Unity Editor instances not found with 8.2.3 (persists even after downgrade to 8.1.6)

[Minssuy99]

Environment

• OS: Windows 11
• Unity Version: 2022.3.62f3
• MCP Version: Initially installed 8.2.3, downgraded to 8.1.6
• Client: Claude Desktop (stdio)

Issue Description

I installed unity-mcp 8.2.3 for the first time, but Claude Desktop cannot detect Unity Editor instances. The issue persists even after downgrading to 8.1.6 and multiple system restarts.

Error Message

{"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"}

Symptoms

• Telemetry ping: Success ✅
• Active Instance: null ❌
• Registered Instances: null ❌
• No error messages in Unity Console
• Unity MCP Bridge appears to be running normally in Unity

What I've Tried

1. Downgraded from 8.2.3 → 8.1.6
2. Complete system reboot (3 times)
3. Verified Unity Editor is running with project open
4. Checked claude_desktop_config.json (stdio configuration is correct)
5. Reinstalled MCP package in Unity Package Manager

"unityMCP": {
  "command": "C:\\WINDOWS\\system32\\cmd.exe",
  "args": [
    "/c",
    "C:\\Users\\MinSeong\\.local\\bin\\uvx.exe",
    "--from",
    "git+https://github.com/CoplayDev/unity-mcp@v8.1.6#subdirectory=Server",
    "mcp-for-unity",
    "--transport",
    "stdio"
  ]
}

Additional Notes

This appears to be related to #447. Since I installed 8.2.3 from the beginning and never had a working version, I cannot confirm if this is a regression or an initial setup issue.

[mikenordby]

Same issue. on VSCode Copilot and Windows 10

[dsarno]

Just double checking -- you guys are pressing "Start Session" in the unity editor right? This is after you start the http server (or stdio).

[mikenordby]

Yes.

[dsarno]

@mikenordby Claude outputs an 'mcp_unity_server_log.json" or something similar to that, in its log directory. Can you check that to see if there are any obvious errors?

[Minssuy99]

@dsarno
Yes, I pressed "Start Session" in stdio mode in Unity Editor, and confirmed the green indicator light is on.

I checked the logs from C:\Users\MinSeong\AppData\Roaming\Claude\logs\mcp-server-unityMCP.log.

WARNING - No Unity instances found on startup
error='No Unity Editor instances found. Please ensure Unity is running with MCP for Unity bridge.'

and all connection attempts fail with:

2025-12-13 05:06:08,032 - mcp-for-unity-server - INFO - MCP for Unity Server starting up
2025-12-13 05:06:08,034 - mcp-for-unity-server - INFO - Initializing Unity connection pool
2025-12-13 05:06:08,035 - mcp-for-unity-server - INFO - Discovered 0 Unity instances (after de-duplication by port)
2025-12-13 05:06:08,035 - mcp-for-unity-server - INFO - Found 0 Unity instances: []
2025-12-13 05:06:08,035 - mcp-for-unity-server - WARNING - No Unity instances found on startup

(The same errors also appear in C:\Users\MinSeong\Library\Application Support\UnityMCP\Logs\unity_mcp_server.log)

Unity Console shows no errors during this time. The MCP bridge appears to be running normally in Unity, but Claude Desktop cannot discover the Unity instance.

Is there any additional configuration needed for the stdio transport to properly detect Unity instances?

[dsarno]

Thanks for the detailed anewer @Minssuy99. One thing to please try

• Check %USERPROFILE%\.unity-mcp\unity-mcp-status-*.json exists + has unity_port.
• Check netstat -ano | findstr :<unity_port> — if Unity is listening on [::1] but not 127.0.0.1, the server’s probe (hardcoded to 127.0.0.1) will discard the instance, causing Active/Registered Instances to be null.
• possible Fix: update _try_probe_unity_mcp to probe localhost/::1 as well as 127.0.0.1.

More troubleshooting detail here. If you can confirm this is the issue we will patch.

[Minssuy99]

@dsarno

I ran the tests you suggested.

The status file %USERPROFILE%\.unity-mcp\unity-mcp-status-*.json exists with

{
  "unity_port": 6400,
  "created_date": "2025-12-13T04:15:00Z",
  "project_path": "C:/Users/MinSeong/Desktop/ToyProject/Assets"
}

Port check shows Unity is listening

C:\Users\MinSeong>netstat -ano | findstr :6400
TCP    127.0.0.1:6400         0.0.0.0:0              LISTENING       27556

Connection tests with Unity running and session started

PS > Test-NetConnection -ComputerName 127.0.0.1 -Port 6400
TcpTestSucceeded : True

PS > Test-NetConnection -ComputerName localhost -Port 6400
WARNING: TCP connect to (::1 : 6400) failed
TcpTestSucceeded : True

PS > Test-NetConnection -ComputerName ::1 -Port 6400
WARNING: TCP connect to (::1 : 6400) failed
TcpTestSucceeded : False

Unity is listening on IPv4 (127.0.0.1) only, and connections to 127.0.0.1 succeed. So this doesn't seem to be the IPv6 issue. The status file exists, Unity is listening on the correct port, and manual connection tests work — but the MCP server still reports "0 Unity instances discovered."

[dsarno]

@Minssuy99 Thanks for trying that. I just tested on Claude Desktop / Windows / stdio, and it works fine. So it's difficult for me to troubleshoot your case. If you're willing to work with your LLM to try to figure out the root cause, and post a report here, we can try to reproduce and fix.

[dsarno]

@Minssuy99 also try running uv cache clean --force in case you have stale versions of the server.

[Minssuy99]

@dsarno Thanks for checking.

I already tried running uv cache clean --force earlier, but it didn’t change the behavior.
I’ll keep investigating the issue with the help of an LLM.
If I find the root cause or manage to resolve the issue, I’ll add a follow-up comment here.