Whether a POSTed request's Accept header admits at least one of the two
media types the Streamable HTTP transport can answer a request with —
application/json (a single JSON reply) or text/event-stream (an SSE
stream). The spec REQUIRES POST clients to send
Accept: application/json, text/event-stream; a request whose Accept
provably excludes BOTH could not consume any response the server may produce,
so it is rejected up front with 406 Not Acceptable. A missing/blank Accept is
permissive (both helpers admit it), matching the GET path's tolerance.
Whether a POSTed request's Accept header admits at least one of the two media types the Streamable HTTP transport can answer a request with — application/json (a single JSON reply) or text/event-stream (an SSE stream). The spec REQUIRES POST clients to send Accept: application/json, text/event-stream; a request whose Accept provably excludes BOTH could not consume any response the server may produce, so it is rejected up front with 406 Not Acceptable. A missing/blank Accept is permissive (both helpers admit it), matching the GET path's tolerance.