Copilot CLI 1.0.79-1 fails MCP initialization when `server/discover` returns `-32602`

Describe the bug

Copilot CLI 1.0.79-1 cannot connect to an MCP server built with FastMCP.

Before MCP initialization completes, the CLI sends a server/discover request. FastMCP does not implement this method and returns -32602 Invalid request parameters. Copilot treats that response as a fatal lifecycle negotiation error and aborts initialization.

The same server and configuration work without issue in Copilot CLI 1.0.78.

Affected version

1.0.79-1

Steps to reproduce the behavior

1. Configure the MCP server as shown in "Additional Context".
2. Start Copilot CLI 1.0.79-1.
3. Observe the CLI sending server/discover.
4. FastMCP rejects the unsupported request.
5. Copilot aborts MCP lifecycle negotiation before initialize completes.

Expected behavior

An unsupported optional server/discover request should not block normal MCP initialization.

If the server returns an error for the probe, Copilot should continue with the standard initialize lifecycle rather than treating the response as fatal.

In particular, this should not depend on the server returning exactly -32601 Method not found. FastMCP currently returns -32602 because the request does not match any supported ClientRequest schema.

Additional context

MCP configuration

Values have been redacted:

{
  "mcpServers": {
    "fabric-rti-mcp": {
      "command": "uvx",
      "args": ["microsoft-fabric-rti-mcp"],
      "env": {
        "KUSTO_SERVICE_URI": "<redacted>",
        "KUSTO_SERVICE_DEFAULT_DB": "<redacted>",
        "FABRIC_API_BASE": "<redacted>"
      }
    }
  }
}

More Details

Another MCP implementation returns:

Method 'server/discover' is not available.

Copilot tolerates that response and continues initialization.

Start by tracing the MCP initialization flow that sends the server/discover request, then compare handling of the -32602 response with the tolerated “Method not found” response described here. Done means an unsupported probe no longer aborts initialization and the FastMCP configuration can complete the standard initialize lifecycle.