[BUG?] FastAPI-MCP tool execution fails with connection errors when running in Docker container

[osushinekotan]

Describe the bug
When running FastAPI-MCP inside a Docker container, tool calls fail with "httpx.ConnectError: All connection attempts failed" errors. The MCP endpoint successfully connects (/mcp endpoint responds with 200 OK) and messages are received (POST to /mcp/messages returns 202 Accepted), but when trying to execute any tool from an MCP client, the internal HTTP client fails to connect to the API endpoints within the same container.

To Reproduce

• https://github.com/osushinekotan/fastapi-mcp-servers

docker compose up

claude desktop

{
  "mcpServers": {
    "servers": {
      "command": "uvx",
      "args": ["mcp-proxy", "http://localhost:8001/mcp"]
    }
  }
}

httpx.ConnectError: All connection attempts failed

System Info

• MacOS