feat: implement Streamable HTTP HttpClient transport and improve error handling (#337)

- Add HttpClientStreamableHttpTransport implementing the 2025-03-26 MCP specification
- Add ResponseSubscribers utility for handling different HTTP response body types
- Refactor HttpClientSseClientTransport to use reactive streams instead of FlowSseClient
  - Remove FlowSseClient in favor of reactive stream patterns
  - Use Disposable-based connection management instead of CountDownLatch
  - Replace message endpoint discovery with Sinks.One approach
- Improve error handling by replacing doOnError/onErrorResume with onErrorComplete
- Add test coverage for new Streamable HTTP transport
- Add resiliency tests for connection handling and session invalidation

BREAKING CHANGE: FlowSseClient has been removed in favor of reactive streams

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

Co-authored-by: Dariusz Jędrzejczyk <dariusz.jedrzejczyk@broadcom.com>

Author: tzolov

mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/client/transport/WebClientStreamableHttpTransport.java

@@ -129,11 +129,10 @@ private DefaultMcpTransportSession createTransportSession() {
 		Function<String, Publisher<Void>> onClose = sessionId -> sessionId == null ? Mono.empty()
 				: webClient.delete().uri(this.endpoint).headers(httpHeaders -> {
 					httpHeaders.add("mcp-session-id", sessionId);
-				})
-					.retrieve()
-					.toBodilessEntity()
-					.doOnError(e -> logger.warn("Got error when closing transport", e))
-					.then();
+				}).retrieve().toBodilessEntity().onErrorComplete(e -> {
+					logger.warn("Got error when closing transport", e);
+					return true;
+				}).then();
 		return new DefaultMcpTransportSession(onClose);
 	}
 
@@ -305,12 +304,12 @@ else if (mediaType.isCompatibleWith(MediaType.APPLICATION_JSON)) {
 					}
 				})
 				.flatMap(jsonRpcMessage -> this.handler.get().apply(Mono.just(jsonRpcMessage)))
-				.onErrorResume(t -> {
+				.onErrorComplete(t -> {
 					// handle the error first
 					this.handleException(t);
 					// inform the caller of sendMessage
 					sink.error(t);
-					return Flux.empty();
+					return true;
 				})
 				.doFinally(s -> {
 					Disposable ref = disposableRef.getAndSet(null);

mcp-spring/mcp-spring-webflux/src/test/java/io/modelcontextprotocol/client/WebClientStreamableHttpAsyncClientTests.java

@@ -1,12 +1,12 @@
 package io.modelcontextprotocol.client;
 
-import io.modelcontextprotocol.client.transport.WebClientStreamableHttpTransport;
-import io.modelcontextprotocol.spec.McpClientTransport;
 import org.junit.jupiter.api.Timeout;
+import org.springframework.web.reactive.function.client.WebClient;
 import org.testcontainers.containers.GenericContainer;
 import org.testcontainers.containers.wait.strategy.Wait;
 
-import org.springframework.web.reactive.function.client.WebClient;
+import io.modelcontextprotocol.client.transport.WebClientStreamableHttpTransport;
+import io.modelcontextprotocol.spec.McpClientTransport;
 
 @Timeout(15)
 public class WebClientStreamableHttpAsyncClientTests extends AbstractMcpAsyncClientTests {

mcp-spring/mcp-spring-webflux/src/test/java/io/modelcontextprotocol/client/WebClientStreamableHttpSyncClientTests.java

@@ -1,12 +1,12 @@
 package io.modelcontextprotocol.client;
 
-import io.modelcontextprotocol.client.transport.WebClientStreamableHttpTransport;
-import io.modelcontextprotocol.spec.McpClientTransport;
 import org.junit.jupiter.api.Timeout;
+import org.springframework.web.reactive.function.client.WebClient;
 import org.testcontainers.containers.GenericContainer;
 import org.testcontainers.containers.wait.strategy.Wait;
 
-import org.springframework.web.reactive.function.client.WebClient;
+import io.modelcontextprotocol.client.transport.WebClientStreamableHttpTransport;
+import io.modelcontextprotocol.spec.McpClientTransport;
 
 @Timeout(15)
 public class WebClientStreamableHttpSyncClientTests extends AbstractMcpSyncClientTests {

mcp-spring/mcp-spring-webmvc/src/test/resources/logback.xml

@@ -9,16 +9,16 @@
     </appender>
 
     <!-- Main MCP package -->
-    <logger name="io.modelcontextprotocol" level="DEBUG"/>
+    <logger name="io.modelcontextprotocol" level="INFO"/>
 
     <!-- Client packages -->
-    <logger name="io.modelcontextprotocol.client" level="DEBUG"/>
+    <logger name="io.modelcontextprotocol.client" level="INFO"/>
 
     <!-- Server transport package -->
-    <logger name="io.modelcontextprotocol.server.transport" level="DEBUG"/>
+    <logger name="io.modelcontextprotocol.server.transport" level="INFO"/>
 
     <!-- Spec package -->
-    <logger name="io.modelcontextprotocol.spec" level="DEBUG"/>
+    <logger name="io.modelcontextprotocol.spec" level="INFO"/>
 
     <!-- Root logger -->
     <root level="INFO">

mcp-test/src/main/java/io/modelcontextprotocol/client/AbstractMcpAsyncClientResiliencyTests.java

@@ -79,7 +79,7 @@ public abstract class AbstractMcpAsyncClientResiliencyTests {
 		host = "http://" + ipAddressViaToxiproxy + ":" + portViaToxiproxy;
 	}
 
-	private static void disconnect() {
+	static void disconnect() {
 		long start = System.nanoTime();
 		try {
 			// disconnect
@@ -96,7 +96,7 @@ private static void disconnect() {
 		}
 	}
 
-	private static void reconnect() {
+	static void reconnect() {
 		long start = System.nanoTime();
 		try {
 			proxy.toxics().get("RESET_UPSTREAM").remove();
@@ -110,7 +110,7 @@ private static void reconnect() {
 		}
 	}
 
-	private static void restartMcpServer() {
+	static void restartMcpServer() {
 		container.stop();
 		container.start();
 	}

mcp/pom.xml

@@ -141,11 +141,11 @@
 
 		<!-- Mockito cannot mock this class: class java.net.http.HttpClient. the bytebuddy helps. -->
 		<dependency>
-            <groupId>net.bytebuddy</groupId>
-            <artifactId>byte-buddy</artifactId>
-            <version>${byte-buddy.version}</version>
-            <scope>test</scope>
-        </dependency>
+			<groupId>net.bytebuddy</groupId>
+			<artifactId>byte-buddy</artifactId>
+			<version>${byte-buddy.version}</version>
+			<scope>test</scope>
+		</dependency>
 		<dependency>
 			<groupId>io.projectreactor</groupId>
 			<artifactId>reactor-test</artifactId>
@@ -202,6 +202,14 @@
 			<scope>test</scope>
 		</dependency>
 
+		<dependency>
+			<groupId>org.testcontainers</groupId>
+			<artifactId>toxiproxy</artifactId>
+			<version>${toxiproxy.version}</version>
+			<scope>test</scope>
+		</dependency>
+
+
 	</dependencies>