Where does the bug appear (feature/product)?
Cursor IDE
Describe the Bug
MCP client wrong handling of http.NOT_FOUND in session management (stateful MCP server).
When Cursor MCP client receives NOT_FOUND response on request with session_id, it MUST start a new session and it didn’t.
According to MCP spec: Transports - Model Context Protocol
- The server MAY terminate the session at any time, after which it MUST respond to requests containing that session ID with HTTP 404 Not Found.
- When a client receives HTTP 404 in response to a request containing an Mcp-Session-Id, it MUST start a new session by sending a new InitializeRequest without a session ID attached.
Steps to Reproduce
- MCP server up and running as stateful streamable-http.
- Cursor configured to use the MCP server - initialization OK, tool working
- Stop the MCP server and start it again without touching Cursor - simulate network outage, server restart, etc.
- In Cursor settings, the MCP indicator turns red
- Now, trigger Agent to call the tools, ends with
{"error": "Error POSTing to endpoint (HTTP 404): Bad Request: No valid session ID provided"} - Reloading the mcp.json file ends with several requests that gets 404 response
Without OFF/ON the MCP server from Cursor settings, there is no way to establish new session with the MCP server
I used modified python mcp=1.12.1 package: streamable_http_manager.py, line 275 to be sure that after connection terminate, the response on client request with session-id which is not known by the server, server response NOT_FOUND with the session_id.
from:
response = Response(
"Bad Request: No valid session ID provided",
status_code=HTTPStatus.BAD_REQUEST,
)
await response(scope, receive, send)
to:
response = Response(
"Bad Request: No valid session ID provided",
status_code=HTTPStatus.NOT_FOUND,
headers={MCP_SESSION_ID_HEADER: request_mcp_session_id},
)
await response(scope, receive, send)
Expected Behavior
When receiving http.NOT_FOUND with session_id, the client MUST start a new session, must send InitializeRequest. This will make the handling of stateful MCP server more robust, more reliable on client-server connection problems
Operating System
Linux
Current Cursor Version (Menu → About Cursor → Copy)
Version: 1.6.45
VSCode Version: 1.99.3
Commit: 3ccce8f55d8cca49f6d28b491a844c699b8719a0
Date: 2025-09-22T18:22:38.013Z
Electron: 34.5.8
Chromium: 132.0.6834.210
Node.js: 20.19.1
V8: 13.2.152.41-electron.0
OS: Linux x64 6.8.0-83-generic
Additional Information
MCP client logs from Cursor->output:
2025-09-24 14:22:26.998 [info] Handling CreateClient action
2025-09-24 14:22:26.998 [info] Creating streamableHttp transport
2025-09-24 14:22:26.998 [info] Connecting to streamableHttp server
2025-09-24 14:22:27.018 [info] No stored tokens found
2025-09-24 14:22:27.043 [info] No stored tokens found
2025-09-24 14:22:27.049 [info] Successfully connected to streamableHttp server
2025-09-24 14:22:27.049 [info] Storing streamableHttp client
2025-09-24 14:22:27.049 [info] CreateClient completed, server stored: true
2025-09-24 14:22:27.055 [info] No stored tokens found
2025-09-24 14:22:33.565 [info] Handling ListOfferings action, server stored: true
2025-09-24 14:22:33.565 [info] Connected to streamableHttp server, fetching offerings
2025-09-24 14:22:33.566 [info] No stored tokens found
2025-09-24 14:22:33.575 [info] No stored tokens found
2025-09-24 14:22:33.586 [info] No stored tokens found
2025-09-24 14:22:33.590 [info] Found 1 tools, 0 prompts, and 0 resources
//Restart MCP server
2025-09-24 14:22:43.496 [error] Client error for command SSE stream disconnected: TypeError: terminated
2025-09-24 14:22:44.498 [info] No stored tokens found
2025-09-24 14:22:44.509 [error] Client error for command fetch failed
2025-09-24 14:22:44.509 [error] Client error for command Failed to reconnect SSE stream: fetch failed
2025-09-24 14:22:46.011 [info] No stored tokens found
2025-09-24 14:22:46.015 [error] Client error for command fetch failed
2025-09-24 14:22:46.015 [error] Client error for command Failed to reconnect SSE stream: fetch failed
2025-09-24 14:22:46.015 [error] Client error for command Maximum reconnection attempts (2) exceeded.
