[Bug] v.9.3.1 + Claude Code Connection Error

[sayplswis]

Hello. I was using version 9.3.0 and everything was working fine, but I updated to 9.3.1 this morning and now I can't connect.

Problem description: I launch the HTTP server (configured for the Claude Code client).
The server starts and the session is active. After that, I launch Claude Code and the server crashes. In the plugin, the "Start Server" button becomes available again and it shows "No Session".

However, the server is still running. Claude Code detects the server, but does not detect the Unity Editor.
If I launch Claude Code first, then launch the server, and then click reconnect, the behavior is the same as the problem described above.

Steps to reproduce

Step 1: Start Server

Unity console `MCP-FOR-UNITY: Started local HTTP server in terminal: C:\Users\Max1m\.local\bin\uvx.exe --prerelease explicit --from "mcpforunityserver>=0.0.0a0" mcp-for-unity --transport http --http-url http://localhost:8080 --project-scoped-tools --pidfile D:\UNITY\WordRace3D\Library\MCPForUnity\RunState\mcp_http_8080.pid --unity-instance-token f1bdbd414d504db8bf3c742327345a37 UnityEngine.Debug:Log (object) MCPForUnity.Editor.Helpers.McpLog:Info (string,bool) (at ./Library/PackageCache/com.coplaydev.unity-mcp@f1a5edbd8324/Editor/Helpers/McpLog.cs:40) MCPForUnity.Editor.Services.ServerManagementService:StartLocalHttpServer () (at ./Library/PackageCache/com.coplaydev.unity-mcp@f1a5edbd8324/Editor/Services/ServerManagementService.cs:318) MCPForUnity.Editor.Windows.Components.Connection.McpConnectionSection/d__53:MoveNext () (at ./Library/PackageCache/com.coplaydev.unity-mcp@f1a5edbd8324/Editor/Windows/Components/Connection/McpConnectionSection.cs:610) System.Runtime.CompilerServices.AsyncVoidMethodBuilder:Startd__53> (MCPForUnity.Editor.Windows.Components.Connection.McpConnectionSection/d__53&) MCPForUnity.Editor.Windows.Components.Connection.McpConnectionSection:OnHttpServerToggleClicked () UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)

Server Console[02/01/26 21:47:19] INFO Starting MCP server 'mcp-for-unity-server' with transport 'http' on server.py:2618
http://localhost:8080/mcp
INFO: Started server process [27528]
INFO: Waiting for application startup.
2026-02-01 21:47:19,064 - docket.worker - INFO - Starting worker 'Max1m#27528' with the following tasks:
2026-02-01 21:47:19,064 - docket.worker - INFO - * trace(message: str, ...)
2026-02-01 21:47:19,064 - docket.worker - INFO - * fail(message: str, ...)
2026-02-01 21:47:19,064 - docket.worker - INFO - * sleep(seconds: float, ...)
2026-02-01 21:47:19,065 - mcp.server.streamable_http_manager - INFO - StreamableHTTP session manager started
INFO: Application startup complete.
INFO: Uvicorn running on http://localhost:8080 (Press CTRL+C to quit)
INFO: ('::1', 50537) - "WebSocket /hub/plugin" [accepted]
INFO: connection open
2026-02-01 21:47:19,497 - transport.plugin_hub - INFO - Plugin registered: WordRace3D (c0b3ac165f58cd02)
2026-02-01 21:47:19,519 - transport.plugin_hub - INFO - Registered 19 tools for session 269f5d3b-140d-4a9a-8793-1cd26b00502d`

Step 2: Start Claude Code

Unity Console `MCP-FOR-UNITY: Server no longer running; ending orphaned session. UnityEngine.Debug:Log (object) MCPForUnity.Editor.Helpers.McpLog:Info (string,bool) (at ./Library/PackageCache/com.coplaydev.unity-mcp@f1a5edbd8324/Editor/Helpers/McpLog.cs:40) MCPForUnity.Editor.Windows.Components.Connection.McpConnectionSection:UpdateConnectionStatus () (at ./Library/PackageCache/com.coplaydev.unity-mcp@f1a5edbd8324/Editor/Windows/Components/Connection/McpConnectionSection.cs:335) MCPForUnity.Editor.Windows.MCPForUnityEditorWindow:OnEditorUpdate () (at ./Library/PackageCache/com.coplaydev.unity-mcp@f1a5edbd8324/Editor/Windows/MCPForUnityEditorWindow.cs:435) UnityEditor.EditorApplication:Internal_CallUpdateFunctions ()

