Handle 202 Accepted response in HTTP client

[atesgoral]

Summary

Second slice of #321 (Streamable HTTP client support).

The Streamable HTTP spec allows a server to respond 202 Accepted when it has received the request but will deliver the response later via an SSE stream. The client currently errors on 202 because the response has no parseable Content-Type. Return { "accepted" => true } so callers can proceed rather than hard-fail on a valid server response.

Actually picking up the deferred response requires listening on an SSE stream (GET-for-SSE), which is a TODO in the main PR and out of scope here. This change is just the graceful-no-hard-error part.

Changes

• lib/mcp/client/http.rb: add elsif response.status == 202 branch to parse_response_body returning { "accepted" => true }
• test/mcp/client/http_test.rb: test for 202 response with empty body

Test plan

• rake test passes (738 runs, 1848 assertions)
• rake rubocop no new offenses on changed files
• New test covers 202 response with empty body

Stack

• Parse SSE responses in HTTP client via event_stream_parser #322 — SSE parsing via event_stream_parser
• This PR — 202 Accepted handling
• (next) — stateful client: connect, session-ID tracking, close/DELETE

🤖 Generated with Claude Code

[koic]

I think the spec defines 202 as the ACK for a notification/response, rather than "request accepted, SSE coming later". The TypeScript and Python SDKs also check status == 202 before Content-Type and return void rather than a Hash sentinel. In the current diff, the 202 branch comes last, so a 202 with any Content-Type header would skip it; the existing test happens to pass only because WebMock leaves Content-Type unset. Also, { "accepted" => true } overlaps with the Hash shape of real JSON-RPC responses. What do you think about something closer to this?

 def parse_response_body(response, method, params)
+  # 202 Accepted is the ACK for a notification/response; no body is expected.
+  # https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#sending-messages-to-the-server
+  return if response.status == 202
+
   content_type = response.headers["Content-Type"]

   if content_type&.include?("text/event-stream")
     parse_sse_response(response.body, method, params)
   elsif content_type&.include?("application/json")
     response.body
-  elsif response.status == 202
-    # Server accepted the request and will deliver the response via an SSE stream.
-    # https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#sending-messages-to-the-server
-    { "accepted" => true }
   else
     raise RequestHandlerError.new(...)
   end
 end

The tests will need a matching update as well.

[atesgoral]

Verified against the spec and the TS/Python SDKs. Updated the diff to check status == 202 before Content-Type dispatch and return nil instead of the Hash sentinel. Also corrected the commit message (202 is the ACK for a notification/response, not "SSE coming later") and cleaned up a stale doc string in examples/streamable_http_server.rb. Thanks for catching this!

[koic]

Thanks!