fix: align tool error handling with MCP specification

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

@@ -4,14 +4,6 @@
 
 package io.modelcontextprotocol;
 
-import static net.javacrumbs.jsonunit.assertj.JsonAssertions.assertThatJson;
-import static net.javacrumbs.jsonunit.assertj.JsonAssertions.json;
-import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
-import static org.assertj.core.api.Assertions.assertWith;
-import static org.awaitility.Awaitility.await;
-import static org.mockito.Mockito.mock;
-
 import java.net.URI;
 import java.net.http.HttpClient;
 import java.net.http.HttpRequest;
@@ -29,9 +21,6 @@
 import java.util.function.Function;
 import java.util.stream.Collectors;
 
-import org.junit.jupiter.params.ParameterizedTest;
-import org.junit.jupiter.params.provider.ValueSource;
-
 import io.modelcontextprotocol.client.McpClient;
 import io.modelcontextprotocol.common.McpTransportContext;
 import io.modelcontextprotocol.server.McpServer;
@@ -56,12 +45,23 @@
 import io.modelcontextprotocol.spec.McpSchema.Role;
 import io.modelcontextprotocol.spec.McpSchema.Root;
 import io.modelcontextprotocol.spec.McpSchema.ServerCapabilities;
+import io.modelcontextprotocol.spec.McpSchema.TextContent;
 import io.modelcontextprotocol.spec.McpSchema.Tool;
 import io.modelcontextprotocol.util.Utils;
 import net.javacrumbs.jsonunit.core.Option;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
+import static net.javacrumbs.jsonunit.assertj.JsonAssertions.assertThatJson;
+import static net.javacrumbs.jsonunit.assertj.JsonAssertions.json;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.Assertions.assertWith;
+import static org.awaitility.Awaitility.await;
+import static org.mockito.Mockito.mock;
+
 public abstract class AbstractMcpClientServerIntegrationTests {
 
 	protected ConcurrentHashMap<String, McpClient.SyncSpec> clientBuilders = new ConcurrentHashMap<>();
@@ -108,8 +108,8 @@ void testCreateMessageWithoutSamplingCapabilities(String clientType) {
 		McpServerFeatures.AsyncToolSpecification tool = McpServerFeatures.AsyncToolSpecification.builder()
 			.tool(Tool.builder().name("tool1").description("tool1 description").inputSchema(emptyJsonSchema).build())
 			.callHandler((exchange, request) -> {
-				exchange.createMessage(mock(McpSchema.CreateMessageRequest.class)).block();
-				return Mono.just(mock(CallToolResult.class));
+				return exchange.createMessage(mock(McpSchema.CreateMessageRequest.class))
+					.then(Mono.just(mock(CallToolResult.class)));
 			})
 			.build();
 
@@ -1434,6 +1434,66 @@ void testStructuredOutputValidationSuccess(String clientType) {
 