//Trigger to Call tool
2025-09-24 14:23:08.097 [info] Handling CallTool action for tool 'nordic_pr_analyzer'
2025-09-24 14:23:08.097 [info] Calling tool 'nordic_pr_analyzer' with toolCallId: tool_ddaec993-ff17-4e81-9fd3-1c531e8920b
2025-09-24 14:23:08.100 [info] No stored tokens found
2025-09-24 14:23:08.106 [error] Client error for command Error POSTing to endpoint (HTTP 400): Bad Request: No valid session ID provided
2025-09-24 14:23:08.106 [error] Error calling tool 'nordic_pr_analyzer': Error POSTing to endpoint (HTTP 400): Bad Request: No valid session ID provided
2025-09-24 14:23:18.466 [info] Handling CreateClient action
2025-09-24 14:23:18.467 [info] CreateClient completed, server stored: true
2025-09-24 14:23:18.486 [info] Handling CreateClient action
2025-09-24 14:23:18.486 [info] CreateClient completed, server stored: true
//Reload mcp.json (make small change in the file and save)
2025-09-24 14:23:31.073 [info] Handling ListOfferings action, server stored: true
2025-09-24 14:23:31.074 [info] Connected to streamableHttp server, fetching offerings
2025-09-24 14:23:31.074 [info] No stored tokens found
2025-09-24 14:23:31.077 [error] Client error for command Error POSTing to endpoint (HTTP 400): Bad Request: No valid session ID provided
2025-09-24 14:23:31.077 [error] Error listing tools: Error POSTing to endpoint (HTTP 400): Bad Request: No valid session ID provided
2025-09-24 14:23:31.087 [info] No stored tokens found
2025-09-24 14:23:31.090 [error] Client error for command Error POSTing to endpoint (HTTP 400): Bad Request: No valid session ID provided
2025-09-24 14:23:31.090 [error] Error listing prompts: Error POSTing to endpoint (HTTP 400): Bad Request: No valid session ID provided
2025-09-24 14:23:31.092 [info] No stored tokens found
2025-09-24 14:23:31.097 [error] Client error for command Error POSTing to endpoint (HTTP 400): Bad Request: No valid session ID provided
2025-09-24 14:23:31.097 [error] Error listing resources: Error POSTing to endpoint (HTTP 400): Bad Request: No valid session ID provided
2025-09-24 14:23:31.097 [info] Found 0 tools, 0 prompts, and 0 resources
...
//Turn OFF and ON of the MCP server from Cursor settings
2025-09-24 14:23:40.041 [info] Handling DeleteClient action
2025-09-24 14:23:40.042 [info] Cleaning up
2025-09-24 14:23:41.498 [info] Handling CreateClient action
2025-09-24 14:23:41.498 [info] Creating streamableHttp transport
2025-09-24 14:23:41.498 [info] Connecting to streamableHttp server
2025-09-24 14:23:41.504 [info] No stored tokens found
2025-09-24 14:23:41.521 [info] No stored tokens found
2025-09-24 14:23:41.525 [info] Successfully connected to streamableHttp server
2025-09-24 14:23:41.525 [info] Storing streamableHttp client
2025-09-24 14:23:41.525 [info] CreateClient completed, server stored: true
2025-09-24 14:23:41.527 [info] No stored tokens found
2025-09-24 14:23:47.919 [info] Handling ListOfferings action, server stored: true
2025-09-24 14:23:47.919 [info] Connected to streamableHttp server, fetching offerings
2025-09-24 14:23:47.920 [info] No stored tokens found
2025-09-24 14:23:47.936 [info] No stored tokens found
2025-09-24 14:23:47.941 [info] No stored tokens found
2025-09-24 14:23:47.947 [info] Found 1 tools, 0 prompts, and 0 resources
Does this stop you from using Cursor
No - Cursor works, but with this issue