Handle 202 Accepted response in HTTP client

Per the Streamable HTTP spec, a server returns 202 Accepted with no
body as the ACK for a JSON-RPC notification or response. The client
previously errored on 202 because it fell through Content-Type
dispatch. Short-circuit to return nil before Content-Type handling so
fire-and-forget messages (e.g. notifications/initialized) no longer
raise.

https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#sending-messages-to-the-server

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: atesgoral

examples/streamable_http_server.rb

@@ -171,7 +171,7 @@ def call(env)
      curl -i http://localhost:9393 -H "Mcp-Session-Id: YOUR_SESSION_ID" \\
        --json '{"jsonrpc":"2.0","method":"tools/call","id":3,"params":{"name":"notification_tool","arguments":{"message":"Hello SSE!", "delay": 2}}}'
 
-  Note: When an SSE stream is active, tool responses will appear in the SSE stream and the POST request will return {"accepted": true}
+  Note: When an SSE stream is active, tool responses will appear in the SSE stream and the POST request will return 202 Accepted with no body.
 
   Press Ctrl+C to stop the server
 MESSAGE

lib/mcp/client/http.rb

@@ -100,6 +100,10 @@ def require_event_stream_parser!
       end
 
       def parse_response_body(response, method, params)
+        # 202 Accepted is the server's ACK for a JSON-RPC notification or 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")

test/mcp/client/http_test.rb

@@ -369,6 +369,21 @@ def test_send_request_parses_sse_error_response
         assert_equal("Invalid request", response.dig("error", "message"))
       end
 
+      def test_send_request_returns_nil_for_202_accepted_response
+        request = {
+          jsonrpc: "2.0",
+          method: "notifications/initialized",
+        }
+
+        stub_request(:post, url)
+          .with(body: request.to_json)
+          .to_return(status: 202, body: "")
+
+        response = client.send_request(request: request)
+
+        assert_nil(response)
+      end
+
       def test_send_request_raises_error_for_sse_without_response
         request = {
           jsonrpc: "2.0",