MCP Server connection fails with 422 "invalid JSON response from dbconnector"

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1

Hi,

I'm trying to connect a custom MCP server to a Retool Agent, but the connection always fails with:

Error: invalid JSON response from dbconnector
HttpError { statusCode: 422, payload: Object }
  statusCode: 422
  error: "Unprocessable Entity"
  message: "invalid JSON response from dbconnector"
  data: null

I've tried:

  • SSE endpoint (/sse)

  • StreamableHTTP endpoint (/mcp)

  • Bearer Token authentication (confirmed working — other clients connect successfully)

  • The server is publicly accessible over HTTPS

The same error occurs with the official Atlassian MCP server, so this doesn't appear to be a problem with my implementation.

A similar issue was reported here with no resolution: Make.com MCP Server

Is MCP server support still in beta? Is there a known fix or workaround?

Thanks

David

4

Now all working.

D.