MCP session is terminated before the request has been executed

I have been working with the Sample Cupcake Server and tried to call it using the OpenAI API. However, no request can be made. I noticed that the session is terminated before the request is made. This only occurs when establishing a session-based connection. In https://community.openai.com/t/openai-mcp-client-starts-to-fail-when-moving-from-sse-to-streamable-http/1275728, the problem was solved with a workaround by no longer allowing the server to work with sessions (Add stateless_http=True to your FastMCP initialization: mcp = FastMCP(“bloomy-mcp”, stateless_http=True)). However, this does not solve the actual problem.

log cupcake-server:
INFO: 52.255.109.109:0 - "GET /mcp/ HTTP/1.0" 200 OK
INFO: 52.255.109.109:0 - "POST /mcp/ HTTP/1.0" 202 Accepted
INFO: 52.255.109.101:0 - "POST /mcp/ HTTP/1.0" 200 OK
session gets terminated here********
INFO: 52.255.109.97:0 - "DELETE /mcp/ HTTP/1.0" 200 OK
*request after session gets terminated
INFO: 52.255.109.102:0 - "POST /mcp/ HTTP/1.0" 404 Not Found
truncated response from server:
"type": "mcp_call",
"error": {
"type": "mcp_protocol_error",
"code": 32600,
"message": "Session terminated"

Start by reproducing the reported session-based flow and inspect the GET, POST, and DELETE /mcp/ sequence in the logs. The fix is done when the request executes after session establishment without premature termination or a subsequent 404 Session terminated response.