[wweir]

Problem

StreamableHttpServerConfig::json_response only applies to the stateless path
(serve_negotiated_request_directly). With a session (the default
legacy_session_mode, and any legacy-protocol client), every POST answer is framed
as text/event-stream, no matter what the client asked for.

That matters because the transport also requires the client to declare both
media types:

Accept: application/json, text/event-stream   → ok
Accept: application/json                      → 406 "Client must accept both ..."

So a client that declares text/event-stream only to satisfy the 406 check, and
then parses nothing but JSON, gets a correct initialize and then stops — it cannot
read any subsequent call. Concretely: Gemini's connector verification does exactly
that, opens a session (initialize + notifications/initialized succeed), and then
never reads tools/list, reporting "cannot connect to this server".

Change

Honour json_response in session mode too: peek the first message of the
request-wise stream.

• terminal response/error → answer application/json (jsonrpc_message_response),
  same as the stateless path already does;
• anything else (priming event, notification, server request) → replay that first
  message in front of the rest and keep the SSE framing, so no message is lost and
  streaming clients are unaffected (json_response defaults to false).

Also updates the json_response doc comment, which claimed stateless-only.

Tests

• cargo check -p rmcp --lib --features server — clean
• cargo test -p rmcp --lib --features server — 229 passed

The behaviour was validated against a real deployment: with this change, a client
that sends Accept: application/json, text/event-stream but only parses JSON
receives initialize, tools/list (~90 KiB catalogue) and tools/call as
application/json with a working Mcp-Session-Id, and proceeds to use the tools.

Note: this is deliberately not relaxing the Accept requirement — that check is
spec-faithful; only the response framing was unreachable for these clients.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	⚠️ Failed 2026-10-29T00:48:23.730124Z We couldn’t complete this review. Comment @codex review to try again.	2c60815	PR opened

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[github-actions]

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