Release version 0.12.1

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

mcp-bom/pom.xml

@@ -7,7 +7,7 @@
     <parent>
         <groupId>io.modelcontextprotocol.sdk</groupId>
         <artifactId>mcp-parent</artifactId>
-        <version>0.12.0-SNAPSHOT</version>
+        <version>0.12.1</version>
     </parent>
 
     <artifactId>mcp-bom</artifactId>

mcp-spring/mcp-spring-webflux/pom.xml

@@ -6,7 +6,7 @@
 	<parent>
 		<groupId>io.modelcontextprotocol.sdk</groupId>
 		<artifactId>mcp-parent</artifactId>
-		<version>0.12.0-SNAPSHOT</version>
+		<version>0.12.1</version>
 		<relativePath>../../pom.xml</relativePath>
 	</parent>
 	<artifactId>mcp-spring-webflux</artifactId>
@@ -25,13 +25,13 @@
 		<dependency>
 			<groupId>io.modelcontextprotocol.sdk</groupId>
 			<artifactId>mcp</artifactId>
-			<version>0.12.0-SNAPSHOT</version>
+			<version>0.12.1</version>
 		</dependency>
 
 		<dependency>
 			<groupId>io.modelcontextprotocol.sdk</groupId>
 			<artifactId>mcp-test</artifactId>
-			<version>0.12.0-SNAPSHOT</version>
+			<version>0.12.1</version>
 			<scope>test</scope>
 		</dependency>
 

mcp-spring/mcp-spring-webmvc/pom.xml

@@ -6,7 +6,7 @@
 	<parent>
 		<groupId>io.modelcontextprotocol.sdk</groupId>
 		<artifactId>mcp-parent</artifactId>
-		<version>0.12.0-SNAPSHOT</version>
+		<version>0.12.1</version>
 		<relativePath>../../pom.xml</relativePath>
 	</parent>
 	<artifactId>mcp-spring-webmvc</artifactId>
@@ -25,7 +25,7 @@
 		<dependency>
 			<groupId>io.modelcontextprotocol.sdk</groupId>
 			<artifactId>mcp</artifactId>
-			<version>0.12.0-SNAPSHOT</version>
+			<version>0.12.1</version>
 		</dependency>
 
 		<dependency>
@@ -37,14 +37,14 @@
 		<dependency>
 			<groupId>io.modelcontextprotocol.sdk</groupId>
 			<artifactId>mcp-test</artifactId>
-			<version>0.12.0-SNAPSHOT</version>
+			<version>0.12.1</version>
 			<scope>test</scope>
 		</dependency>
 
 		<dependency>
 			<groupId>io.modelcontextprotocol.sdk</groupId>
 			<artifactId>mcp-spring-webflux</artifactId>
-			<version>0.12.0-SNAPSHOT</version>
+			<version>0.12.1</version>
 			<scope>test</scope>
 		</dependency>
 

mcp-test/pom.xml

@@ -6,7 +6,7 @@
 	<parent>
 		<groupId>io.modelcontextprotocol.sdk</groupId>
 		<artifactId>mcp-parent</artifactId>
-		<version>0.12.0-SNAPSHOT</version>
+		<version>0.12.1</version>
 	</parent>
 	<artifactId>mcp-test</artifactId>
 	<packaging>jar</packaging>
@@ -24,7 +24,7 @@
 		<dependency>
 			<groupId>io.modelcontextprotocol.sdk</groupId>
 			<artifactId>mcp</artifactId>
-			<version>0.12.0-SNAPSHOT</version>
+			<version>0.12.1</version>
 		</dependency>
 
 		<dependency>

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.BiFunction;
 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.PromptReference;
 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 @@ public abstract class AbstractMcpClientServerIntegrationTests {
 		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 exchange.createMessage(mock(McpSchema.CreateMessageRequest.class))
-				return Mono.just(mock(CallToolResult.class));
+					.then(Mono.just(mock(CallToolResult.class)));
 			})
 			.build();
 
@@ -1434,6 +1434,66 @@ public abstract class AbstractMcpClientServerIntegrationTests {
 
 	@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.Map;
 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;
 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 @@ public abstract class AbstractStatelessIntegrationTests {
 		}
 	}
 
