[BUG][MCP]: POST /mcp returns 405 after v0.9 to v1.0 migration — TRANSPORT_TYPE config mismatch

[ahujayogesh2003-maker]

🐞 Bug Summary

After migrating from v0.9 to v1.0.0-RC1, MCP tool calls fail when using streamable_http transport. The /mcp endpoint returns 405 Method Not Allowed with allow: GET in the response header. The same code worked without any issues on v0.9.
The suggested fixes from Discussion #3434 — adding the Accept header and using a trailing slash — did not resolve the issue.

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🧩 Affected Component

Select the area of the project impacted:

• [ yes ] mcpgateway - API
• mcpgateway - UI (admin panel)
• mcpgateway.wrapper - stdio wrapper
• Federation or Transports
• CLI, Makefiles, or shell scripts
• Container setup (Docker/Podman/Compose)
• Other (explain below)

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🔁 Steps to Reproduce

1. Migrate MCP Gateway from v0.9 to v1.0.0-RC1
2. Connect using langchain_mcp_adapters with streamable_http transport
3. Call client.get_tools()

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🤔 Expected Behavior

POST /mcp is accepted and returns tools as it did in v0.9.

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📓 Logs / Error Output

ExceptionGroup('unhandled errors in a TaskGroup', [HTTPStatusError("Client error 
'405 Method Not Allowed' for url '.../servers/{id}/mcp/'")])

📓 Code

Original code that worked on v0.9:

client = MultiServerMCPClient(
    {
        "gateway": {
            "transport": "streamable_http",
            "url": f"{mcpgw_url}/servers/{mcp_virtual_server}/mcp",
            "headers": {"Authorization": f"Bearer {mcpgw_token}"},
        }
    }
)
tools = await client.get_tools()

Code after applying suggested fixes — still fails:

client = MultiServerMCPClient(
    {
        "gateway": {
            "transport": "streamable_http",
            "url": f"{mcpgw_url}/servers/{mcp_virtual_server}/mcp/",
            "headers": {
                "Authorization": f"Bearer {mcpgw_token}",
                "Accept": "application/json, text/event-stream",
            },
        }
    }
)
tools = await client.get_tools()

🧠 Environment Info

You can retrieve most of this from the /version endpoint.

Key	Value
Version or commit	v1.0.0-RC1
Runtime	e.g. Python 3.12
Platform / OS	EKS
langchain-mcp-adapters	0.2.1
mcp	1.26

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🧩 Additional Context (optional)

Add any configuration details, flags, or related issues.

[crivetimihai]

Investigated. The allow: GET in your 405 response header indicates the SSE handler (GET-only) is catching your POST request instead of the streamable HTTP handler. In v1.0, Docker Compose default changed to TRANSPORT_TYPE=streamablehttp. If your deployment carried over TRANSPORT_TYPE=sse from v0.9, POST requests to /servers/{id}/mcp will get 405.

Fix: set TRANSPORT_TYPE=streamablehttp or TRANSPORT_TYPE=all.

I verified POST works correctly on our test instance with the correct transport config. See also discussion #3434 where this was analyzed. Could you confirm your TRANSPORT_TYPE setting?