Handle 202 Accepted response in HTTP client

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 previously errored on 202 because the response
has no parseable Content-Type. Return `{ "accepted" => true }` so the
caller can decide how to proceed.

Actually picking up the deferred response requires listening on an SSE
stream (GET-for-SSE), which is not yet implemented. This PR only
prevents the hard error on a valid server response.

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

Author: atesgoral

lib/mcp/client/http.rb

@@ -106,6 +106,10 @@ 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 an SSE stream.
+          # https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#sending-messages-to-the-server
+          { "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

@@ -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_accepted_for_202_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_equal({ "accepted" => true }, response)
+      end
+
       def test_send_request_raises_error_for_sse_without_response
         request = {
           jsonrpc: "2.0",