[Bug]: MCP server reports "No Unity Editor instances found" although Unity MCP Bridge is active and server connection is healthy (macOS / ALCOM)

[Rutjaer]

What happened?

Environment
OS: macOS Tahoe 26.5.2 (25F84)
Hardware: MacBook Air M1 (Apple Silicon, 2020)
Unity: 2022.3.22f1
Unity installation: Apple Silicon native installation via Unity Hub
Project type: VRChat development project
Project management: ALCOM (macOS alternative workflow to VRChat Creator Companion)
Client: Claude Desktop
MCP client transport: stdio
MCP for Unity plugin versions tested:
10.1.0
10.1.2
mcpforunityserver versions tested:
10.1.0
10.1.0

 Python Version 3.11.15

Problem
Claude Desktop cannot access the running Unity Editor through MCP.
All Unity-related MCP requests fail with:
No Unity Editor instances found.
Please ensure Unity is running with MCP for Unity bridge.
Affected commands include:
manage_scene
read_console
set_active_instance
Explicit instance selection also fails:
No Unity instance found on port XXXX.
Available: none.

Confirmed working components
Unity MCP status
The MCP for Unity window shows:
Transport:
HTTP Local

URL:
http://127.0.0.1:8080

Session:
Active
VRChat_MCP_Test

Server Health:
Healthy
The local MCP server starts successfully.

HTTP server
The local server responds:
curl http://127.0.0.1:8080
Result:
Not Found
This confirms that the HTTP server is reachable.

Running Unity MCP server process
The Unity-started MCP process is active:
mcpforunityserver==10.1.2

mcp-for-unity
--transport http
--http-url http://127.0.0.1:8080
--project-scoped-tools
--unity-instance-token XXXXX
The process remains running and Unity reports an active session.

Important observation
There appear to be two separate MCP server processes.
Unity-started process
mcpforunityserver==10.1.2

--transport http
--http-url http://127.0.0.1:8080
This process is connected to the Unity project.

Claude Desktop started process
mcpforunityserver==10.1.2

--transport stdio
This process starts successfully but reports:
No Unity Editor instances found.
The issue appears to be between MCP server discovery and Unity Editor registration.
The current communication path appears to be:
Claude Desktop
|
| stdio
|
MCP server process
|
X
Unity MCP Bridge / Unity instance discovery
|
Unity Editor
The individual components work, but the Unity instance is not discovered by the MCP process used by Claude.

Steps already tested
The following have been tested:
Unity MCP package update
Upgrade from MCP for Unity 10.1.0 to 10.1.2
Clean new Unity test project
Multiple Unity restarts
Multiple Claude Desktop restarts
Full system restart
Manual cleanup of old MCP processes
Port conflict checks
Manual MCP server start from terminal
Removal and restoration of --offline option
Manual Claude HTTP configuration attempt
Unity HTTP Local transport testing
Different MCP server versions

Claude Desktop configuration
Current configuration generated by MCP for Unity:
{
"mcpServers": {
"unityMCP": {
"command": "/Users/xxxxxxxxxxx/.local/bin/uvx",
"args": [
"--offline",
"--from",
"mcpforunityserver==10.1.2",
"mcp-for-unity",
"--transport",
"stdio"
],
"type": "stdio"
}
}
}
"Configure All Detected Clients" was tested multiple times.
The Unity MCP configuration reports:
[Claude Desktop] auto-selected Stdio transport
(client does not support Http)
Manual HTTP configuration was also tested but did not establish a working connection with Claude Desktop.

Unity MCP Advanced settings
Current relevant settings:
Server Health:
Healthy

Auto-Start Server on Editor Load:
Enabled

Allow LAN Bind (HTTP Local):
Disabled

Allow Insecure Remote HTTP:
Disabled
No separate Unity Bridge start/accept/registration button is visible.
Available MCP tabs:
Connect
Tools
Resources
Generative
Deps
Advanced

Additional observation
The automatic client configuration appears to create a stdio MCP server for Claude Desktop, while Unity itself starts an HTTP Local MCP server.
The two processes appear independent.