Server Console[02/01/26 21:47:19] INFO Starting MCP server 'mcp-for-unity-server' with transport 'http' on server.py:2618
http://localhost:8080/mcp
INFO: Started server process [27528]
INFO: Waiting for application startup.
2026-02-01 21:47:19,064 - docket.worker - INFO - Starting worker 'Max1m#27528' with the following tasks:
2026-02-01 21:47:19,064 - docket.worker - INFO - * trace(message: str, ...)
2026-02-01 21:47:19,064 - docket.worker - INFO - * fail(message: str, ...)
2026-02-01 21:47:19,064 - docket.worker - INFO - * sleep(seconds: float, ...)
2026-02-01 21:47:19,065 - mcp.server.streamable_http_manager - INFO - StreamableHTTP session manager started
INFO: Application startup complete.
INFO: Uvicorn running on http://localhost:8080 (Press CTRL+C to quit)
INFO: ('::1', 50537) - "WebSocket /hub/plugin" [accepted]
INFO: connection open
2026-02-01 21:47:19,497 - transport.plugin_hub - INFO - Plugin registered: WordRace3D (c0b3ac165f58cd02)
2026-02-01 21:47:19,519 - transport.plugin_hub - INFO - Registered 19 tools for session 269f5d3b-140d-4a9a-8793-1cd26b00502d
2026-02-01 21:50:44,089 - mcp.server.streamable_http_manager - INFO - Created new transport with session ID: cf7e211ec3314e21888b881bdc67413b
INFO: ::1:58224 - "POST /mcp HTTP/1.1" 200 OK
INFO: ::1:58224 - "POST /mcp HTTP/1.1" 202 Accepted
INFO: ::1:58224 - "GET /mcp HTTP/1.1" 200 OK
2026-02-01 21:50:44,118 - asyncio - ERROR - Task exception was never retrieved
future: <Task finished name='Task-355' coro=<IocpProactor.accept..accept_coro() done, defined at C:\Users\Max1m\AppData\Roaming\uv\python\cpython-3.12.12-windows-x86_64-none\Lib\asyncio\windows_events.py:563> exception=OSError(22, 'Указанное сетевое имя более недоступно', None, 64, None)>
Traceback (most recent call last):
File "C:\Users\Max1m\AppData\Roaming\uv\python\cpython-3.12.12-windows-x86_64-none\Lib\asyncio\windows_events.py", line 566, in accept_coro
await future
File "C:\Users\Max1m\AppData\Roaming\uv\python\cpython-3.12.12-windows-x86_64-none\Lib\asyncio\windows_events.py", line 803, in _poll
value = callback(transferred, key, ov)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Max1m\AppData\Roaming\uv\python\cpython-3.12.12-windows-x86_64-none\Lib\asyncio\windows_events.py", line 555, in finish_accept
ov.getresult()
OSError: [WinError 64] Указанное сетевое имя более недоступно
2026-02-01 21:50:44,119 - asyncio - ERROR - Accept failed on a socket
socket: <asyncio.TransportSocket fd=1200, family=2, type=1, proto=0, laddr=('127.0.0.1', 8080)>
Traceback (most recent call last):
File "C:\Users\Max1m\AppData\Roaming\uv\python\cpython-3.12.12-windows-x86_64-none\Lib\asyncio\proactor_events.py", line 846, in loop
conn, addr = f.result()
^^^^^^^^^^
OSError: [WinError 64] Указанное сетевое имя более недоступно
INFO: ::1:58226 - "POST /mcp HTTP/1.1" 200 OK
INFO: ::1:58228 - "POST /mcp HTTP/1.1" 200 OK
INFO: ::1:58230 - "POST /mcp HTTP/1.1" 200 OK
2026-02-01 21:50:45,451 - transport.plugin_hub - INFO - Plugin session 269f5d3b-140d-4a9a-8793-1cd26b00502d disconnected (1005)`

[dsarno]

Some people got stuck in this state when we updated the MCP to a new version a few days ago. Two things should fix it.

1. Make sure that your Unity Package Manager is pointing to this address (the main branch of the repo) when you install: https://github.com/CoplayDev/unity-mcp.git?path=/MCPForUnity#main

2. Unregister Claude Code in the Client section of the Unity MCP window. The Register again. You will likely need to exit and restart Claude Code -- also stop and restart the Unity MCP Server after you do this. That should fix it.

