Accept a 2xx acknowledgment of a notification without a body

lib/mcp/client/http.rb

@@ -427,7 +427,7 @@ def send_request(request:)
             nil
           end
 
-          body = resolve_response_body(stream, response, method, params)
+          body = resolve_response_body(stream, response, method, params, notification: !(request[:id] || request["id"]))
 
           # Every way a response arrives (JSON body, SSE event, resumed stream) ends here. Checked before
           # anything is learned from the body, and only for a request: a notification awaits no response.
@@ -1107,7 +1107,7 @@ def require_faraday!
 
       # Determines the logical JSON-RPC body of the exchange after the POST stream finished
       # (or was aborted because the response already arrived).
-      def resolve_response_body(stream, response, method, params)
+      def resolve_response_body(stream, response, method, params, notification: false)
         return stream.response if stream.response
 
         # 202 Accepted is the server's ACK for a JSON-RPC notification or response; no body is expected.
@@ -1141,6 +1141,10 @@ def resolve_response_body(stream, response, method, params)
           # added by the Faraday customizer may have parsed it already.
           response.body.is_a?(String) ? parse_json_buffer(response.body, method, params) : response.body
         else
+          # A notification awaits no response, so a server that acknowledges it with 200 and no body
+          # instead of 202 has still accepted it. The TypeScript and Python SDKs also accept this.
+          return if notification && response.success?
+
           raise RequestHandlerError.new(
             "Unsupported Content-Type: #{content_type.inspect}. Expected application/json or text/event-stream.",
             { method: method, params: params },

test/mcp/client/http_test.rb

@@ -831,6 +831,58 @@ def test_send_request_returns_nil_for_202_accepted_response
         assert_nil(response)
       end
 
+      def test_send_request_returns_nil_for_notification_acknowledged_with_empty_200
+        request = {
+          jsonrpc: "2.0",
+          method: "notifications/initialized",
+        }
+
+        stub_request(:post, url)
+          .with(body: request.to_json)
+          .to_return(status: 200, body: "")
+
+        response = client.send_request(request: request)
+
+        assert_nil(response)
+      end
+
+      def test_send_request_raises_error_for_notification_answered_with_empty_4xx_or_5xx
+        request = {
+          jsonrpc: "2.0",
+          method: "notifications/initialized",
+        }
+
+        [400, 500].each do |status|
+          stub_request(:post, url)
+            .with(body: request.to_json)
+            .to_return(status: status, body: "")
+
+          error = assert_raises(RequestHandlerError) do
+            client.send_request(request: request)
+          end
+
+          assert_equal(status, error.original_error.response[:status])
+        end
+      end
+
+      def test_send_request_raises_error_for_request_answered_with_empty_200
+        request = {
+          jsonrpc: "2.0",
+          id: "test_id",
+          method: "tools/list",
+        }
+
+        stub_request(:post, url)
+          .with(body: request.to_json)
+          .to_return(status: 200, body: "")
+
+        error = assert_raises(RequestHandlerError) do
+          client.send_request(request: request)
+        end
+
+        assert_equal(:unsupported_media_type, error.error_type)
+      end
+
       def test_send_request_raises_error_for_sse_without_response
         request = {
           jsonrpc: "2.0",
@@ -2249,6 +2301,31 @@ def test_connect_performs_initialize_handshake
         assert_equal({ "name" => "test-server", "version" => "1.0" }, result["serverInfo"])
       end
 
+      def test_connect_accepts_initialized_notification_acknowledged_with_empty_200
+        stub_request(:post, url)
+          .with { |req| JSON.parse(req.body)["method"] == "initialize" }
+          .to_return(
+            status: 200,
+            headers: { "Content-Type" => "application/json" },
+            body: {
+              jsonrpc: "2.0",
+              result: {
+                protocolVersion: "2025-11-25",
+                capabilities: {},
+                serverInfo: { name: "test-server", version: "1.0" },
+              },
+            }.to_json,
+          )
+        stub_request(:post, url)
+          .with { |req| JSON.parse(req.body)["method"] == "notifications/initialized" }
+          .to_return(status: 200, body: "")
+
+        result = client.connect
+
+        assert_predicate(client, :connected?)
+        assert_equal("2025-11-25", result["protocolVersion"])
+      end
+
       def test_connect_caches_server_info
         stub_initialize
         stub_notification