fix: handle empty JSON responses in ResponseSubscribers

- Remove length check in hookOnComplete() to always emit AggregateResponseEvent
- Ensures proper completion handling regardless of response content length
- Add test for empty application/json responses with 200 OK status

Author: codezkk

mcp/src/main/java/io/modelcontextprotocol/client/transport/ResponseSubscribers.java

@@ -238,10 +238,9 @@ protected void hookOnNext(String line) {
 
 		@Override
 		protected void hookOnComplete() {
-			if (this.eventBuilder.length() > 0) {
-				String data = this.eventBuilder.toString();
-				this.sink.next(new AggregateResponseEvent(responseInfo, data));
-			}
+			String data = this.eventBuilder.toString();
+			this.sink.next(new AggregateResponseEvent(responseInfo, data));
+
 			this.sink.complete();
 		}
 

mcp/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportEmptyJsonResponseTest.java

@@ -0,0 +1,92 @@
+/*
+ * Copyright 2024-2025 the original author or authors.
+ */
+
+package io.modelcontextprotocol.client.transport;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.atLeastOnce;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+
+import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.net.URI;
+import java.net.URISyntaxException;
+
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.Timeout;
+
+import com.sun.net.httpserver.HttpServer;
+
+import io.modelcontextprotocol.server.transport.TomcatTestUtil;
+import io.modelcontextprotocol.spec.McpSchema;
+import io.modelcontextprotocol.spec.ProtocolVersions;
+import reactor.test.StepVerifier;
+
+/**
+ * Handles emplty application/json response with 200 OK status code.
+ *
+ * @author codezkk
+ */
+public class HttpClientStreamableHttpTransportEmptyJsonResponseTest {
+
+	static int PORT = TomcatTestUtil.findAvailablePort();
+
+	static String host = "http://localhost:" + PORT;
+
+	static HttpServer server;
+
+	@BeforeAll
+	static void startContainer() throws IOException {
+
+		server = HttpServer.create(new InetSocketAddress(PORT), 0);
+
+		// Empty, 200 OK response for the /mcp endpoint
+		server.createContext("/mcp", exchange -> {
+			exchange.getResponseHeaders().set("Content-Type", "application/json");
+			exchange.sendResponseHeaders(200, 0);
+			exchange.close();
+		});
+
+		server.setExecutor(null);
+		server.start();
+	}
+
+	@AfterAll
+	static void stopContainer() {
+		server.stop(1);
+	}
+
+	/**
+	 * Regardless of the response (even if the response is null and the content-type is
+	 * present), notify should handle it correctly.
+	 */
+	@Test
+	@Timeout(3)
+	void testNotificationInitialized() throws URISyntaxException {
+
+		var uri = new URI(host + "/mcp");
+		var mockRequestCustomizer = mock(SyncHttpRequestCustomizer.class);
+		var transport = HttpClientStreamableHttpTransport.builder(host)
+			.httpRequestCustomizer(mockRequestCustomizer)
+			.build();
+
+		var initializeRequest = new McpSchema.InitializeRequest(ProtocolVersions.MCP_2025_03_26,
+				McpSchema.ClientCapabilities.builder().roots(true).build(),
+				new McpSchema.Implementation("Spring AI MCP Client", "0.3.1"));
+		var testMessage = new McpSchema.JSONRPCRequest(McpSchema.JSONRPC_VERSION, McpSchema.METHOD_INITIALIZE,
+				"test-id", initializeRequest);
+
+		StepVerifier.create(transport.sendMessage(testMessage)).verifyComplete();
+
+		// Verify the customizer was called
+		verify(mockRequestCustomizer, atLeastOnce()).customize(any(), eq("GET"), eq(uri), eq(
+				"{\"jsonrpc\":\"2.0\",\"method\":\"initialize\",\"id\":\"test-id\",\"params\":{\"protocolVersion\":\"2025-03-26\",\"capabilities\":{\"roots\":{\"listChanged\":true}},\"clientInfo\":{\"name\":\"Spring AI MCP Client\",\"version\":\"0.3.1\"}}}"));
+
+	}
+
+}