[Bug] Auto-registration not working with `cforge run --expose-streamable-http`

[gabe-l-hart]

Describe the bug

When running cforge run --expose-streamable-http, the auto-registration feature fails.

Platform

Please provide details about the environment you are using, including the following:

• Interpreter version:
• Library version:

Sample Code

cforge run --stdio "docker run --rm -i ghcr.io/ibm/fast-time-server:latest -transport=stdio" --temporary --port 9005 --expose-streamable-http

Expected behavior

This is success for an SSE server. I would expect w/ Streamable HTTP it would hit /mcp instead of /sse.

INFO:     127.0.0.1:60652 - "GET /sse HTTP/1.1" 200 OK
INFO:     127.0.0.1:60654 - "POST /message?session_id=73d1253fe90a4ceeb104ef2338b3ffad HTTP/1.1" 202 Accepted
INFO:     127.0.0.1:60654 - "POST /message?session_id=73d1253fe90a4ceeb104ef2338b3ffad HTTP/1.1" 202 Accepted
2026-02-26T20:18:23.489309 [mcp.s:DBUG] Received message: root=InitializedNotification(method='notifications/initialized', params=None, jsonrpc='2.0')
INFO:     127.0.0.1:60654 - "POST /message?session_id=73d1253fe90a4ceeb104ef2338b3ffad HTTP/1.1" 202 Accepted
2026-02-26T20:18:23.490848 [mcp.s:DBUG] Received message: <mcp.shared.session.RequestResponder object at 0x109fd6ab0>
2026-02-26T20:18:23.491007 [mcp.s:INFO] Processing request of type ListToolsRequest
2026-02-26T20:18:23.491038 [mcp.s:DBUG] Dispatching request of type ListToolsRequest
2026-02-26T20:18:23.491413 [mcp.s:DBUG] Response sent
✓ Server registered successfully (ID: a390052350174b44b17a5da736e967aa)

Observed behavior

Registering server 'stdio-server-9005' at http://127.0.0.1:9005/sse...
INFO:     127.0.0.1:60889 - "GET /sse HTTP/1.1" 404 Not Found
Warning: Failed to register server: API request failed (503): {"message":"Unable to connect to gateway"}
Continuing without registration...