modelcontextprotocol · tzolov · Aug 4, 2025 · Aug 4, 2025
diff --git a/.../main/java/io/modelcontextprotocol/client/transport/WebClientStreamableHttpTransport.java b/.../main/java/io/modelcontextprotocol/client/transport/WebClientStreamableHttpTransport.java
@@ -34,6 +34,7 @@
 import io.modelcontextprotocol.spec.McpTransportSession;
 import io.modelcontextprotocol.spec.McpTransportSessionNotFoundException;
 import io.modelcontextprotocol.spec.McpTransportStream;
+import io.modelcontextprotocol.spec.ProtocolVersions;
 import io.modelcontextprotocol.util.Assert;
 import io.modelcontextprotocol.util.Utils;
 import reactor.core.Disposable;
@@ -71,7 +72,7 @@ public class WebClientStreamableHttpTransport implements McpClientTransport {
 
 	private static final Logger logger = LoggerFactory.getLogger(WebClientStreamableHttpTransport.class);
 
-	private static final String MCP_PROTOCOL_VERSION = "2025-03-26";
+	private static final String MCP_PROTOCOL_VERSION = ProtocolVersions.MCP_2025_03_26;
 
 	private static final String DEFAULT_ENDPOINT = "/mcp";
 
@@ -111,8 +112,8 @@ private WebClientStreamableHttpTransport(ObjectMapper objectMapper, WebClient.Bu
 	}
 
 	@Override
-	public String protocolVersion() {
-		return MCP_PROTOCOL_VERSION;
+	public List<String> protocolVersions() {
+		return List.of(ProtocolVersions.MCP_2024_11_05, ProtocolVersions.MCP_2025_03_26);
 	}
 
 	/**

diff --git a/...lux/src/main/java/io/modelcontextprotocol/client/transport/WebFluxSseClientTransport.java b/...lux/src/main/java/io/modelcontextprotocol/client/transport/WebFluxSseClientTransport.java
@@ -5,6 +5,7 @@
 package io.modelcontextprotocol.client.transport;
 
 import java.io.IOException;
+import java.util.List;
 import java.util.function.BiConsumer;
 import java.util.function.Function;
 
@@ -16,6 +17,7 @@
 import io.modelcontextprotocol.spec.McpError;
 import io.modelcontextprotocol.spec.McpSchema;
 import io.modelcontextprotocol.spec.McpSchema.JSONRPCMessage;
+import io.modelcontextprotocol.spec.ProtocolVersions;
 import io.modelcontextprotocol.util.Assert;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -65,7 +67,7 @@ public class WebFluxSseClientTransport implements McpClientTransport {
 
 	private static final Logger logger = LoggerFactory.getLogger(WebFluxSseClientTransport.class);
 
-	private static final String MCP_PROTOCOL_VERSION = "2024-11-05";
+	private static final String MCP_PROTOCOL_VERSION = ProtocolVersions.MCP_2024_11_05;
 
 	/**
 	 * Event type for JSON-RPC messages received through the SSE connection. The server
@@ -172,8 +174,8 @@ public WebFluxSseClientTransport(WebClient.Builder webClientBuilder, ObjectMappe
 	}
 
 	@Override
-	public String protocolVersion() {
-		return MCP_PROTOCOL_VERSION;
+	public List<String> protocolVersions() {
+		return List.of(MCP_PROTOCOL_VERSION);
 	}
 
 	/**

diff --git a/...main/java/io/modelcontextprotocol/server/transport/WebFluxSseServerTransportProvider.java b/...main/java/io/modelcontextprotocol/server/transport/WebFluxSseServerTransportProvider.java
@@ -6,6 +6,7 @@
 
 import java.io.IOException;
 import java.time.Duration;
+import java.util.List;
 import java.util.concurrent.ConcurrentHashMap;
 
 import com.fasterxml.jackson.core.type.TypeReference;
@@ -15,6 +16,7 @@
 import io.modelcontextprotocol.spec.McpServerSession;
 import io.modelcontextprotocol.spec.McpServerTransport;
 import io.modelcontextprotocol.spec.McpServerTransportProvider;
+import io.modelcontextprotocol.spec.ProtocolVersions;
 import io.modelcontextprotocol.util.Assert;
 import io.modelcontextprotocol.util.KeepAliveScheduler;
 
