After update to 8.2.1, unity instance always null

[GameXLife]

I use claude desktop app with unity-mcp.
Yesterday I use unity-mcp very well, and I found unity-mcp had a v8.2.1 update hint.
After I update and restart unity-mcp, claude desktop app cant control unity anymore.
I let it check what happened, claude response "Telemetry ping is success", but Active Instance and Registered Instances are both null.

Maybe I should downgrade to v8.1.6?

[Jordonh18]

1. What operating system and Unity version are you using?
2. The output from Claude Desktop logs (Help → View Logs) when the connection fails
3. Any error messages in Unity's Console window during connection attempts

[GameXLife]

1. What operating system and Unity version are you using?

   2. The output from Claude Desktop logs (Help → View Logs) when the connection fails

   3. Any error messages in Unity's Console window during connection attempts

1. Windows 11 , Unity 6000.3.0f1 LTS

2. mcp.log and mcp-server-UnityMCP.log from claude desktop app
   2025-12-09T15:06:18.079Z [info] [unityMCP] Message from client: {"method":"tools/call","params":{"name":"execute_menu_item","arguments":{"menu_path":"Help/About Unity"}},"jsonrpc":"2.0","id":26}
   2025-12-09T15:06:18.395Z [info] [unityMCP] Message from server: {"jsonrpc":"2.0","id":26,"result":{"content":[{"type":"text","text":"{"success":false,"message":null,"error":"No Unity Editor instances found. Please ensure Unity is running with MCP for Unity bridge.","data":null,"hint":null}"}],"structuredContent":{"success":false,"message":null,"error":"No Unity Editor instances found. Please ensure Unity is running with MCP for Unity bridge.","data":null,"hint":null},"isError":false}}
   2025-12-09T15:06:42.238Z [info] [unityMCP] Message from client: {"method":"tools/call","params":{"name":"manage_prefabs","arguments":{"action":"open_stage","prefab_path":"Assets/Prefabs"}},"jsonrpc":"2.0","id":27}
   2025-12-09T15:06:42.540Z [info] [unityMCP] Message from server: {"jsonrpc":"2.0","id":27,"result":{"content":[{"type":"text","text":"{"success":false,"message":"success=False message=None error='No Unity Editor instances found. Please ensure Unity is running with MCP for Unity bridge.' data=None hint=None"}"}],"structuredContent":{"success":false,"message":"success=False message=None error='No Unity Editor instances found. Please ensure Unity is running with MCP for Unity bridge.' data=None hint=None"},"isError":false}}
   2025-12-09T15:07:15.568Z [info] [unityMCP] Message from client: {"method":"tools/call","params":{"name":"batch_execute","arguments":{"commands":[{"tool":"manage_editor","params":{"action":"telemetry_status"}},{"tool":"manage_scene","params":{"action":"get_active"}},{"tool":"debug_request_context","params":{}}]}},"jsonrpc":"2.0","id":28}
   2025-12-09T15:07:15.872Z [info] [unityMCP] Message from server: {"jsonrpc":"2.0","id":28,"result":{"content":[{"type":"text","text":"{"success":false,"message":null,"error":"No Unity Editor instances found. Please ensure Unity is running with MCP for Unity bridge.","data":null,"hint":null}"}],"structuredContent":{"success":false,"message":null,"error":"No Unity Editor instances found. Please ensure Unity is running with MCP for Unity bridge.","data":null,"hint":null},"isError":false}}

3.No error message in unity console. No message appears during claude trying control unity.

Addional image: my unity mcp window and session status.

[GameXLife]

I try to downgrade to v8.1.6, and still shows unity instance is null.
So I realize that cant detect unity connection status is not because unity-mcp.
After restart computer, unity connection status can be detected correctly...

Seems this issue can be closed.

[Minssuy99]

The same issue occurred for me as well. It happened on Windows 11 with Unity 2022.3.62f3.
{"success":false,"message":"success=False message=None error='No Unity Editor instances found. Please ensure Unity is running with MCP for Unity bridge.' data=None hint=None"}
Just like in your case, there were no error messages or debug logs in the console, and it still didn't work even after I rebooted my system. MCP version is 8.2.3, and I also downgraded to 8.1.6, but the issue persists.
When I check the connection, "Telemetry ping is success" but Active Instance and Registered Instances are both null, just like your case.