Enhance roots functionality and improve error handling

- Replace Utils.isEmpty check with null check for roots map
- Add specialized error messages for method not found errors
- Extend McpError to include JSONRPCError information
- Add comprehensive tests for roots functionality including:
  - Empty roots list handling
  - Missing capability scenarios
  - Multiple consumers
  - Server closure with active subscriptions

mcp/src/main/java/org/springframework/ai/mcp/client/McpAsyncClient.java

@@ -150,13 +150,13 @@ public class McpAsyncClient {
 
 		this.transport = transport;
 
-		this.roots = !Utils.isEmpty(roots) ? new ConcurrentHashMap<>(roots) : new ConcurrentHashMap<>();
+		this.roots = roots != null ? new ConcurrentHashMap<>(roots) : new ConcurrentHashMap<>();
 
 		// Request Handlers
 		Map<String, RequestHandler> requestHanlers = new HashMap<>();
 
 		// Roots List Request Handler
-		if (!Utils.isEmpty(this.roots) && this.clientCapabilities.roots() != null) {
+		if (this.roots != null && this.clientCapabilities.roots() != null) {
 			requestHanlers.put(McpSchema.METHOD_ROOTS_LIST, rootsListRequestHandler());
 		}
 

mcp/src/main/java/org/springframework/ai/mcp/spec/DefaultMcpSession.java

@@ -128,9 +128,11 @@ public class DefaultMcpSession implements McpSession {
 		this.requestHandlers.putAll(requestHandlers);
 		this.notificationHandlers.putAll(notificationHandlers);
 
-		// TODO: consider mono.transformDeferredContextual where the Context contains the
+		// TODO: consider mono.transformDeferredContextual where the Context contains
+		// the
 		// Observation associated with the individual message - it can be used to
-		// create child Observation and emit it together with the message to the consumer
+		// create child Observation and emit it together with the message to the
+		// consumer
 		this.connection = this.transport.connect(mono -> mono.doOnNext(message -> {
 			if (message instanceof McpSchema.JSONRPCResponse response) {
 				logger.info("Received Response: {}", response);
@@ -169,9 +171,10 @@ public class DefaultMcpSession implements McpSession {
 		return Mono.defer(() -> {
 			var handler = this.requestHandlers.get(request.method());
 			if (handler == null) {
+				MethodNotFoundError error = getMethodNotFoundError(request.method());
 				return Mono.just(new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(), null,
 						new McpSchema.JSONRPCResponse.JSONRPCError(McpSchema.ErrorCodes.METHOD_NOT_FOUND,
-								"Method not found: " + request.method(), null)));
+								error.message(), error.data())));
 			}
 
 			return handler.handle(request.params())
@@ -183,6 +186,19 @@ public class DefaultMcpSession implements McpSession {
 		});
 	}
 
+	record MethodNotFoundError(String method, String message, Object data) {
+	}
+
+	public static MethodNotFoundError getMethodNotFoundError(String method) {
+		switch (method) {
+			case McpSchema.METHOD_ROOTS_LIST:
+				return new MethodNotFoundError(method, "Roots not supported",
+						Map.of("reason", "Client does not have roots capability"));
+			default:
+				return new MethodNotFoundError(method, "Method not found: " + method, null);
+		}
+	}
+
 	/**
 	 * Handles an incoming JSON-RPC notification by routing it to the appropriate handler.
 	 * @param notification The incoming JSON-RPC notification

mcp/src/main/java/org/springframework/ai/mcp/spec/McpError.java

@@ -15,10 +15,23 @@
 */
 package org.springframework.ai.mcp.spec;
 
+import org.springframework.ai.mcp.spec.McpSchema.JSONRPCResponse.JSONRPCError;
+
 public class McpError extends RuntimeException {
 
+	private JSONRPCError jsonRpcError;
+
+	public McpError(JSONRPCError jsonRpcError) {
+		super(jsonRpcError.message());
+		this.jsonRpcError = jsonRpcError;
+	}
+
 	public McpError(Object error) {
 		super(error.toString());
 	}
 
+	public JSONRPCError getJsonRpcError() {
+		return jsonRpcError;
+	}
+
 }