@@ -219,8 +221,8 @@ public WebFluxSseServerTransportProvider(ObjectMapper objectMapper, String baseU
 	}
 
 	@Override
-	public String protocolVersion() {
-		return "2024-11-05";
+	public List<String> protocolVersions() {
+		return List.of(ProtocolVersions.MCP_2024_11_05);
 	}
 
 	@Override

diff --git a/...va/io/modelcontextprotocol/server/transport/WebFluxStreamableServerTransportProvider.java b/...va/io/modelcontextprotocol/server/transport/WebFluxStreamableServerTransportProvider.java
@@ -14,6 +14,7 @@
 import io.modelcontextprotocol.spec.McpStreamableServerSession;
 import io.modelcontextprotocol.spec.McpStreamableServerTransport;
 import io.modelcontextprotocol.spec.McpStreamableServerTransportProvider;
+import io.modelcontextprotocol.spec.ProtocolVersions;
 import io.modelcontextprotocol.server.McpTransportContext;
 import io.modelcontextprotocol.util.Assert;
 import io.modelcontextprotocol.util.KeepAliveScheduler;
@@ -96,8 +97,8 @@ private WebFluxStreamableServerTransportProvider(ObjectMapper objectMapper, Stri
 	}
 
 	@Override
-	public String protocolVersion() {
-		return "2025-03-26";
+	public List<String> protocolVersions() {
+		return List.of(ProtocolVersions.MCP_2024_11_05, ProtocolVersions.MCP_2025_03_26);
 	}
 
 	@Override

diff --git a/.../main/java/io/modelcontextprotocol/server/transport/WebMvcSseServerTransportProvider.java b/.../main/java/io/modelcontextprotocol/server/transport/WebMvcSseServerTransportProvider.java
@@ -6,6 +6,7 @@
 
 import java.io.IOException;
 import java.time.Duration;
+import java.util.List;
 import java.util.UUID;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.locks.ReentrantLock;
@@ -16,6 +17,7 @@
 import io.modelcontextprotocol.spec.McpSchema;
 import io.modelcontextprotocol.spec.McpServerTransport;
 import io.modelcontextprotocol.spec.McpServerTransportProvider;
+import io.modelcontextprotocol.spec.ProtocolVersions;
 import io.modelcontextprotocol.spec.McpServerSession;
 import io.modelcontextprotocol.util.Assert;
 import io.modelcontextprotocol.util.KeepAliveScheduler;
@@ -210,8 +212,8 @@ public WebMvcSseServerTransportProvider(ObjectMapper objectMapper, String baseUr
 	}
 
 	@Override
-	public String protocolVersion() {
-		return "2024-11-05";
+	public List<String> protocolVersions() {
+		return List.of(ProtocolVersions.MCP_2024_11_05);
 	}
 
 	@Override

diff --git a/...ava/io/modelcontextprotocol/server/transport/WebMvcStreamableServerTransportProvider.java b/...ava/io/modelcontextprotocol/server/transport/WebMvcStreamableServerTransportProvider.java
@@ -32,6 +32,7 @@
 import io.modelcontextprotocol.spec.McpStreamableServerSession;
 import io.modelcontextprotocol.spec.McpStreamableServerTransport;
 import io.modelcontextprotocol.spec.McpStreamableServerTransportProvider;
+import io.modelcontextprotocol.spec.ProtocolVersions;
 import io.modelcontextprotocol.util.Assert;
 import io.modelcontextprotocol.util.KeepAliveScheduler;
 import reactor.core.publisher.Flux;
@@ -142,8 +143,8 @@ private WebMvcStreamableServerTransportProvider(ObjectMapper objectMapper, Strin
 	}
 
 	@Override
-	public String protocolVersion() {
-		return "2025-03-26";
+	public List<String> protocolVersions() {
+		return List.of(ProtocolVersions.MCP_2024_11_05, ProtocolVersions.MCP_2025_03_26);
 	}
 
