refactor(client): improve validation and remove server methods

mcp-test/src/main/java/io/modelcontextprotocol/MockMcpTransport.java

@@ -4,7 +4,9 @@
 
 package io.modelcontextprotocol;
 
-import java.util.concurrent.atomic.AtomicInteger;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.BiConsumer;
 import java.util.function.Function;
 
 import com.fasterxml.jackson.core.type.TypeReference;
@@ -14,47 +16,53 @@
 import io.modelcontextprotocol.spec.ServerMcpTransport;
 import io.modelcontextprotocol.spec.McpSchema.JSONRPCNotification;
 import io.modelcontextprotocol.spec.McpSchema.JSONRPCRequest;
-import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import reactor.core.publisher.Sinks;
-import reactor.core.scheduler.Schedulers;
 
-@SuppressWarnings("unused")
+/**
+ * A mock implementation of the {@link ClientMcpTransport} and {@link ServerMcpTransport}
+ * interfaces.
+ */
 public class MockMcpTransport implements ClientMcpTransport, ServerMcpTransport {
 
-	private final AtomicInteger inboundMessageCount = new AtomicInteger(0);
+	private final Sinks.Many<McpSchema.JSONRPCMessage> inbound = Sinks.many().unicast().onBackpressureBuffer();
+
+	private final List<McpSchema.JSONRPCMessage> sent = new ArrayList<>();
 
-	private final Sinks.Many<McpSchema.JSONRPCMessage> outgoing = Sinks.many().multicast().onBackpressureBuffer();
+	private final BiConsumer<MockMcpTransport, McpSchema.JSONRPCMessage> interceptor;
 
-	private final Sinks.Many<McpSchema.JSONRPCMessage> inbound = Sinks.many().unicast().onBackpressureBuffer();
+	public MockMcpTransport() {
+		this((t, msg) -> {
+		});
+	}
 
-	private final Flux<McpSchema.JSONRPCMessage> outboundView = outgoing.asFlux().cache(1);
+	public MockMcpTransport(BiConsumer<MockMcpTransport, McpSchema.JSONRPCMessage> interceptor) {
+		this.interceptor = interceptor;
+	}
 
 	public void simulateIncomingMessage(McpSchema.JSONRPCMessage message) {
 		if (inbound.tryEmitNext(message).isFailure()) {
-			throw new RuntimeException("Failed to emit message " + message);
+			throw new RuntimeException("Failed to process incoming message " + message);
 		}
-		inboundMessageCount.incrementAndGet();
 	}
 
 	@Override
 	public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {
-		if (outgoing.tryEmitNext(message).isFailure()) {
-			return Mono.error(new RuntimeException("Can't emit outgoing message " + message));
-		}
+		sent.add(message);
+		interceptor.accept(this, message);
 		return Mono.empty();
 	}
 
 	public McpSchema.JSONRPCRequest getLastSentMessageAsRequest() {
-		return (JSONRPCRequest) outboundView.blockFirst();
+		return (JSONRPCRequest) getLastSentMessage();
 	}
 
-	public McpSchema.JSONRPCNotification getLastSentMessageAsNotifiation() {
-		return (JSONRPCNotification) outboundView.blockFirst();
+	public McpSchema.JSONRPCNotification getLastSentMessageAsNotification() {
+		return (JSONRPCNotification) getLastSentMessage();
 	}
 
 	public McpSchema.JSONRPCMessage getLastSentMessage() {
-		return outboundView.blockFirst();
+		return !sent.isEmpty() ? sent.get(sent.size() - 1) : null;
 	}
 
 	private volatile boolean connected = false;
@@ -66,7 +74,6 @@ public Mono<Void> connect(Function<Mono<McpSchema.JSONRPCMessage>, Mono<McpSchem
 		}
 		connected = true;
 		return inbound.asFlux()
-			.publishOn(Schedulers.boundedElastic())
 			.flatMap(message -> Mono.just(message).transform(handler))
 			.doFinally(signal -> connected = false)
 			.then();
@@ -76,8 +83,8 @@ public Mono<Void> connect(Function<Mono<McpSchema.JSONRPCMessage>, Mono<McpSchem
 	public Mono<Void> closeGracefully() {
 		return Mono.defer(() -> {
 			connected = false;
-			outgoing.tryEmitComplete();
 			inbound.tryEmitComplete();
+			// Wait for all subscribers to complete
 			return Mono.empty();
 		});
 	}
@@ -87,4 +94,4 @@ public <T> T unmarshalFrom(Object data, TypeReference<T> typeRef) {
 		return new ObjectMapper().convertValue(data, typeRef);
 	}
 
