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fix: return HTTP 404 for unknown session IDs instead of 400 #1808

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fix: return HTTP 404 for unknown session IDs instead of 400 #1808

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19 changes: 16 additions & 3 deletions src/mcp/server/streamable_http_manager.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@
from __future__ import annotations

import contextlib
import json
import logging
from collections.abc import AsyncIterator
from http import HTTPStatus
Expand Down Expand Up @@ -277,9 +278,21 @@ async def run_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORE
# Handle the HTTP request and return the response
await http_transport.handle_request(scope, receive, send)
else: # pragma: no cover
# Invalid session ID
# Unknown or expired session ID - return 404 per MCP spec
# Match TypeScript SDK exactly: jsonrpc, error, id order
error_body = json.dumps(
{
"jsonrpc": "2.0",
"error": {
"code": -32001,
"message": "Session not found",
},
"id": None,
}
)
response = Response(
"Bad Request: No valid session ID provided",
status_code=HTTPStatus.BAD_REQUEST,
content=error_body,
status_code=HTTPStatus.NOT_FOUND,
media_type="application/json",
)
await response(scope, receive, send)