René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"FastMCP read_resource() returns incorrect error code when resource not found","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\nIt appears to be unclear as to how an MCP server should respond to a read resource request if the resource is not found, however, FastMCP returns error code `0`, which seems likely incorrect regardless.\n\nIn the Error Handling part of the [protocol specification](https://modelcontextprotocol.io/specification/2025-06-18/server/resources#error-handling) it states:\n\u003e Resource not found: `-32002`\n\nHowever, the python SDK's FastMCP [attempts to raise a ResourceError](https://github.com/modelcontextprotocol/python-sdk/blob/de2289dc93c2efb6d6b055eace20bcbfe61bce5c/src/mcp/server/fastmcp/server.py#L372), except it appears that code would never get executed because a [ValueError is raised in ResourceManager](https://github.com/modelcontextprotocol/python-sdk/blob/de2289dc93c2efb6d6b055eace20bcbfe61bce5c/src/mcp/server/fastmcp/resources/resource_manager.py#L103) first.\n\nThe net result of this is that servers implemented using FastMCP return the following for resource-not-found:\n```json\n{\"jsonrpc\":\"2.0\",\"id\":7,\"error\":{\"code\":0,\"message\":\"Unknown resource: https://example.com/does-not-exist\"}}\n```\n\nFor the purposes of comparison, I compared this to the (presumably reference) typescript implementation, which raises an MCP error with [InvalidParams: code -32602](https://github.com/modelcontextprotocol/typescript-sdk/blob/main/src/types.ts#L121) - [see here](https://github.com/modelcontextprotocol/typescript-sdk/blob/9757ace06efd6c9cdcb40ee6430ef4c956e6cbf8/src/server/mcp.ts#L353)\n\n`-32602` is notably _similar_ to `-32002`, but _also_ seemingly incorrect (though in a different way ☹).\n\nAs things stand today, it is rather difficult to reliably write clients that behave predictably due to this. I understand this may be more of a protocol specification problem than a python-sdk problem, but error code `0` seems incorrect either way, so thought I'd report it.\n\n### Example Code\n\nBasic server (anything will do):\n\n```Python\nfrom mcp.server.fastmcp import FastMCP\n\nmcp = FastMCP('Pydantic AI MCP Server')\nlog_level = 'unset'\n\n\n@mcp.resource('resource://user_name.txt', mime_type='text/plain')\nasync def user_name_resource() -\u003e str:\n    return 'Alice'\n\n\nif __name__ == '__main__':\n    mcp.run(transport='streamable-http')\n```\n\nThen try to fetch any resource via whatever MCP client you like, eg (manual curl):\n```bash\nMCP_URL=\"http://localhost:8000/mcp\"\nSESSION_ID=\"\"\n\n# Step 1: Initialize and get session ID\necho \"1. Initializing MCP server...\"\nINIT_RESPONSE=$(curl -s -D - -X POST \"$MCP_URL\" \\\n  -H \"Content-Type: application/json\" \\\n  -H \"Accept: application/json, text/event-stream\" \\\n  -d '{\n    \"jsonrpc\": \"2.0\",\n    \"method\": \"initialize\",\n    \"params\": {\n      \"protocolVersion\": \"2024-11-05\",\n      \"capabilities\": {},\n      \"clientInfo\": {\n        \"name\": \"test-mcp-script\",\n        \"version\": \"1.0.0\"\n      }\n    },\n    \"id\": 1\n  }')\n\n# Extract session ID from headers\nSESSION_ID=$(echo \"$INIT_RESPONSE\" | grep -i 'mcp-session-id:' | cut -d' ' -f2 | tr -d '\\r')\n\n# Try to read resource:\ncurl -s -X POST \"$MCP_URL\" \\\n  -H \"Content-Type: application/json\" \\\n  -H \"Accept: application/json, text/event-stream\" \\\n  -H \"Mcp-Session-Id: $SESSION_ID\" \\\n  -d '{\n    \"jsonrpc\": \"2.0\",\n    \"method\": \"resources/read\",\n    \"params\": {\n      \"uri\": \"https://example.com/does-not-exist\"\n    },\n    \"id\": 7\n  }'\n\n# \u003e event: message\n# \u003e data: {\"jsonrpc\":\"2.0\",\"id\":7,\"error\":{\"code\":0,\"message\":\"Unknown resource: https://example.com/does-not-exist\"}}\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\n`main`\n```","author":{"url":"https://github.com/fennb","@type":"Person","name":"fennb"},"datePublished":"2025-11-05T09:31:33.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/1579/python-sdk/issues/1579"}