MCP Server can't not detected in Claude.app

[minsongbc]

2 cases

1. Installing via Smithery
   mcp-server is on, and Unity also connected successfully, but can't not detected UnityEditor in Claude.app.

2. Try to without Smithery
   Can't not detected mcp server attachment.
   This is my config.

{
  "mcpServers": {
    "unity-mcp": {
      "command": "node",
      "args": ["~/unitymcp2/assets/unitymcp/unity-mcp-server/build/index.js"]
    }
  }
}

The path is correct.

[Zogrus]

Installing via Smithery
mcp-server is on, and Unity also connected successfully, but can't not detected UnityEditor in Claude.app.

me, too.
Node.js:v22.11.0, npm :10.9.1, Unity:2022.3.47f1

I'm encountering connection issues between Claude and our Unity project via MCP. The Unity and MCP Server connection appears to be working (the "UnityMCP Debug" window shows "Connected" status), but Claude cannot communicate properly with Unity.
I've examined the MCP log files by using Claude Desktop's "Developer" menu > "Open MCP log file" option and found that the following errors keep appearing repeatedly:

2025-03-19T15:47:09.880Z [info] [unitymcp] Message from client: {"method":"prompts/list","params":{},"jsonrpc":"2.0","id":33}
2025-03-19T15:47:10.509Z [info] [unitymcp] Message from server: {"jsonrpc":"2.0","id":33,"error":{"code":-32601,"message":"Method not found"}}
2025-03-19T15:47:14.216Z [info] [unitymcp] Message from client: {"method":"resources/list","params":{},"jsonrpc":"2.0","id":34}
2025-03-19T15:47:14.957Z [info] [unitymcp] Message from server: {"jsonrpc":"2.0","id":34,"error":{"code":-32601,"message":"Method not found"}}

The logs show that Claude (client) is attempting to call methods like resources/list and prompts/list, but the MCP server is consistently returning error code -32601 ("Method not found"). This suggests an API mismatch - Claude is requesting methods that aren't implemented or are named differently in our current MCP server implementation.

[minsongbc]

This problem is unityConnection is always null, when still unityConnect is connected.
I checked unityConnection null state using interval every 1 sec. It's happened only inside CallToolRequestSchema.

setInterval(() => {
  console.error(`client is null : ${this.unityConnection == null}`);
}, 1000);

index.ts line 339

if (!this.unityConnection) {
  throw new McpError(
    ErrorCode.InternalError,
    'Unity Editor is not connected. Please ensure the Unity Editor is running and the UnityMCP window is open.'
  );
}

[SolarisNeko]

same error.

2025-03-20T09:41:28.247Z [info] [unitymcp] Message from client: {"method":"prompts/list","params":{},"jsonrpc":"2.0","id":62}
2025-03-20T09:41:28.538Z [info] [unitymcp] Message from server: {"jsonrpc":"2.0","id":62,"error":{"code":-32601,"message":"Method not found"}}

how to fix it ?

the problem = Unity <-> NodeJS MCP Server (Ok)

Claude <-xxxx-> NodeJS MCP Server (Error by RPC)

[cleoag]

same error for me to on windows claude client

[Poivre-hxx]

same to me

[Poivre-hxx]

need unity6, not unity 5

[Zogrus]

Successful Connection Between Claude Desktop and Unity via MCP Server

I've successfully connected Claude Desktop with Unity through the MCP server! I managed to get it working after making a few modifications to the implementation.

This solution worked in my environment, and while I implemented it with assistance from Cursor (so there might be some unnecessary code), I wanted to share it since many people seem to be struggling with this same issue.

Environment

• Macbook Pro M3 Max (macOS: Sonoma 14.4)
• Unity 2022.3.47f1 and Unity 6000.0.32f1

Prerequisites

1. Do NOT install using Smithery.
2. Follow the README instructions for the Prerequisites, Unity Plugin Setup, and MCP Server Setup.
3. I placed the unity-mcp-server folder directly in the Unity project's Assets folder.
4. If you encounter the following error when opening your Unity project:

   Assets/UnityMCPPlugin/Editor/UnityMCPConnection.cs(10,7): error CS0246: The type or namespace name 'Newtonsoft' could not be found (are you missing a using directive or an assembly reference?)
   

   Resolve it by installing the Newtonsoft package via Unity's Package Manager: Add package from git URL... com.unity.nuget.newtonsoft-json and click "Add".
5. Once your Unity project opens without errors, close Unity for now.
6. Do NOT manually start the server (the claude_desktop_config.json configuration will have Claude Desktop start the MCP server).
7. Download and install Claude Desktop for Mac from https://claude.ai/download. Launch it to verify Claude works, then close it.