 	@ParameterizedTest(name = "{0} : {displayName} ")
 	@ValueSource(strings = { "httpclient", "webflux" })
+	void testStructuredOutputWithInHandlerError(String clientType) {
+		var clientBuilder = clientBuilders.get(clientType);
+
+		// Create a tool with output schema
+		Map<String, Object> outputSchema = Map.of(
+				"type", "object", "properties", Map.of("result", Map.of("type", "number"), "operation",
+						Map.of("type", "string"), "timestamp", Map.of("type", "string")),
+				"required", List.of("result", "operation"));
+
+		Tool calculatorTool = Tool.builder()
+			.name("calculator")
+			.description("Performs mathematical calculations")
+			.outputSchema(outputSchema)
+			.build();
+
+		// Handler that throws an exception to simulate an error
+		McpServerFeatures.SyncToolSpecification tool = McpServerFeatures.SyncToolSpecification.builder()
+			.tool(calculatorTool)
+			.callHandler((exchange, request) -> {
+
+				return CallToolResult.builder()
+					.isError(true)
+					.content(List.of(new TextContent("Error calling tool: Simulated in-handler error")))
+					.build();
+			})
+			.build();
+
+		var mcpServer = prepareSyncServerBuilder().serverInfo("test-server", "1.0.0")
+			.capabilities(ServerCapabilities.builder().tools(true).build())
+			.tools(tool)
+			.build();
+
+		try (var mcpClient = clientBuilder.build()) {
+			InitializeResult initResult = mcpClient.initialize();
+			assertThat(initResult).isNotNull();
+
+			// Verify tool is listed with output schema
+			var toolsList = mcpClient.listTools();
+			assertThat(toolsList.tools()).hasSize(1);
+			assertThat(toolsList.tools().get(0).name()).isEqualTo("calculator");
+			// Note: outputSchema might be null in sync server, but validation still works
+
+			// Call tool with valid structured output
+			CallToolResult response = mcpClient
+				.callTool(new McpSchema.CallToolRequest("calculator", Map.of("expression", "2 + 3")));
+
+			assertThat(response).isNotNull();
+			assertThat(response.isError()).isTrue();
+			assertThat(response.content()).isNotEmpty();
+			assertThat(response.content())
+				.containsExactly(new McpSchema.TextContent("Error calling tool: Simulated in-handler error"));
+			assertThat(response.structuredContent()).isNull();
+		}
+		finally {
+			mcpServer.closeGracefully();
+		}
+	}
+
+	@ParameterizedTest(name = "{0} : {displayName} ")
+	@ValueSource(strings = { "httpclient" })
 	void testStructuredOutputValidationFailure(String clientType) {
 
 		var clientBuilder = clientBuilders.get(clientType);

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

@@ -4,12 +4,6 @@
 
 package io.modelcontextprotocol;
 
-import static net.javacrumbs.jsonunit.assertj.JsonAssertions.assertThatJson;
-import static net.javacrumbs.jsonunit.assertj.JsonAssertions.json;
-import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
-import static org.awaitility.Awaitility.await;
-
 import java.net.URI;
 import java.net.http.HttpClient;
 import java.net.http.HttpRequest;
@@ -20,9 +14,6 @@
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicReference;
 
-import org.junit.jupiter.params.ParameterizedTest;
-import org.junit.jupiter.params.provider.ValueSource;
-
 import io.modelcontextprotocol.client.McpClient;
 import io.modelcontextprotocol.server.McpServer.StatelessAsyncSpecification;
 import io.modelcontextprotocol.server.McpServer.StatelessSyncSpecification;
@@ -33,10 +24,19 @@
 import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
 import io.modelcontextprotocol.spec.McpSchema.InitializeResult;
 import io.modelcontextprotocol.spec.McpSchema.ServerCapabilities;
+import io.modelcontextprotocol.spec.McpSchema.TextContent;
 import io.modelcontextprotocol.spec.McpSchema.Tool;
 import net.javacrumbs.jsonunit.core.Option;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 import reactor.core.publisher.Mono;
 
+import static net.javacrumbs.jsonunit.assertj.JsonAssertions.assertThatJson;
+import static net.javacrumbs.jsonunit.assertj.JsonAssertions.json;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.awaitility.Awaitility.await;
+
 public abstract class AbstractStatelessIntegrationTests {
 
 	protected ConcurrentHashMap<String, McpClient.SyncSpec> clientBuilders = new ConcurrentHashMap<>();
@@ -350,6 +350,64 @@ void testStructuredOutputValidationSuccess(String clientType) {
 		}
 	}
 
