Claude Desktop stalls indefinitely when invoking stdio MCP server tools

What Should Happen

When asking Claude to call a tool from a connected stdio MCP server (e.g., "Please call the list_meeting_transcripts_and_recordings tool now"), Claude Desktop should send a tools/call JSON-RPC message to the MCP server and return the result. The server is connected, tools are discovered successfully, and the same tool call works perfectly via MCP Inspector v0.21.1.

Error Messages / Logs

No error messages — the request silently stalls with the spinner. The MCP server log (~/Library/Logs/Claude/mcp-server-webex.log) shows successful initialization and tool discovery but no tools/call message is ever received:

[webex] [info] Server started and connected successfully
[webex] [info] Message from client: {"method":"tools/list",...}
[webex] [info] Message from server: {"jsonrpc":"2.0","id":1,"result":{"tools":[...]}}
// No tools/call message ever appears after this

Steps to Reproduce

1. Add a stdio MCP server to claude_desktop_config.json:

{
  "mcpServers": {
    "webex": {
      "command": "/Users/username/.local/bin/uv",
      "args": ["run", "--directory", "/path/to/project", "python", "webex_mcp.py"],
      "env": { "WEBEX_ACCESS_TOKEN": "<valid_token>" }
    }
  }
}

1. Restart Claude Desktop — server connects successfully (confirmed in mcp-server-webex.log)
2. Open a new conversation
3. Type: "Please call the list_meeting_transcripts_and_recordings tool now"
4. Claude responds with text but then the spinner hangs indefinitely — no tool call is dispatched
5. Same behavior occurs with "always trigger" enabled for the tools in connector settings
6. The identical server and token work correctly in MCP Inspector v0.21.1 (returns 30 items successfully)

Environment

• Claude Desktop: 1.2.234 (2d1855) 2026-04-01T07:58:22.000Z
• OS: macOS Darwin 25.4.0
• MCP Server: FastMCP 3.2.0, Python 3.12, stdio transport
• Other active integrations: Google Calendar, Gmail, Asana, Miro, Canva, MS365, Box

Additional Context

• The MCP server exposes 2 tools: list_meeting_transcripts_and_recordings and download_meeting_transcript
• Server initialization handshake completes fully (protocol version negotiation, tools/list, prompts/list, resources/list all succeed)
• Non-tool text responses work fine in the same conversation (e.g., "hi" gets an immediate response)
• The stall persists across new conversations and full app restarts
• MCP Inspector v0.21.1 confirms the server works end-to-end via stdio — successfully returned 30 meeting transcripts/recordings from the Webex API
• The tools/call JSON-RPC message is never sent by Claude Desktop, confirmed by monitoring the MCP server logs