local\\Programs\\UnityMCP\\UnityMcpServer\\src",
        "run",
        "server.py"
      ]
    }
  }
}

• Manually run the same command in PowerShell to confirm it launches:

"C:\Users\YOU\AppData\Local\Microsoft\WinGet\Links\uv.exe" --directory "C:\Users\YOU\AppData\Local\Programs\UnityMCP\UnityMcpServer\src" run server.py

If that runs without error, restart Cursor and it should connect.

Why this happens

• On Windows, multiple uv.exe can exist (WinGet Packages path, a WinGet Links shim, Python Scripts, etc.). The Links shim is the most stable target for GUI apps to launch.
• Prior versions of the auto-config could pick the first found path and re-write config on refresh. Choosing a path via the MCP window pins a known‑good absolute path and prevents auto-rewrites.

Extra notes

• Restart Cursor after changing mcp.json; it doesn’t always hot-reload that file.
• If you also have a project-scoped .cursor\\mcp.json in your Unity project folder, that file overrides the global one.

Why pin the WinGet Links shim (and not the Packages path)

• Windows often has multiple uv.exe installs and GUI clients (Cursor/Windsurf/VSCode) may launch with a reduced PATH. Using an absolute path is safer than "command": "uv".
• WinGet publishes stable launch shims in these locations:
  • User scope: %LOCALAPPDATA%\Microsoft\WinGet\Links\uv.exe
  • Machine scope: C:\Program Files\WinGet\Links\uv.exe These shims survive upgrades and are intended as the portable entrypoints. See the WinGet notes: discussion • how to find installs
• The Packages root is where payloads live and can change across updates, so avoid pointing your config at it.

Recommended practice

• Prefer the WinGet Links shim paths above. If present, select one via “Choose uv Install Location”.
• If the unity window keeps rewriting to a different uv.exe, pick the Links shim again; MCP for Unity saves a pinned override and will stop auto-rewrites.
• If neither Links path exists, a reasonable fallback is ~/.local/bin/uv.exe (uv tools bin) or a Scoop shim, but Links is preferred for stability.

References

• WinGet portable Links: GitHub discussion
• WinGet install locations: Super User
• GUI client PATH caveats (Cursor): Cursor community thread
• uv tools install location (~/.local/bin): Astral docs