+	@ParameterizedTest(name = "{0} : {displayName} ")
+	@ValueSource(strings = { "httpclient", "webflux" })
+	void testStructuredOutputWithInHandlerError(String clientType) {
+		var clientBuilder = clientBuilders.get(clientType);
+
+		// Create a tool with output schema
+		Map<String, Object> outputSchema = Map.of(
+				"type", "object", "properties", Map.of("result", Map.of("type", "number"), "operation",
+						Map.of("type", "string"), "timestamp", Map.of("type", "string")),
+				"required", List.of("result", "operation"));
+
+		Tool calculatorTool = Tool.builder()
+			.name("calculator")
+			.description("Performs mathematical calculations")
+			.outputSchema(outputSchema)
+			.build();
+
+		// Handler that throws an exception to simulate an error
+		McpStatelessServerFeatures.SyncToolSpecification tool = McpStatelessServerFeatures.SyncToolSpecification
+			.builder()
+			.tool(calculatorTool)
+			.callHandler((exchange, request) -> CallToolResult.builder()
+				.isError(true)
+				.content(List.of(new TextContent("Error calling tool: Simulated in-handler error")))
+				.build())
+			.build();
+
+		var mcpServer = prepareSyncServerBuilder().serverInfo("test-server", "1.0.0")
+			.capabilities(ServerCapabilities.builder().tools(true).build())
+			.tools(tool)
+			.build();
+
+		try (var mcpClient = clientBuilder.build()) {
+			InitializeResult initResult = mcpClient.initialize();
+			assertThat(initResult).isNotNull();
+
+			// Verify tool is listed with output schema
+			var toolsList = mcpClient.listTools();
+			assertThat(toolsList.tools()).hasSize(1);
+			assertThat(toolsList.tools().get(0).name()).isEqualTo("calculator");
+			// Note: outputSchema might be null in sync server, but validation still works
+
+			// Call tool with valid structured output
+			CallToolResult response = mcpClient
+				.callTool(new McpSchema.CallToolRequest("calculator", Map.of("expression", "2 + 3")));
+
+			assertThat(response).isNotNull();
+			assertThat(response.isError()).isTrue();
+			assertThat(response.content()).isNotEmpty();
+			assertThat(response.content())
+				.containsExactly(new McpSchema.TextContent("Error calling tool: Simulated in-handler error"));
+			assertThat(response.structuredContent()).isNull();
+		}
+		finally {
+			mcpServer.closeGracefully();
+		}
+	}
+
 	@ParameterizedTest(name = "{0} : {displayName} ")
 	@ValueSource(strings = { "httpclient", "webflux" })
 	void testStructuredOutputValidationFailure(String clientType) {

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

@@ -29,10 +29,12 @@
 import io.modelcontextprotocol.spec.McpError;
 import io.modelcontextprotocol.spec.McpSchema;
 import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
+import io.modelcontextprotocol.spec.McpSchema.JSONRPCResponse;
 import io.modelcontextprotocol.spec.McpSchema.LoggingLevel;
 import io.modelcontextprotocol.spec.McpSchema.LoggingMessageNotification;
 import io.modelcontextprotocol.spec.McpSchema.ResourceTemplate;
 import io.modelcontextprotocol.spec.McpSchema.SetLevelRequest;
+import io.modelcontextprotocol.spec.McpSchema.TextContent;
 import io.modelcontextprotocol.spec.McpSchema.Tool;
 import io.modelcontextprotocol.spec.McpServerSession;
 import io.modelcontextprotocol.spec.McpServerTransportProvider;
@@ -376,6 +378,11 @@ public Mono<CallToolResult> apply(McpAsyncServerExchange exchange, McpSchema.Cal
 
 			return this.delegateCallToolResult.apply(exchange, request).map(result -> {
 
+				if (result.isError() != null && result.isError()) {
+					// If the tool call resulted in an error, skip further validation
+					return result;
+				}
+
 				if (outputSchema == null) {
 					if (result.structuredContent() != null) {
 						logger.warn(
@@ -507,11 +514,11 @@ private McpRequestHandler<CallToolResult> toolsCallRequestHandler() {
 				.findAny();
 
 			if (toolSpecification.isEmpty()) {
-				return Mono.error(new McpError("Tool not found: " + callToolRequest.name()));
+				return Mono.error(new McpError(new JSONRPCResponse.JSONRPCError(McpSchema.ErrorCodes.INVALID_PARAMS,
+						"Unknown tool: invalid_tool_name", "Tool not found: " + callToolRequest.name())));
 			}
 
-			return toolSpecification.map(tool -> Mono.defer(() -> tool.callHandler().apply(exchange, callToolRequest)))
-				.orElse(Mono.error(new McpError("Tool not found: " + callToolRequest.name())));
+			return toolSpecification.get().callHandler().apply(exchange, callToolRequest);
 		};
 	}
 

