unified mcp endpoint not reachable

[Sozhan308]

I am trying to launch couple of mcp servers with metamcp and create an unified endpoint. curl request is already failing for the /mcp unified endpoint.
surprisingly the /sse endpoint works fine

:~/repo/metamcp$ curl -v http://localhost:12008/metamcp/mcp/sse

• Trying 127.0.0.1:12008...
• Connected to localhost (127.0.0.1) port 12008 (#0)

GET /metamcp/mcp/sse HTTP/1.1
Host: localhost:12008
User-Agent: curl/7.81.0
Accept: /

• Mark bundle as not supporting multiuse
  < HTTP/1.1 200 OK
  < x-powered-by: Express
  < access-control-allow-origin: *
  < access-control-allow-credentials: true
  < x-frame-options: DENY
  < x-content-type-options: nosniff
  < x-xss-protection: 1; mode=block
  < referrer-policy: strict-origin-when-cross-origin
  < content-security-policy: default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; connect-src 'self'; frame-ancestors 'none';
  < vary: Origin
  < content-type: text/event-stream
  < cache-control: no-cache, no-transform
  < connection: keep-alive
  < date: Thu, 13 Nov 2025 12:07:44 GMT
  < transfer-encoding: chunked
  <
  event: endpoint
  data: /metamcp/mcp/message?sessionId=ae37b65b-f648-4881-9cd4-999b212b843e

^C
:~/repo/metamcp$ curl -v http://localhost:12008/metamcp/mcp/mcp

• Trying 127.0.0.1:12008...
• Connected to localhost (127.0.0.1) port 12008 (#0)

GET /metamcp/mcp/mcp HTTP/1.1
Host: localhost:12008
User-Agent: curl/7.81.0
Accept: /

• Mark bundle as not supporting multiuse
  < HTTP/1.1 404 Not Found
  < x-powered-by: Express
  < access-control-allow-origin: *
  < access-control-allow-credentials: true
  < x-frame-options: DENY
  < x-content-type-options: nosniff
  < x-xss-protection: 1; mode=block
  < referrer-policy: strict-origin-when-cross-origin
  < content-security-policy: default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; connect-src 'self'; frame-ancestors 'none';
  < vary: Origin
  < date: Thu, 13 Nov 2025 12:07:49 GMT
  < connection: close
  < content-length: 17
  <
• Closing connection 0
  Session not found

[Stavitsky]

Hey @Sozhan308 👋

It looks like the endpoint is fine. The 404 is simply telling you that there is no active MCP session yet. The unified MCP endpoint only works after a session is created, and that happens with a POST initialize request. A GET request cannot start a session, so the server replies with Session not found.

Here is a working example you can try (replace test endpoint name in URL with mcp from your example):

curl -X POST \
  -H "Accept: application/json, text/event-stream" \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "initialize",
    "params": {
      "protocolVersion": "1.0",
      "capabilities": {
        "tools": {},
        "resources": {},
        "roots": {}
      },
      "clientInfo": {
        "name": "curl-test",
        "version": "0.0.1"
      }
    }
  }' \
  http://localhost:12008/metamcp/test/mcp

If you want to see what is happening internally, take a quick look at the metamcp logs. When the server returns “Session not found”, you will see the same note there. This is normal behavior for the HTTP streaming MCP transport.

P.S. You can also check the official MCP documentation, where the process is described clearly, starting with the POST session initialization request.