René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"ClosedResourceError when using FastMCP(..., stateless_http=True, json_response=True) with MCP Inspector","articleBody":"### Initial Checks\n\n- [x] I confirm that I'm using the latest version of MCP Python SDK\n- [x] I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue\n\n### Description\n\n# Summary\n\nRunning the example MCP server fastmcp_quickstart.py with:\n`FastMCP(\"Demo\", stateless_http=True, json_response=True)\n`\nand connecting to it using MCP Inspector triggers an exception inside the server’s message router:\n`ClosedResourceError`\n\nThis occurs immediately after the Inspector connects.\n\n# Steps to Reproduce\n1. Use the example server from the Python MCP SDK:\n`uv run --with mcp examples/snippets/servers/fastmcp_quickstart.py\n`\n2. Ensure the server instance is initialized as:\n`FastMCP(\"Demo\", stateless_http=True, json_response=True)\n`\n3. Open MCP Inspector and connect to this server.\n4. Observe the server error.\n\n# Environment\n- MCP Python SDK version: latest\n- Python version: 3.13\n- OS: macOS (Apple Silicon)\n- MCP Inspector version: latest\n\n### Example Code\n\n```Python\n\"\"\"\nFastMCP quickstart example.\n\nRun from the repository root:\n    uv run examples/snippets/servers/fastmcp_quickstart.py\n\"\"\"\n\nfrom mcp.server.fastmcp import FastMCP\n\n# Create an MCP server\nmcp = FastMCP(\"Demo\", stateless_http=True, json_response=True)\n\n\n# Add an addition tool\n@mcp.tool()\ndef add(a: int, b: int) -\u003e int:\n    \"\"\"Add two numbers\"\"\"\n    return a + b\n\n\n# Add a dynamic greeting resource\n@mcp.resource(\"greeting://{name}\")\ndef get_greeting(name: str) -\u003e str:\n    \"\"\"Get a personalized greeting\"\"\"\n    return f\"Hello, {name}!\"\n\n\n# Add a prompt\n@mcp.prompt()\ndef greet_user(name: str, style: str = \"friendly\") -\u003e str:\n    \"\"\"Generate a greeting prompt\"\"\"\n    styles = {\n        \"friendly\": \"Please write a warm, friendly greeting\",\n        \"formal\": \"Please write a formal, professional greeting\",\n        \"casual\": \"Please write a casual, relaxed greeting\",\n    }\n\n    return f\"{styles.get(style, styles['friendly'])} for someone named {name}.\"\n\n\n# Run with streamable HTTP transport\nif __name__ == \"__main__\":\n    mcp.run(transport=\"streamable-http\")\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\nPython: 3.13\nMCP Python SDK: 1.22.1.dev4+b19fa6f\n```","author":{"url":"https://github.com/yabramow","@type":"Person","name":"yabramow"},"datePublished":"2025-11-24T14:14:33.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/1658/python-sdk/issues/1658"}