Custom MCP Server not working because of SSE Post Request

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Hi :slight_smile:
I'am trying to connect my Custom MCP Server as a resource to Retool. The SSE Endpoint is exposed and working fine for other clients (like LibreChat). But on the Retool configuration page I get the following errors:

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My server is using FastMCP and I can see in the logs ""POST /sse HTTP/1.1" 405 Method Not Allowed". As fas as I know the SSE-Standard does NOT support POST requests...why would Retool use this to check the connection? How can I make Retool work with the FastMCP server?

Thanks for your time and ideas!

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Hello @Nightmare!

I noticed this myself while working with FastMCP. As I understand it, Retool supports a few different aspects of MCP. We support streamable transport as well as SSE. Right now there's no way to tell Retool in advance which type of server you support, so the POST you see is Retool first trying to see if your server supports streamable and then if that fails with a 4XX status, it falls back to GET and SSE over HTTP transport.

Are you seeing successful calls to your tool as well? When you're configuring the URL as a Retool resource, you might need to add the /sse path to the URL as well.

Taylor

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Thanks for the fast reply!

Actually I can see at my tool that there is a successful GET request after the disallowed POST request:
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But the Retool console still shows the following error:
" 1. sseError: Object

  1. message: "Error POSTing to endpoint (HTTP 404): ""
4

I also tried to switch my MCP Server to "streamble-http" but this returns the following error:

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The MCP Server is running behind a reverse proxy (where the proxy_pass is "http://localhost..." - could this cause the issue? The reverse proxy itself is https configured.

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Good find! Pretty certain this error is thrown by the underlying library we use as a MCP client. This can be tricky to debug.. do you know the raw response your MCP server is giving to that first POST to /mcp? It sounds possible that that FastMCP is returning the HTTP-based URL instead of returning one that still goes through the proxy.