"Client failed to initialize by explicit API call" bugfix

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

@@ -237,10 +237,10 @@ 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));
-			}
+			// sink.next() is must
+			String data = this.eventBuilder.toString();
+			this.sink.next(new AggregateResponseEvent(responseInfo, data));
+
 			this.sink.complete();
 		}
 

mcp/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportFor200Test.java

@@ -0,0 +1,91 @@
+/*
+ * Copyright 2024-2025 the original author or authors.
+ */
+
+package io.modelcontextprotocol.client.transport;
+
+import com.sun.net.httpserver.HttpServer;
+import io.modelcontextprotocol.spec.McpSchema;
+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 reactor.test.StepVerifier;
+
+import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.net.URI;
+import java.net.URISyntaxException;
+import java.util.concurrent.TimeUnit;
+import java.util.function.Consumer;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.atLeastOnce;
+import static org.mockito.Mockito.verify;
+
+/**
+ * Tests for the "Client failed to initialize by explicit API call" problem.
+ *
+ * @author codezkk
+ */
+public class HttpClientStreamableHttpTransportFor200Test {
+
+	static String host = "http://localhost:3001";
+
+	static HttpServer server;
+
+	@BeforeAll
+	static void startContainer() throws IOException {
+		server = HttpServer.create(new InetSocketAddress(3001), 0);
+		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.
+	 * @throws URISyntaxException
+	 */
+	@Test
+	@Timeout(value = 3, unit = TimeUnit.SECONDS)
+	void testNotificationInitialized() throws URISyntaxException {
+
+		var uri = new URI(host + "/mcp");
+		var mockRequestCustomizer = mock(SyncHttpRequestCustomizer.class);
+		var transport = HttpClientStreamableHttpTransport.builder(host)
+			.httpRequestCustomizer(mockRequestCustomizer)
+			.build();
+
+		withTransport(transport, (t) -> {
+			var testNotification = new McpSchema.JSONRPCNotification(McpSchema.JSONRPC_VERSION,
+					McpSchema.METHOD_NOTIFICATION_INITIALIZED, null);
+
+			StepVerifier.create(t.sendMessage(testNotification)).verifyComplete();
+
+			verify(mockRequestCustomizer, atLeastOnce()).customize(any(), eq("GET"), eq(uri),
+					eq("{\"jsonrpc\":\"2.0\",\"method\":\"notifications/initialized\"}"));
+		});
+	}
+
+	void withTransport(HttpClientStreamableHttpTransport transport, Consumer<HttpClientStreamableHttpTransport> c) {
+		try {
+			c.accept(transport);
+		}
+		finally {
+			StepVerifier.create(transport.closeGracefully()).verifyComplete();
+		}
+	}
+
+}