 	@Override

diff --git a/mcp/src/main/java/io/modelcontextprotocol/client/LifecycleInitializer.java b/mcp/src/main/java/io/modelcontextprotocol/client/LifecycleInitializer.java
@@ -290,7 +290,8 @@ public <T> Mono<T> withIntitialization(String actionName, Function<Initializatio
 				.timeout(this.initializationTimeout)
 				.onErrorResume(ex -> {
 					logger.warn("Failed to initialize", ex);
-					return Mono.error(new McpError("Client failed to initialize " + actionName));
+					return Mono.error(
+							new McpError("Client failed to initialize " + actionName + " due to: " + ex.getMessage()));
 				})
 				.flatMap(operation);
 		});

diff --git a/mcp/src/main/java/io/modelcontextprotocol/client/McpAsyncClient.java b/mcp/src/main/java/io/modelcontextprotocol/client/McpAsyncClient.java
@@ -272,9 +272,9 @@ public class McpAsyncClient {
 		notificationHandlers.put(McpSchema.METHOD_NOTIFICATION_PROGRESS,
 				asyncProgressNotificationHandler(progressConsumersFinal));
 
-		this.initializer = new LifecycleInitializer(clientCapabilities, clientInfo,
-				List.of(transport.protocolVersion()), initializationTimeout, ctx -> new McpClientSession(requestTimeout,
-						transport, requestHandlers, notificationHandlers, con -> con.contextWrite(ctx)));
+		this.initializer = new LifecycleInitializer(clientCapabilities, clientInfo, transport.protocolVersions(),
+				initializationTimeout, ctx -> new McpClientSession(requestTimeout, transport, requestHandlers,
+						notificationHandlers, con -> con.contextWrite(ctx)));
 		this.transport.setExceptionHandler(this.initializer::handleException);
 	}
 

diff --git a/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java b/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
@@ -10,6 +10,7 @@
 import java.net.http.HttpRequest;
 import java.net.http.HttpResponse;
 import java.time.Duration;
+import java.util.List;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Consumer;
@@ -25,6 +26,7 @@
 import io.modelcontextprotocol.spec.McpClientTransport;
 import io.modelcontextprotocol.spec.McpError;
 import io.modelcontextprotocol.spec.McpSchema;
+import io.modelcontextprotocol.spec.ProtocolVersions;
 import io.modelcontextprotocol.spec.McpSchema.JSONRPCMessage;
 import io.modelcontextprotocol.util.Assert;
 import io.modelcontextprotocol.util.Utils;
@@ -62,7 +64,7 @@
  */
 public class HttpClientSseClientTransport implements McpClientTransport {
 
-	private static final String MCP_PROTOCOL_VERSION = "2024-11-05";
+	private static final String MCP_PROTOCOL_VERSION = ProtocolVersions.MCP_2024_11_05;
 
 	private static final String MCP_PROTOCOL_VERSION_HEADER_NAME = "MCP-Protocol-Version";
 
@@ -217,8 +219,8 @@ public HttpClientSseClientTransport(HttpClient.Builder clientBuilder, HttpReques
 	}
 
 	@Override
-	public String protocolVersion() {
-		return MCP_PROTOCOL_VERSION;
+	public List<String> protocolVersions() {
+		return List.of(ProtocolVersions.MCP_2024_11_05);
 	}
 
 	/**

diff --git a/...main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java b/...main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
@@ -35,6 +35,7 @@
 import io.modelcontextprotocol.spec.McpTransportSession;
 import io.modelcontextprotocol.spec.McpTransportSessionNotFoundException;
 import io.modelcontextprotocol.spec.McpTransportStream;
+import io.modelcontextprotocol.spec.ProtocolVersions;
 import io.modelcontextprotocol.util.Assert;
 import io.modelcontextprotocol.util.Utils;
 import reactor.core.Disposable;
@@ -73,7 +74,7 @@ public class HttpClientStreamableHttpTransport implements McpClientTransport {
 
 	private static final Logger logger = LoggerFactory.getLogger(HttpClientStreamableHttpTransport.class);
 
-	private static final String MCP_PROTOCOL_VERSION = "2025-03-26";
+	private static final String MCP_PROTOCOL_VERSION = ProtocolVersions.MCP_2025_03_26;
 
 	private static final String DEFAULT_ENDPOINT = "/mcp";
 