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

@@ -11,7 +11,9 @@
 import io.modelcontextprotocol.spec.McpError;
 import io.modelcontextprotocol.spec.McpSchema;
 import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
+import io.modelcontextprotocol.spec.McpSchema.JSONRPCResponse;
 import io.modelcontextprotocol.spec.McpSchema.ResourceTemplate;
+import io.modelcontextprotocol.spec.McpSchema.TextContent;
 import io.modelcontextprotocol.spec.McpSchema.Tool;
 import io.modelcontextprotocol.spec.McpStatelessServerTransport;
 import io.modelcontextprotocol.util.Assert;
@@ -249,6 +251,11 @@ public Mono<CallToolResult> apply(McpTransportContext transportContext, McpSchem
 
 			return this.delegateHandler.apply(transportContext, request).map(result -> {
 
+				if (result.isError() != null && result.isError()) {
+					// If the tool call resulted in an error, skip further validation
+					return result;
+				}
+
 				if (outputSchema == null) {
 					if (result.structuredContent() != null) {
 						logger.warn(
@@ -375,11 +382,11 @@ private McpStatelessRequestHandler<CallToolResult> toolsCallRequestHandler() {
 				.findAny();
 
 			if (toolSpecification.isEmpty()) {
-				return Mono.error(new McpError("Tool not found: " + callToolRequest.name()));
+				return Mono.error(new McpError(new JSONRPCResponse.JSONRPCError(McpSchema.ErrorCodes.INVALID_PARAMS,
+						"Unknown tool: invalid_tool_name", "Tool not found: " + callToolRequest.name())));
 			}
 
-			return toolSpecification.map(tool -> tool.callHandler().apply(ctx, callToolRequest))
-				.orElse(Mono.error(new McpError("Tool not found: " + callToolRequest.name())));
+			return toolSpecification.get().callHandler().apply(ctx, callToolRequest);
 		};
 	}
 

mcp/src/main/java/io/modelcontextprotocol/util/Utils.java

@@ -9,6 +9,7 @@
 import java.net.URI;
 import java.util.Collection;
 import java.util.Map;
+import java.util.Objects;
 
 /**
  * Miscellaneous utility methods.
@@ -107,4 +108,19 @@ private static boolean isUnderBaseUri(URI baseUri, URI endpointUri) {
 		return endpointPath.startsWith(basePath);
 	}
 
+	/**
+	 * Finds the root cause of the given throwable by traversing the cause chain.
+	 * @param throwable The throwable to analyze
+	 * @return The root cause throwable
+	 * @throws NullPointerException if the provided throwable is null
+	 */
+	public static Throwable findRootCause(Throwable throwable) {
+		Objects.requireNonNull(throwable);
+		Throwable rootCause = throwable;
+		while (rootCause.getCause() != null && rootCause.getCause() != rootCause) {
+			rootCause = rootCause.getCause();
+		}
+		return rootCause;
+	}
+
 }

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

@@ -17,7 +17,7 @@
  * @author Christian Tzolov
  * @author Dariusz Jędrzejczyk
  */
-@Timeout(15) // Giving extra time beyond the client timeout
+@Timeout(25) // Giving extra time beyond the client timeout
 class StdioMcpAsyncClientTests extends AbstractMcpAsyncClientTests {
 
 	@Override
@@ -40,4 +40,9 @@ protected Duration getInitializationTimeout() {
 		return Duration.ofSeconds(20);
 	}
 
+	@Override
+	protected Duration getRequestTimeout() {
+		return Duration.ofSeconds(25);
+	}
+
 }

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

@@ -25,7 +25,7 @@
  * @author Christian Tzolov
  * @author Dariusz Jędrzejczyk
  */
-@Timeout(15) // Giving extra time beyond the client timeout
+@Timeout(25) // Giving extra time beyond the client timeout
 class StdioMcpSyncClientTests extends AbstractMcpSyncClientTests {
 
 	@Override
@@ -71,4 +71,9 @@ protected Duration getInitializationTimeout() {
 		return Duration.ofSeconds(10);
 	}
 
+	@Override
+	protected Duration getRequestTimeout() {
+		return Duration.ofSeconds(25);
+	}
+
 }