Power Platform Community Forum Thread Details

Hey guys. I'm using a customer MCP connector in my agent flow but I'm getting an error like "Error Message: The connector 'mcp' returned an HTTP error with code 400. Error Code: ConnectorRequestFailure". Looking into my MCP logs I notice that the 400 error is causing because for some reason the "accept" header is not being sent in the request: "{"jsonrpc":"2.0","error":{"code":-32600,"message":"Client MUST accept either application/json, text/event-stream or both mediaTypes"}}". I've included that "accept" parameter in the step where the MCP is called:

Now, this issue occurs with the first call of my MCP in the flow only. After that, the following request and executions in my flow are working fine :( . Does anyone get the same issue? If so, what did you do to fix it? I really appreciate any help on this topic.

Thanks and happy day guys.

Hi there,

This usually happens because the first MCP request doesn’t include the Accept header, even if you added it in the step.

With MCP (Streamable HTTP), the client must send:

Accept: application/json, text/event-stream

Putting it in the flow step is often not enough — you need to define it in the custom connector (OpenAPI) or enforce it with a Set HTTP Header policy.

The reason it only fails on the first call is that the initial request is handled differently (handshake), and headers from the step aren’t applied yet.

Hope this helps!
Paolo

✅ Did this solve your issue? → Accept as Solution
👍 Partially helpful? → Click "Yes" on "Was this reply helpful?" or drop a Like!

Want more tips on Power Platform & AI? Follow me here:

🔗 LinkedIn: https://www.linkedin.com/in/paoloasnaghi/
▶️ YouTube: https://www.youtube.com/@BeyondThePlatforms
📸 Instagram: https://www.instagram.com/beyond_the_platforms/
🌐 Website: https://www.beyondtheplatforms.com/