@@ -135,8 +136,8 @@ private HttpClientStreamableHttpTransport(ObjectMapper objectMapper, HttpClient
 	}
 
 	@Override
-	public String protocolVersion() {
-		return MCP_PROTOCOL_VERSION;
+	public List<String> protocolVersions() {
+		return List.of(ProtocolVersions.MCP_2024_11_05, ProtocolVersions.MCP_2025_03_26);
 	}
 
 	public static Builder builder(String baseUri) {

diff --git a/mcp/src/main/java/io/modelcontextprotocol/server/McpAsyncServer.java b/mcp/src/main/java/io/modelcontextprotocol/server/McpAsyncServer.java
@@ -145,7 +145,7 @@ public class McpAsyncServer {
 		Map<String, McpRequestHandler<?>> requestHandlers = prepareRequestHandlers();
 		Map<String, McpNotificationHandler> notificationHandlers = prepareNotificationHandlers(features);
 
-		this.protocolVersions = List.of(mcpTransportProvider.protocolVersion());
+		this.protocolVersions = mcpTransportProvider.protocolVersions();
 
 		mcpTransportProvider.setSessionFactory(transport -> new McpServerSession(UUID.randomUUID().toString(),
 				requestTimeout, transport, this::asyncInitializeRequestHandler, requestHandlers, notificationHandlers));
@@ -170,7 +170,7 @@ public class McpAsyncServer {
 		Map<String, McpRequestHandler<?>> requestHandlers = prepareRequestHandlers();
 		Map<String, McpNotificationHandler> notificationHandlers = prepareNotificationHandlers(features);
 
-		this.protocolVersions = List.of(mcpTransportProvider.protocolVersion());
+		this.protocolVersions = mcpTransportProvider.protocolVersions();
 
 		mcpTransportProvider.setSessionFactory(new DefaultMcpStreamableServerSessionFactory(requestTimeout,
 				this::asyncInitializeRequestHandler, requestHandlers, notificationHandlers));

diff --git a/mcp/src/main/java/io/modelcontextprotocol/server/McpStatelessAsyncServer.java b/mcp/src/main/java/io/modelcontextprotocol/server/McpStatelessAsyncServer.java
@@ -118,7 +118,7 @@ public class McpStatelessAsyncServer {
 			requestHandlers.put(McpSchema.METHOD_COMPLETION_COMPLETE, completionCompleteRequestHandler());
 		}
 
-		this.protocolVersions = List.of(mcpTransport.protocolVersion());
+		this.protocolVersions = new ArrayList<>(mcpTransport.protocolVersions());
 
 		McpStatelessServerHandler handler = new DefaultMcpStatelessServerHandler(requestHandlers, Map.of());
 		mcpTransport.setMcpHandler(handler);

diff --git a/.../java/io/modelcontextprotocol/server/transport/HttpServletSseServerTransportProvider.java b/.../java/io/modelcontextprotocol/server/transport/HttpServletSseServerTransportProvider.java
@@ -8,6 +8,7 @@
 import java.io.IOException;
 import java.io.PrintWriter;
 import java.time.Duration;
+import java.util.List;
 import java.util.Map;
 import java.util.UUID;
 import java.util.concurrent.ConcurrentHashMap;
@@ -20,6 +21,7 @@
 import io.modelcontextprotocol.spec.McpServerSession;
 import io.modelcontextprotocol.spec.McpServerTransport;
 import io.modelcontextprotocol.spec.McpServerTransportProvider;
+import io.modelcontextprotocol.spec.ProtocolVersions;
 import io.modelcontextprotocol.util.Assert;
 import io.modelcontextprotocol.util.KeepAliveScheduler;
 import jakarta.servlet.AsyncContext;
@@ -180,8 +182,8 @@ public HttpServletSseServerTransportProvider(ObjectMapper objectMapper, String b
 	}
 
 	@Override
-	public String protocolVersion() {
-		return "2024-11-05";
+	public List<String> protocolVersions() {
+		return List.of(ProtocolVersions.MCP_2024_11_05);
 	}
 
