Handle 202 Accepted response when server delivers via SSE stream

When an SSE stream is active, the server responds with 202 Accepted
(no Content-Type, empty body) to POST requests, delivering the actual
response via the SSE stream. Previously this raised an unsupported
Content-Type error.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: atesgoral

lib/mcp/client/http.rb

@@ -134,6 +134,9 @@ def parse_response_body(response, method, params)
           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 SSE stream
+          { "accepted" => true }
         else
           raise RequestHandlerError.new(
             "Unsupported Content-Type: #{content_type.inspect}. Expected application/json or text/event-stream.",

test/mcp/client/http_test.rb

@@ -265,6 +265,15 @@ def test_post_raises_error_for_sse_without_response
         assert_equal(:parse_error, error.error_type)
       end
 
+      def test_post_returns_accepted_for_202_with_no_content_type
+        stub_request(:post, url)
+          .to_return(status: 202, body: "")
+
+        response = transport.post(body: {})
+
+        assert_equal({ "accepted" => true }, response.body)
+      end
+
       def test_delete_sends_request_with_headers
         stub_request(:delete, url)
           .with(