-}
+}

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

@@ -274,9 +274,6 @@ void testNotificationHandlers() {
 
 		assertThatCode(() -> {
 			client.initialize().block();
-			// Trigger notifications
-			client.sendResourcesListChanged().block();
-			client.promptListChangedNotification().block();
 			client.closeGracefully().block();
 		}).doesNotThrowAnyException();
 	}

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

@@ -258,9 +258,6 @@ void testNotificationHandlers() {
 
 		assertThatCode(() -> {
 			client.initialize();
-			// Trigger notifications
-			client.sendResourcesListChanged();
-			client.promptListChangedNotification();
 			client.close();
 		}).doesNotThrowAnyException();
 	}

mcp/src/main/java/io/modelcontextprotocol/client/McpAsyncClient.java

@@ -296,6 +296,14 @@ public McpSchema.Implementation getServerInfo() {
 		return this.serverInfo;
 	}
 
+	/**
+	 * Check if the client-server connection is initialized.
+	 * @return true if the client-server connection is initialized
+	 */
+	public boolean isInitialized() {
+		return this.serverCapabilities != null;
+	}
+
 	/**
 	 * Get the client capabilities that define the supported features and functionality.
 	 * @return The client capabilities
@@ -456,6 +464,12 @@ private RequestHandler<CreateMessageResult> samplingCreateMessageHandler() {
 	 * (false/absent)
 	 */
 	public Mono<McpSchema.CallToolResult> callTool(McpSchema.CallToolRequest callToolRequest) {
+		if (!this.isInitialized()) {
+			return Mono.error(new McpError("Client must be initialized before calling tools"));
+		}
+		if (this.serverCapabilities.tools() == null) {
+			return Mono.error(new McpError("Server does not provide tools capability"));
+		}
 		return this.mcpSession.sendRequest(McpSchema.METHOD_TOOLS_CALL, callToolRequest, CALL_TOOL_RESULT_TYPE_REF);
 	}
 
@@ -477,6 +491,12 @@ public Mono<McpSchema.ListToolsResult> listTools() {
 	 * Optional cursor for pagination if more tools are available
 	 */
 	public Mono<McpSchema.ListToolsResult> listTools(String cursor) {
+		if (!this.isInitialized()) {
+			return Mono.error(new McpError("Client must be initialized before listing tools"));
+		}
+		if (this.serverCapabilities.tools() == null) {
+			return Mono.error(new McpError("Server does not provide tools capability"));
+		}
 		return this.mcpSession.sendRequest(McpSchema.METHOD_TOOLS_LIST, new McpSchema.PaginatedRequest(cursor),
 				LIST_TOOLS_RESULT_TYPE_REF);
 	}
@@ -532,6 +552,12 @@ public Mono<McpSchema.ListResourcesResult> listResources() {
 	 * @return A Mono that completes with the list of resources result
 	 */
 	public Mono<McpSchema.ListResourcesResult> listResources(String cursor) {
+		if (!this.isInitialized()) {
+			return Mono.error(new McpError("Client must be initialized before listing resources"));
+		}
+		if (this.serverCapabilities.resources() == null) {
+			return Mono.error(new McpError("Server does not provide the resources capability"));
+		}
 		return this.mcpSession.sendRequest(McpSchema.METHOD_RESOURCES_LIST, new McpSchema.PaginatedRequest(cursor),
 				LIST_RESOURCES_RESULT_TYPE_REF);
 	}
@@ -551,6 +577,12 @@ public Mono<McpSchema.ReadResourceResult> readResource(McpSchema.Resource resour
 	 * @return A Mono that completes with the resource content
 	 */
 	public Mono<McpSchema.ReadResourceResult> readResource(McpSchema.ReadResourceRequest readResourceRequest) {
+		if (!this.isInitialized()) {
+			return Mono.error(new McpError("Client must be initialized before reading resources"));
+		}
+		if (this.serverCapabilities.resources() == null) {
+			return Mono.error(new McpError("Server does not provide the resources capability"));
+		}
 		return this.mcpSession.sendRequest(McpSchema.METHOD_RESOURCES_READ, readResourceRequest,
 				READ_RESOURCE_RESULT_TYPE_REF);
 	}
@@ -575,19 +607,16 @@ public Mono<McpSchema.ListResourceTemplatesResult> listResourceTemplates() {
 	 * @return A Mono that completes with the list of resource templates result
 	 */
 	public Mono<McpSchema.ListResourceTemplatesResult> listResourceTemplates(String cursor) {
+		if (!this.isInitialized()) {
+			return Mono.error(new McpError("Client must be initialized before listing resource templates"));
+		}
+		if (this.serverCapabilities.resources() == null) {
+			return Mono.error(new McpError("Server does not provide the resources capability"));
+		}
 		return this.mcpSession.sendRequest(McpSchema.METHOD_RESOURCES_TEMPLATES_LIST,
 				new McpSchema.PaginatedRequest(cursor), LIST_RESOURCE_TEMPLATES_RESULT_TYPE_REF);
 	}
 