+	@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/pom.xml

@@ -6,7 +6,7 @@
 	<parent>
 		<groupId>io.modelcontextprotocol.sdk</groupId>
 		<artifactId>mcp-parent</artifactId>
-		<version>0.12.0-SNAPSHOT</version>
+		<version>0.12.1</version>
 	</parent>
 	<artifactId>mcp</artifactId>
 	<packaging>jar</packaging>

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

@@ -29,10 +29,12 @@ import io.modelcontextprotocol.spec.McpClientSession;
 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 class McpAsyncServer {
 
 			return this.delegateCallToolResult.apply(exchange, request).map(result -> {
 
+				if (Boolean.TRUE.equals(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 @@ public class McpAsyncServer {
 				.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)))
+			return toolSpecification.get().callHandler().apply(exchange, callToolRequest);
-				.orElse(Mono.error(new McpError("Tool not found: " + callToolRequest.name())));
 		};
 	}
 

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

@@ -11,7 +11,9 @@ import io.modelcontextprotocol.spec.JsonSchemaValidator;
 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 class McpStatelessAsyncServer {
 
 			return this.delegateHandler.apply(transportContext, request).map(result -> {
 
+				if (Boolean.TRUE.equals(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 @@ public class McpStatelessAsyncServer {
 				.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))
+			return toolSpecification.get().callHandler().apply(ctx, callToolRequest);
-				.orElse(Mono.error(new McpError("Tool not found: " + callToolRequest.name())));
 		};
 	}
 

mcp/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java

@@ -35,6 +35,7 @@ import java.util.function.Function;
  *
  * @author Christian Tzolov
  * @author Dariusz Jędrzejczyk
+ * @author Yanming Zhou
  */
 public class McpClientSession implements McpSession {
 
@@ -146,13 +147,20 @@ public class McpClientSession implements McpSession {
 
 	private void handle(McpSchema.JSONRPCMessage message) {
 		if (message instanceof McpSchema.JSONRPCResponse response) {
-			logger.debug("Received Response: {}", response);
+			logger.debug("Received response: {}", response);
-			var sink = pendingResponses.remove(response.id());
+			if (response.id() != null) {
-			if (sink == null) {
+				var sink = pendingResponses.remove(response.id());
-				logger.warn("Unexpected response for unknown id {}", response.id());
+				if (sink == null) {
+					logger.warn("Unexpected response for unknown id {}", response.id());
+				}
+				else {
+					sink.success(response);
+				}
 			}
 			else {
-				sink.success(response);
+				logger.error("Discarded MCP request response without session id. "
+						+ "This is an indication of a bug in the request sender code that can lead to memory "
+						+ "leaks as pending requests will never be completed.");
 			}
 		}
 		else if (message instanceof McpSchema.JSONRPCRequest request) {

mcp/src/main/java/io/modelcontextprotocol/spec/McpServerSession.java

@@ -204,13 +204,20 @@ public class McpServerSession implements McpLoggableSession {
 			// TODO handle errors for communication to without initialization happening
 			// first
 			if (message instanceof McpSchema.JSONRPCResponse response) {
-				logger.debug("Received Response: {}", response);
+				logger.debug("Received response: {}", response);
-				var sink = pendingResponses.remove(response.id());
+				if (response.id() != null) {
-				if (sink == null) {
+					var sink = pendingResponses.remove(response.id());
-					logger.warn("Unexpected response for unknown id {}", response.id());
+					if (sink == null) {
+						logger.warn("Unexpected response for unknown id {}", response.id());
+					}
+					else {
+						sink.success(response);
+					}
 				}
 				else {
-					sink.success(response);
+					logger.error("Discarded MCP request response without session id. "
+							+ "This is an indication of a bug in the request sender code that can lead to memory "
+							+ "leaks as pending requests will never be completed.");
 				}
 				return Mono.empty();
 			}

mcp/src/main/java/io/modelcontextprotocol/spec/McpStreamableServerSession.java

@@ -21,6 +21,7 @@ import io.modelcontextprotocol.common.McpTransportContext;
 import io.modelcontextprotocol.server.McpAsyncServerExchange;
 import io.modelcontextprotocol.server.McpNotificationHandler;
 import io.modelcontextprotocol.server.McpRequestHandler;
+import io.modelcontextprotocol.spec.McpSchema.ErrorCodes;
 import io.modelcontextprotocol.util.Assert;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -33,6 +34,7 @@ import reactor.core.publisher.MonoSink;
  * capability without the insight into the transport-specific details of HTTP handling.
  *
  * @author Dariusz Jędrzejczyk
+ * @author Yanming Zhou
  */
 public class McpStreamableServerSession implements McpLoggableSession {
 
