René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"sse_app() ignores mount prefix, resulting in 404 from client","articleBody":"**Describe the bug**\n\nWhen mounting sse_app() from FastMCP with a URL prefix using Starlette’s Mount, the SSE stream still returns the default /messages/ endpoint without the prefix. This causes the MCP client to resolve an incorrect URL (e.g., /messages/ instead of /mcp/messages/), resulting in a 404 error.\n\n**To Reproduce**\n\nSteps to reproduce the behavior:\n\n1. Define an MCP server as below:\n\n```python\nfrom mcp.server.fastmcp import FastMCP\nfrom starlette.applications import Starlette\nfrom starlette.routing import Mount\n\nmcp = FastMCP(\n    # sse_path=\"/sse\",  # default\n    # message_path=\"/messages/\",  # default\n)\n\n\napplication = Starlette(\n    routes=[\n        Mount(\"/mcp\", app=mcp.sse_app()),  # HERE !!!\n    ]\n)\n```\n\n2. Start the server and navigate to http://127.0.0.1:8000/mcp/sse in your browser\n3. Observe the response in the browser. You will receive an SSE event like:\n\n```\nevent: endpoint\ndata: /messages/?session_id=...\n```\n\nAt this point, the MCP client performs a `urljoin` operation between the SSE URL (http://127.0.0.1:8000/mcp/sse) and the endpoint path (/messages/). This causes the resolved endpoint URL to become http://127.0.0.1:8000/messages/.\n\n4. The client tries to connect to http://127.0.0.1:8000/messages/, which results in a 404.\n\n**Expected behavior**\n\nThe SSE stream should return the correct full path reflecting the prefix, e.g.:\n\n```\ndata: /mcp/messages/?session_id=...\n```\n\nThis would allow the client to connect to the actual valid message endpoint.\n\n**Screenshots**\nIf applicable, add screenshots to help explain your problem.\n\n**Desktop (please complete the following information):**\n\n - OS: macOS\n - Browser : Arc\n - Version : 1.87.1 (60573)\n\n**Smartphone (please complete the following information):**\n - Device: N/A\n - OS: N/A\n - Browser : N/A\n - Version : N/A\n\n**Additional context**\n\nThis behavior seems to originate from hardcoded endpoint generation inside sse_app():\n\nhttps://github.com/modelcontextprotocol/python-sdk/blob/c2ca8e03e046908935d089a2ceed4e80b0c29a24/src/mcp/server/sse.py#L98\n\nIt would be great to have support for specifying a prefix in sse_app() or for the prefix to be auto-detected from the ASGI scope.\n\nThank you!","author":{"url":"https://github.com/allieus","@type":"Person","name":"allieus"},"datePublished":"2025-04-02T16:00:51.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":13},"url":"https://github.com/412/python-sdk/issues/412"}