-	/**
-	 * List Changed Notification. When the list of available resources changes, servers
-	 * that declared the listChanged capability SHOULD send a notification.
-	 * @return A Mono that completes when the notification is sent
-	 */
-	public Mono<Void> sendResourcesListChanged() {
-		return this.mcpSession.sendNotification(McpSchema.METHOD_NOTIFICATION_RESOURCES_LIST_CHANGED);
-	}
-
 	/**
 	 * Subscriptions. The protocol supports optional subscriptions to resource changes.
 	 * Clients can subscribe to specific resources and receive notifications when they
@@ -660,16 +689,6 @@ public Mono<GetPromptResult> getPrompt(GetPromptRequest getPromptRequest) {
 		return this.mcpSession.sendRequest(McpSchema.METHOD_PROMPT_GET, getPromptRequest, GET_PROMPT_RESULT_TYPE_REF);
 	}
 
-	/**
-	 * (Server) An optional notification from the server to the client, informing it that
-	 * the list of prompts it offers has changed. This may be issued by servers without
-	 * any previous subscription from the client.
-	 * @return A Mono that completes when the notification is sent
-	 */
-	public Mono<Void> promptListChangedNotification() {
-		return this.mcpSession.sendNotification(McpSchema.METHOD_NOTIFICATION_PROMPTS_LIST_CHANGED);
-	}
-
 	private NotificationHandler asyncPromptsChangeNotificationHandler(
 			List<Function<List<McpSchema.Prompt>, Mono<Void>>> promptsChangeConsumers) {
 		return params -> listPrompts().flatMap(listPromptsResult -> Flux.fromIterable(promptsChangeConsumers)

mcp/src/main/java/io/modelcontextprotocol/client/McpSyncClient.java

@@ -284,14 +284,6 @@ public McpSchema.ListResourceTemplatesResult listResourceTemplates() {
 		return this.delegate.listResourceTemplates().block();
 	}
 
-	/**
-	 * List Changed Notification. When the list of available resources changes, servers
-	 * that declared the listChanged capability SHOULD send a notification:
-	 */
-	public void sendResourcesListChanged() {
-		this.delegate.sendResourcesListChanged().block();
-	}
-
 	/**
 	 * Subscriptions. The protocol supports optional subscriptions to resource changes.
 	 * Clients can subscribe to specific resources and receive notifications when they
@@ -329,15 +321,6 @@ public GetPromptResult getPrompt(GetPromptRequest getPromptRequest) {
 		return this.delegate.getPrompt(getPromptRequest).block();
 	}
 
-	/**
-	 * (Server) An optional notification from the server to the client, informing it that
-	 * the list of prompts it offers has changed. This may be issued by servers without
-	 * any previous subscription from the client.
-	 */
-	public void promptListChangedNotification() {
-		this.delegate.promptListChangedNotification().block();
-	}
-
 	/**
 	 * Client can set the minimum logging level it wants to receive from the server.
 	 * @param loggingLevel the min logging level

mcp/src/main/java/io/modelcontextprotocol/server/McpAsyncServer.java

@@ -188,9 +188,13 @@ private DefaultMcpSession.RequestHandler<McpSchema.InitializeResult> asyncInitia
 					initializeRequest.protocolVersion(), initializeRequest.capabilities(),
 					initializeRequest.clientInfo());
 
+			// The server MUST respond with the highest protocol version it supports if
+			// it does not support the requested (e.g. Client) version.
 			String serverProtocolVersion = this.protocolVersions.get(this.protocolVersions.size() - 1);
 
 			if (this.protocolVersions.contains(initializeRequest.protocolVersion())) {
+				// If the server supports the requested protocol version, it MUST respond
+				// with the same version.
 				serverProtocolVersion = initializeRequest.protocolVersion();
 			}
 			else {

mcp/src/test/java/io/modelcontextprotocol/MockMcpTransport.java

@@ -4,7 +4,9 @@
 
 package io.modelcontextprotocol;
 
-import java.util.concurrent.atomic.AtomicInteger;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.BiConsumer;
 import java.util.function.Function;
 
 import com.fasterxml.jackson.core.type.TypeReference;
@@ -14,50 +16,53 @@
 import io.modelcontextprotocol.spec.ServerMcpTransport;
 import io.modelcontextprotocol.spec.McpSchema.JSONRPCNotification;
 import io.modelcontextprotocol.spec.McpSchema.JSONRPCRequest;
-import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import reactor.core.publisher.Sinks;
-import reactor.core.scheduler.Schedulers;
 