@@ -214,19 +216,30 @@ public class McpStreamableServerSession implements McpLoggableSession {
 	 */
 	public Mono<Void> accept(McpSchema.JSONRPCResponse response) {
 		return Mono.defer(() -> {
-			var stream = this.requestIdToStream.get(response.id());
+			logger.debug("Received response: {}", response);
-			if (stream == null) {
+
-				return Mono.error(new McpError("Unexpected response for unknown id " + response.id())); // TODO
+			if (response.id() != null) {
-																										// JSONize
+				var stream = this.requestIdToStream.get(response.id());
-			}
+				if (stream == null) {
-			// TODO: encapsulate this inside the stream itself
+					return Mono.error(McpError.builder(ErrorCodes.INTERNAL_ERROR)
-			var sink = stream.pendingResponses.remove(response.id());
+						.message("Unexpected response for unknown id " + response.id())
-			if (sink == null) {
+						.build());
-				return Mono.error(new McpError("Unexpected response for unknown id " + response.id())); // TODO
+				}
-																										// JSONize
+				// TODO: encapsulate this inside the stream itself
+				var sink = stream.pendingResponses.remove(response.id());
+				if (sink == null) {
+					return Mono.error(McpError.builder(ErrorCodes.INTERNAL_ERROR)
+						.message("Unexpected response for unknown id " + response.id())
+						.build());
+				}
+				else {
+					sink.success(response);
+				}
 			}
 			else {
-				sink.success(response);
+				logger.error("Discarded MCP request response without session id. "
+						+ "This is an indication of a bug in the request sender code that can lead to memory "
+						+ "leaks as pending requests will never be completed.");
 			}
 			return Mono.empty();
 		});

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

@@ -4,12 +4,12 @@
 
 package io.modelcontextprotocol.util;
 
-import reactor.util.annotation.Nullable;
-
 import java.net.URI;
 import java.util.Collection;
 import java.util.Map;
 
+import reactor.util.annotation.Nullable;
+
 /**
  * Miscellaneous utility methods.
  *

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

@@ -14,10 +14,17 @@ import org.junit.jupiter.api.Timeout;
 /**
  * Tests for the {@link McpAsyncClient} with {@link StdioClientTransport}.
  *
+ * <p>
+ * These tests use npx to download and run the MCP "everything" server locally. The first
+ * test execution will download the everything server scripts and cache them locally,
+ * which can take more than 15 seconds. Subsequent test runs will use the cached version
+ * and execute faster.
+ *
  * @author Christian Tzolov
  * @author Dariusz Jędrzejczyk
  */
-@Timeout(15) // Giving extra time beyond the client timeout
+@Timeout(25) // Giving extra time beyond the client timeout to account for initial server
+				// download
 class StdioMcpAsyncClientTests extends AbstractMcpAsyncClientTests {
 
 	@Override
@@ -40,4 +47,9 @@ class StdioMcpAsyncClientTests extends AbstractMcpAsyncClientTests {
 		return Duration.ofSeconds(20);
 	}
 
+	@Override
+	protected Duration getRequestTimeout() {
+		return Duration.ofSeconds(25);
+	}
+
 }

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

@@ -22,10 +22,17 @@ import static org.assertj.core.api.Assertions.assertThat;
 /**
  * Tests for the {@link McpSyncClient} with {@link StdioClientTransport}.
  *
+ * <p>
+ * These tests use npx to download and run the MCP "everything" server locally. The first
+ * test execution will download the everything server scripts and cache them locally,
+ * which can take more than 15 seconds. Subsequent test runs will use the cached version
+ * and execute faster.
+ *
  * @author Christian Tzolov
  * @author Dariusz Jędrzejczyk
  */
-@Timeout(15) // Giving extra time beyond the client timeout
+@Timeout(25) // Giving extra time beyond the client timeout to account for initial server
+				// download
 class StdioMcpSyncClientTests extends AbstractMcpSyncClientTests {
 
 	@Override
@@ -71,4 +78,9 @@ class StdioMcpSyncClientTests extends AbstractMcpSyncClientTests {
 		return Duration.ofSeconds(10);
 	}
 
+	@Override
+	protected Duration getRequestTimeout() {
+		return Duration.ofSeconds(25);
+	}
+
 }

mcp/src/test/java/io/modelcontextprotocol/server/AbstractMcpClientServerIntegrationTests.java