 	/**

diff --git a/...o/modelcontextprotocol/server/transport/HttpServletStreamableServerTransportProvider.java b/...o/modelcontextprotocol/server/transport/HttpServletStreamableServerTransportProvider.java
@@ -28,6 +28,7 @@
 import io.modelcontextprotocol.spec.McpStreamableServerSession;
 import io.modelcontextprotocol.spec.McpStreamableServerTransport;
 import io.modelcontextprotocol.spec.McpStreamableServerTransportProvider;
+import io.modelcontextprotocol.spec.ProtocolVersions;
 import io.modelcontextprotocol.util.Assert;
 import io.modelcontextprotocol.util.KeepAliveScheduler;
 import jakarta.servlet.AsyncContext;
@@ -155,8 +156,8 @@ private HttpServletStreamableServerTransportProvider(ObjectMapper objectMapper,
 	}
 
 	@Override
-	public String protocolVersion() {
-		return "2025-03-26";
+	public List<String> protocolVersions() {
+		return List.of(ProtocolVersions.MCP_2024_11_05, ProtocolVersions.MCP_2025_03_26);
 	}
 
 	@Override

diff --git a/mcp/src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java b/mcp/src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java
@@ -10,6 +10,7 @@
 import java.io.InputStreamReader;
 import java.io.OutputStream;
 import java.nio.charset.StandardCharsets;
+import java.util.List;
 import java.util.concurrent.Executors;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.Function;
@@ -22,6 +23,7 @@
 import io.modelcontextprotocol.spec.McpServerSession;
 import io.modelcontextprotocol.spec.McpServerTransport;
 import io.modelcontextprotocol.spec.McpServerTransportProvider;
+import io.modelcontextprotocol.spec.ProtocolVersions;
 import io.modelcontextprotocol.util.Assert;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -89,8 +91,8 @@ public StdioServerTransportProvider(ObjectMapper objectMapper, InputStream input
 	}
 
 	@Override
-	public String protocolVersion() {
-		return "2024-11-05";
+	public List<String> protocolVersions() {
+		return List.of(ProtocolVersions.MCP_2024_11_05);
 	}
 
 	@Override

diff --git a/mcp/src/main/java/io/modelcontextprotocol/spec/McpSchema.java b/mcp/src/main/java/io/modelcontextprotocol/spec/McpSchema.java
@@ -45,7 +45,7 @@ private McpSchema() {
 	}
 
 	@Deprecated
-	public static final String LATEST_PROTOCOL_VERSION = "2025-03-26";
+	public static final String LATEST_PROTOCOL_VERSION = ProtocolVersions.MCP_2025_03_26;
 
 	public static final String JSONRPC_VERSION = "2.0";
 

diff --git a/mcp/src/main/java/io/modelcontextprotocol/spec/McpServerTransportProviderBase.java b/mcp/src/main/java/io/modelcontextprotocol/spec/McpServerTransportProviderBase.java
@@ -4,6 +4,7 @@
 
 package io.modelcontextprotocol.spec;
 
+import java.util.List;
 import java.util.Map;
 
 import reactor.core.publisher.Mono;
@@ -63,8 +64,8 @@ default void close() {
 	 * Returns the protocol version supported by this transport provider.
 	 * @return the protocol version as a string
 	 */
-	default String protocolVersion() {
-		return "2024-11-05";
+	default List<String> protocolVersions() {
+		return List.of(ProtocolVersions.MCP_2024_11_05);
 	}
 
 }
diff --git a/mcp/src/main/java/io/modelcontextprotocol/spec/McpStatelessServerTransport.java b/mcp/src/main/java/io/modelcontextprotocol/spec/McpStatelessServerTransport.java
@@ -4,6 +4,8 @@
 
 package io.modelcontextprotocol.spec;
 
+import java.util.List;
+
 import io.modelcontextprotocol.server.McpStatelessServerHandler;
 import reactor.core.publisher.Mono;
 
@@ -26,8 +28,8 @@ default void close() {
 	 */
 	Mono<Void> closeGracefully();
 
-	default String protocolVersion() {
-		return "2025-03-26";
+	default List<String> protocolVersions() {
+		return List.of(ProtocolVersions.MCP_2025_03_26);
 	}
 
 }