Question
Why does the MCP server process started by Claude Desktop fail to discover the running Unity Editor instance although:
Unity MCP reports an active session
The HTTP server is healthy
The local port is reachable
The Unity MCP process has a valid Unity instance token
Could this be an issue with:
Unity Bridge registration/discovery
MCP instance registry sharing between HTTP and stdio transports
macOS Apple Silicon compatibility
ALCOM project workflow differences compared to VCC/VPM workflow

Available additional information
I can provide:
Unity MCP logs
server-launch logs
process listings
complete configuration files
screenshots of Unity MCP settings
--unity-instance-token 53a3axxxxxxxxxxxxxc3

Reproduction steps

Steps to reproduce

1. Open the Unity project (Unity 2022.3.22f1) with MCP for Unity installed and wait until the MCP window reports:Server Health: Healthy
   Session: Active

2. Start Claude Desktop (configured by MCP for Unity using the generated stdio configuration).

3. Ask Claude to execute any Unity MCP tool (for example: manage_scene, get_active_scene, read_console, or set_active_instance).

4. Observe the response.

Expected behavior
Claude should detect the running Unity Editor instance and execute the requested Unity MCP tool.
Actual behavior
Every Unity MCP request fails with:
"No Unity Editor instances found. Please ensure Unity is running with MCP for Unity bridge."

Additional attempts such as set_active_instance with explicit ports (8080/8090) also fail with:
"No Unity instance found on port XXXX. Available: none."

This occurs even though:
Server Health is Healthy
Session is Active
the HTTP server is reachable on http://127.0.0.1:8080
the MCP server process is running with a valid --unity-instance-token
the issue is reproducible in a newly created test project.

Unity version

2022.3 22f1

MCP for Unity package version

10.1.2

Python server version

3.11.15

MCP client

Claude Desktop

Transport

stdio

OS

macOS

Relevant logs / console output

Checks

• I searched existing issues and did not find a duplicate
• I included logs / steps to reproduce

[Salehtorkanloo]

Hello, I think I had the same problem as you in Windows 10 and unity 6.0.3, I've installed last version of python and uv and set up mcp in unity editor and installed claude desktop, I followed the instructions in this video : https://www.youtube.com/watch?v=AXh1Tx1zDCk untill the step ("Verify in claude" 2:42), the claude wasn't connected to unity. to fix this, open claude desktop navigate to Files>Settings>Developer then press Edit Config to open the config file with .json format. add this section to the end of the line before last '}' character:

"mcpServers": {
    "unityMCP": {
      "command": "uvx",
      "args": [
        "--prerelease",
        "explicit",
        "--from",
        "mcpforunityserver>=0.0.0a0",
        "mcp-for-unity",
        "--transport",
        "stdio"
      ]
    }
  }

save the file and close claude desktop from task manager and open it again now if you open unity and write a prompt in claude and say create a gameobject it should be working, it worked for me I hope works for you too.

[Rutjaer]

Hallo, ich glaube, ich hatte das gleiche Problem wie Sie in Windows 10 und Einheit 6.0.3, ich habe die letzte Version von Python und uv installiert und mcp in Unity-Editor eingerichtet und den Claude-Desktop installiert, ich habe die Anweisungen in diesem Video befolgt : https://www.youtube.com/watch? v=AXh1Tx1zDCk bis zum Schritt ("Verify in claude" 2:42), das Claude wurde nicht mit der Einheit verbunden. um dies zu beheben, öffnen Sie claude desktop navigieren Sie zu Dateien>Einstellungen>Developer drücken Sie dann Config bearbeiten, um die Konfigurationsdatei mit dem .json-Format zu öffnen. Fügen Sie diesen Abschnitt am Ende der Zeile vor dem letzten '}' Zeichen hinzu:

"mcpServers": {
    "unityMCP": {
      "command": "uvx",
      "args": [
        "--prerelease",
        "explicit",
        "--from",
        "mcpforunityserver>=0.0.0a0",
        "mcp-for-unity",
        "--transport",
        "stdio"
      ]
    }
  }

Speichern Sie die Datei und schließen Sie den Claude-Desktop vom Task-Manager und öffnen Sie ihn jetzt erneut, wenn Sie die Einheit öffnen und eine Eingabeaufforderung in Claude schreiben und sagen, erstellen Sie ein Gameobject, es sollte funktionieren, es hat für mich funktioniert, ich hoffe, dass es auch für Sie funktioniert.

Vielen Dank für Ihre Antwort. Ich habe es wie vorgeschlagen versucht und die json Datei angepasst. Leider wirft Calude den gleichen Fehler weiter aus. Das Problem ist soweit eingekreist, dass es aussieht, als würde die Anfrage von Claude auf Gleis 5 ankommen, während Unity auf Gleis 1 wartet. Vielleicht verdeutlicht das nochmals das Problem.