 /**
  * A mock implementation of the {@link ClientMcpTransport} and {@link ServerMcpTransport}
  * interfaces.
  */
 public class MockMcpTransport implements ClientMcpTransport, ServerMcpTransport {
 
-	private final AtomicInteger inboundMessageCount = new AtomicInteger(0);
+	private final Sinks.Many<McpSchema.JSONRPCMessage> inbound = Sinks.many().unicast().onBackpressureBuffer();
 
-	private final Sinks.Many<McpSchema.JSONRPCMessage> outgoing = Sinks.many().multicast().onBackpressureBuffer();
+	private final List<McpSchema.JSONRPCMessage> sent = new ArrayList<>();
 
-	private final Sinks.Many<McpSchema.JSONRPCMessage> inbound = Sinks.many().unicast().onBackpressureBuffer();
+	private final BiConsumer<MockMcpTransport, McpSchema.JSONRPCMessage> interceptor;
 
-	private final Flux<McpSchema.JSONRPCMessage> outboundView = outgoing.asFlux().cache(1);
+	public MockMcpTransport() {
+		this((t, msg) -> {
+		});
+	}
+
+	public MockMcpTransport(BiConsumer<MockMcpTransport, McpSchema.JSONRPCMessage> interceptor) {
+		this.interceptor = interceptor;
+	}
 
 	public void simulateIncomingMessage(McpSchema.JSONRPCMessage message) {
 		if (inbound.tryEmitNext(message).isFailure()) {
-			throw new RuntimeException("Failed to emit message " + message);
+			throw new RuntimeException("Failed to process incoming message " + message);
 		}
-		inboundMessageCount.incrementAndGet();
 	}
 
 	@Override
 	public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {
-		if (outgoing.tryEmitNext(message).isFailure()) {
-			return Mono.error(new RuntimeException("Can't emit outgoing message " + message));
-		}
+		sent.add(message);
+		interceptor.accept(this, message);
 		return Mono.empty();
 	}
 
 	public McpSchema.JSONRPCRequest getLastSentMessageAsRequest() {
-		return (JSONRPCRequest) outboundView.blockFirst();
+		return (JSONRPCRequest) getLastSentMessage();
 	}
 
-	public McpSchema.JSONRPCNotification getLastSentMessageAsNotifiation() {
-		return (JSONRPCNotification) outboundView.blockFirst();
+	public McpSchema.JSONRPCNotification getLastSentMessageAsNotification() {
+		return (JSONRPCNotification) getLastSentMessage();
 	}
 
 	public McpSchema.JSONRPCMessage getLastSentMessage() {
-		return outboundView.blockFirst();
+		return !sent.isEmpty() ? sent.get(sent.size() - 1) : null;
 	}
 
 	private volatile boolean connected = false;
@@ -69,7 +74,6 @@ public Mono<Void> connect(Function<Mono<McpSchema.JSONRPCMessage>, Mono<McpSchem
 		}
 		connected = true;
 		return inbound.asFlux()
-			.publishOn(Schedulers.boundedElastic())
 			.flatMap(message -> Mono.just(message).transform(handler))
 			.doFinally(signal -> connected = false)
 			.then();
@@ -79,7 +83,6 @@ public Mono<Void> connect(Function<Mono<McpSchema.JSONRPCMessage>, Mono<McpSchem
 	public Mono<Void> closeGracefully() {
 		return Mono.defer(() -> {
 			connected = false;
-			outgoing.tryEmitComplete();
 			inbound.tryEmitComplete();
 			// Wait for all subscribers to complete
 			return Mono.empty();

mcp/src/test/java/io/modelcontextprotocol/client/AbstractMcpAsyncClientTests.java

@@ -275,9 +275,6 @@ void testNotificationHandlers() {
 
 		assertThatCode(() -> {
 			client.initialize().block();
-			// Trigger notifications
-			client.sendResourcesListChanged().block();
-			client.promptListChangedNotification().block();
 			client.closeGracefully().block();
 		}).doesNotThrowAnyException();
 	}

mcp/src/test/java/io/modelcontextprotocol/client/AbstractMcpSyncClientTests.java

@@ -259,9 +259,6 @@ void testNotificationHandlers() {
 
 		assertThatCode(() -> {
 			client.initialize();
-			// Trigger notifications
-			client.sendResourcesListChanged();
-			client.promptListChangedNotification();
 			client.close();
 		}).doesNotThrowAnyException();
 	}