HTTP Streamable must return JsonResponse for single requests

[Wolfgang-check24]

What happened?

According https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#sending-messages-to-the-server and https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#sequence-diagram the server returns on a "single" request a simple http request type json/application and on multiple / batched ones as text/event-stream

How to reproduce the bug

curl --location 'http://localhost:8000/mcp' \
--header 'Content-Type: application/json' \
--data '{
  "jsonrpc": "2.0",
  "method": "tools/call",
  "id": 5,
  "params": {
    "name": "hello-world",
    "arguments": {
      "name": "HTTP SSE Developer"
    }
  }
}'

Package Version

1.3.1

PHP Version

8.2.0

Symfony Version

6.4, 7.2

Which operating systems does this happen with?

macOS

Notes

No response

[Wolfgang-check24]

to be honest impact quite minor probably

[klapaudius]

Hello Wolfgang,

You're absolutely right about the fact that I didn't implement strictly the specification on this part. The decision was made to ensure the code remains easily maintainable, especially since the MCP Inspector supports this methodology. Additionally, I conducted tests using Cursor and open-webui/mcpo, all of which functioned as expected.

I will keep the issue open for future consideration when it becomes appropriate to adhere strictly to the specification.

Regards

[klapaudius]

Hi, this issue is now part of the #43 New MCP Specification version 2025-06-18 Roadmap