Modifications

1. Edit Claude Desktop Configuration

With Claude Desktop closed, modify ~/Library/Application Support/Claude/claude_desktop_config.json as follows (I've also added filesystem capability):

{
  "mcpServers": {
    "filesystem": {
      "command": "npx",
      "args": [
        "-y",
        "@modelcontextprotocol/server-filesystem",
        "/Users/[YourUserName]/[PathToYourUnityProject]"
      ]
    },
    "unitymcp": {
      "command": "node",
      "args": [
        "/Users/[YourUserName]/[PathToYourUnityProject]/Assets/unity-mcp-server/build/index.js"
      ],
      "env": {
        "UNITYMCP_PORT": "8080",
        "UNITYMCP_HOST": "localhost",
        "UNITYMCP_PROTOCOL": "ws"
      }
    }
  }
}

2. Modify Unity MCP Connection Script

Update the ConnectToServer() method in Assets/UnityMCPPlugin/Editor/UnityMCPConnection.cs as follows:

try
{
    Debug.Log($"[UnityMCP] Attempting to connect to MCP Server at {serverUri}");
    Debug.Log($"[UnityMCP] Current Unity version: {Application.unityVersion}");
    Debug.Log($"[UnityMCP] Current platform: {Application.platform}");
    
    webSocket = new ClientWebSocket();
    webSocket.Options.KeepAliveInterval = TimeSpan.FromSeconds(30);
    
    var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));
    using var linkedCts = CancellationTokenSource.CreateLinkedTokenSource(cts.Token, timeout.Token);
    
    await webSocket.ConnectAsync(serverUri, linkedCts.Token);
    isConnected = true;
    Debug.Log("[UnityMCP] Successfully connected to MCP Server");
    
    // Send initial state after successful connection
    var initialState = GetEditorState();
    var message = JsonConvert.SerializeObject(new
    {
        type = "editorState",
        data = initialState
    });
    var buffer = Encoding.UTF8.GetBytes(message);
    await webSocket.SendAsync(new ArraySegment<byte>(buffer), WebSocketMessageType.Text, true, cts.Token);
    
    StartReceiving();
    StartSendingEditorState();
}

3. Modify MCP Server

Update the run() method in Assets/unity-mcp-server/src/index.ts:

async run() {
    try {
      // Set up the MCP protocol's standard input/output (stdio) transport
      const transport = new StdioServerTransport();
      
      // Error handling for standard input/output
      transport.onerror = (error) => {
        console.error('[Unity MCP] Transport error:', error);
      };

      // Connect the MCP server to standard input/output (for Claude Desktop)
      await Promise.race([
        this.server.connect(transport),
        new Promise((_, reject) => 
          setTimeout(() => reject(new Error('MCP connection timeout')), 10000)
        )
      ]);

      console.error('[Unity MCP] Server running on stdio for Claude Desktop');
      
      // Also set up WebSocket server (for Unity Editor)
      await new Promise<void>((resolve, reject) => {
        const timeout = setTimeout(() => {
          reject(new Error('WebSocket server startup timeout'));
        }, 5000);

        this.wsServer.once('listening', () => {
          clearTimeout(timeout);
          console.error('[Unity MCP] WebSocket server is ready on port 8080 for Unity');
          resolve();
        });

        this.wsServer.once('error', (error) => {
          clearTimeout(timeout);
          reject(error);
        });
      });

      console.error('[Unity MCP] Server initialization complete - dual mode (stdio + WebSocket)');
    } catch (error) {
      console.error('[Unity MCP] Failed to start server:', error);
      process.exit(1);
    }
  }

4. Build the MCP Server

Run the following in the Assets/unity-mcp-server directory:

npm run build

This will compile the TypeScript code and reflect your changes to Assets/unity-mcp-server/build/index.js.

5. Check for Port Conflicts

Run lsof -i :8080 in a terminal to check if any process is using port 8080. If you see output like this:

COMMAND   PID         USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
node    37304 [UserName]   14u  IPv6 0x5cc4b189f5868b8b      0t0  TCP *:http-alt (LISTEN)

Kill the process to avoid port conflicts:

kill -9 37304

Verify that port 8080 is free by running lsof -i :8080 again. It should show no results.

6. Launch Unity and Check Connection Status

Start Unity and open the UnityMCP Debug Window (UnityMCP > Debug Window). Confirm that the Connection Status is "Disconnected" (since Claude Desktop hasn't been launched yet).

7. Launch Claude Desktop

Start Claude Desktop. If the hammer icon at the bottom of the chat shows "14", it means both filesystem (11 tools) and unitymcp (3 tools) are recognized. Check the UnityMCP Debug Window in Unity - if the Connection Status changes to "Connected", the setup is successful.

8. Test the Connection

Now you can interact with Unity through Claude Desktop. Try commands like:

• "Connect to Unity."
• "Place a sphere in the Unity scene."
• "Arrange six cubes around the sphere."
• "Adjust lighting and fog to create a sunset atmosphere."
• "Save this scene and create a new one called SampleTest2."

Note: Unity's scene view might not update while you're typing in Claude, but clicking on Unity to focus it should refresh the view.

---

This implementation allows Claude Desktop to start the MCP server and establish a dual-mode connection (stdio for Claude and WebSocket for Unity). The key modifications involve proper connection handling and error management in both the Unity plugin and the MCP server.