StreamableHTTPError: Streamable HTTP error: Failed to open SSE stream: Not Found

[demian85]

When running the MCP inspector with my local server using http streaming as specified in the example in this repo, an error is displayed in the console:

🔍 MCP Inspector is up and running at http://127.0.0.1:6274 🚀
New StreamableHttp connection request
Query parameters: {"url":"http://localhost:3081/mcp","transportType":"streamable-http"}
Created StreamableHttp server transport
Created StreamableHttp client transport
Client <-> Proxy  sessionId: 2b0b3905-6f2a-4741-8201-c93dd2866a9f
Connection refused. Is the MCP server running?
New StreamableHttp connection request
Query parameters: {"url":"http://localhost:3081/mcp","transportType":"streamable-http"}
Created StreamableHttp server transport
Created StreamableHttp client transport
Client <-> Proxy  sessionId: d4343810-8d2f-4138-a531-6b118af1f139
Received POST message for sessionId d4343810-8d2f-4138-a531-6b118af1f139
Received GET message for sessionId d4343810-8d2f-4138-a531-6b118af1f139
Error from MCP server: StreamableHTTPError: Streamable HTTP error: Failed to open SSE stream: Not Found
    at StreamableHTTPClientTransport._startOrAuthSse (file:///Users/demian/.npm/_npx/5a9d879542beca3a/node_modules/@modelcontextprotocol/sdk/dist/esm/client/streamableHttp.js:106:23)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5) {
  code: 404
}

Although the inspector works, I'm trying to connect my server to a Librechat instance but it fails to connect and logs show:

LibreChat         | 2025-11-21 13:21:32 info: [MCP][Rlink-DB] Creating streamable-http transport: http://localhost:3081/mcp
LibreChat         | 2025-11-21 13:21:32 error: [MCP][Rlink-DB] Transport error: fetch failed
LibreChat         | 2025-11-21 13:21:32 info: [MCP][Rlink-DB] Streamable-http transport closed
LibreChat         | 2025-11-21 13:21:32 error: [MCP][Rlink-DB] Connection failed: fetch failed
LibreChat         | 2025-11-21 13:21:36 info: [MCP][Rlink-DB] Creating streamable-http transport: http://localhost:3081/mcp
LibreChat         | 2025-11-21 13:21:36 error: [MCP][Rlink-DB] Transport error: fetch failed
LibreChat         | 2025-11-21 13:21:36 info: [MCP][Rlink-DB] Streamable-http transport closed
LibreChat         | 2025-11-21 13:21:36 error: [MCP][Rlink-DB] Connection failed: fetch failed
LibreChat         | 2025-11-21 13:21:36 error: [MCP][Rlink-DB] Failed to connect after 3 attempts fetch failed
LibreChat         | 2025-11-21 13:21:36 error: [MCP][Rlink-DB] Failed to initialize: fetch failed
LibreChat         | 2025-11-21 13:21:36 info: MCP servers initialized successfully. Added 0 MCP tools.

To Reproduce
Run the quick sample code from this repo locally and run mcp inspector.

Is the example wrong? What am I missing? Is it a Librechat issue?

Thanks.