[TheSmallKiwi]

In Claude code using the /mcp action to reconnect to UnityMCP reliably causes this error in the FastMCP process:
2026-02-01 12:10:13,508 - asyncio - ERROR - Task exception was never retrieved future: <Task finished name='Task-88' coro=<IocpProactor.accept.<locals>.accept_coro() done, defined at C:\Users\devon\AppData\Roaming\uv\python\cpython-3.13.11-windows-x86_64-none\Lib\asyncio\windows_events.py:564> exception=OSError(22, 'The specified network name is no longer available', None, 64, None)> Traceback (most recent call last): File "C:\Users\devon\AppData\Roaming\uv\python\cpython-3.13.11-windows-x86_64-none\Lib\asyncio\windows_events.py", line 567, in accept_coro await future File "C:\Users\devon\AppData\Roaming\uv\python\cpython-3.13.11-windows-x86_64-none\Lib\asyncio\windows_events.py", line 804, in _poll value = callback(transferred, key, ov) File "C:\Users\devon\AppData\Roaming\uv\python\cpython-3.13.11-windows-x86_64-none\Lib\asyncio\windows_events.py", line 556, in finish_accept ov.getresult() ~~~~~~~~~~~~^^ OSError: [WinError 64] The specified network name is no longer available 2026-02-01 12:10:13,509 - asyncio - ERROR - Accept failed on a socket socket: <asyncio.TransportSocket fd=940, family=2, type=1, proto=0, laddr=('127.0.0.1', 8080)> Traceback (most recent call last): File "C:\Users\devon\AppData\Roaming\uv\python\cpython-3.13.11-windows-x86_64-none\Lib\asyncio\proactor_events.py", line 845, in loop conn, addr = f.result() ~~~~~~~~^^ OSError: [WinError 64] The specified network name is no longer available

Even after following the above steps. This causes the MCP For Unity plugin to lose the session.

[dsarno]

@TheSmallKiwi try to go to advanced tab of the Unity MCP window and check the force fresh install box, then restart MCP server, and see if that helps

[TheSmallKiwi]

@TheSmallKiwi try to go to advanced tab of the Unity MCP window and check the force fresh install box, then restart MCP server, and see if that helps

This was already selected. Crucially, the FastMCP process continues to run after the above error, but MCP For Unity plugin no longer can detect the server, testing Server Health on the Advanced tab reports Unknown.

[u-satoru]

Immediately after starting the MCP server, it connects to the Unity Editor.
However, when connecting to the MCP server from Claude Code, the connection between the MCP server and the Unity Editor is lost.

The MCP server remains running.

[dsarno]

Please share logs from the mcp server, and screenshots of what you see when you do /mcp in Claude code + enter on the mcp name, plus screenshot of UnityMCP window.

http local mode for server is easier to debug. Remember if using Claude code you have to unregister and then register it, exit Claude code, and restart it. This is tripping a lot of people up but it really should work.

[dsarno]

Also, Claude code is very good at helping troubleshoot stuff like this, so everyone please get help from your friendly Claude code!

[TheSmallKiwi]

I have had success by dropping down to the previous server version. I manually launch the server using the command supplied in the /Server/readme and the plugin sees it. Connecting with Claude code does not cause an error and I can use UnityMCP as before.

[dsarno]

Good clues. Can you paste the sever command that's working and the one that wasn't please?

[TheSmallKiwi]

The working server command: uvx --from mcpforunityserver mcp-for-unity --transport http --http-url http://localhost:8080

The not working server command: uvx --no-cache --refresh --prerelease explicit --from "mcpforunityserver>=0.0.0a0" mcp-for-unity --transport http --http-url http://localhost:8080 --project-scoped-tools
(from the plugin)

[dsarno]

Make sure advanced setting in MCP editor window to "use beta" is turned off and restart server, and register Claude code.

[sayplswis]

When I encountered this problem and was looking for a solution, I had already seen advice that re-registering would help fix it. However, that didn't help me, so I created an issue. Perhaps I did something wrong, or there is a difference between registering with the server on or server off.

Now I was more careful and solved the problem thanks to your advice.

Attached is a list of steps that 100% solved the problem:

1. Turn off Claude Code.
2. Start the server in the plugin.
3. Unregister Claude Code.
4. Register Claude Code.
5. Stop the server.
6. Start the server.
7. Start Claude Code.