@@ -4,15 +4,6 @@
 
 package io.modelcontextprotocol.server;
 
-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 io.modelcontextprotocol.common.McpTransportContext;
 import java.net.URI;
 import java.net.http.HttpClient;
 import java.net.http.HttpRequest;
@@ -30,10 +21,8 @@ import java.util.function.BiFunction;
 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.spec.McpError;
 import io.modelcontextprotocol.spec.McpSchema;
 import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
@@ -52,12 +41,23 @@ import io.modelcontextprotocol.spec.McpSchema.PromptReference;
 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<>();
@@ -104,8 +104,8 @@ public abstract class AbstractMcpClientServerIntegrationTests {
 		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 exchange.createMessage(mock(McpSchema.CreateMessageRequest.class))
-				return Mono.just(mock(CallToolResult.class));
+					.then(Mono.just(mock(CallToolResult.class)));
 			})
 			.build();
 
@@ -1428,6 +1428,63 @@ public abstract class AbstractMcpClientServerIntegrationTests {
 		}
 	}
 
+	@ParameterizedTest(name = "{0} : {displayName} ")
+	@ValueSource(strings = { "httpclient" })
+	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 returns an error result
+		McpServerFeatures.SyncToolSpecification tool = McpServerFeatures.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" })
 	void testStructuredOutputValidationFailure(String clientType) {

mcp/src/test/java/io/modelcontextprotocol/server/HttpServletSseIntegrationTests.java

@@ -4,12 +4,18 @@
 
 package io.modelcontextprotocol.server;
 
-import static org.assertj.core.api.Assertions.assertThat;
-
-import io.modelcontextprotocol.common.McpTransportContext;
 import java.time.Duration;
 import java.util.Map;
 
+import com.fasterxml.jackson.databind.ObjectMapper;
+import io.modelcontextprotocol.client.McpClient;
+import io.modelcontextprotocol.client.transport.HttpClientSseClientTransport;
+import io.modelcontextprotocol.common.McpTransportContext;
+import io.modelcontextprotocol.server.McpServer.AsyncSpecification;
+import io.modelcontextprotocol.server.McpServer.SyncSpecification;
+import io.modelcontextprotocol.server.transport.HttpServletSseServerTransportProvider;
+import io.modelcontextprotocol.server.transport.TomcatTestUtil;
+import jakarta.servlet.http.HttpServletRequest;
 import org.apache.catalina.LifecycleException;
 import org.apache.catalina.LifecycleState;
 import org.apache.catalina.startup.Tomcat;
@@ -17,15 +23,7 @@ import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Timeout;
 
-import com.fasterxml.jackson.databind.ObjectMapper;
+import static org.assertj.core.api.Assertions.assertThat;
-
-import io.modelcontextprotocol.client.McpClient;
-import io.modelcontextprotocol.client.transport.HttpClientSseClientTransport;
-import io.modelcontextprotocol.server.McpServer.AsyncSpecification;
-import io.modelcontextprotocol.server.McpServer.SyncSpecification;
-import io.modelcontextprotocol.server.transport.HttpServletSseServerTransportProvider;
-import io.modelcontextprotocol.server.transport.TomcatTestUtil;
-import jakarta.servlet.http.HttpServletRequest;
 
 @Timeout(15)
 class HttpServletSseIntegrationTests extends AbstractMcpClientServerIntegrationTests {

mcp/src/test/java/io/modelcontextprotocol/server/HttpServletStatelessIntegrationTests.java

@@ -4,6 +4,13 @@
 
 package io.modelcontextprotocol.server;
 
+import java.time.Duration;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.BiFunction;
+
 import com.fasterxml.jackson.databind.ObjectMapper;
 import io.modelcontextprotocol.client.McpClient;
 import io.modelcontextprotocol.client.transport.HttpClientStreamableHttpTransport;
@@ -11,16 +18,17 @@ import io.modelcontextprotocol.common.McpTransportContext;
 import io.modelcontextprotocol.server.transport.HttpServletStatelessServerTransport;
 import io.modelcontextprotocol.server.transport.TomcatTestUtil;
 import io.modelcontextprotocol.spec.HttpHeaders;
-import io.modelcontextprotocol.spec.McpError;
 import io.modelcontextprotocol.spec.McpSchema;
 import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
 import io.modelcontextprotocol.spec.McpSchema.CompleteRequest;
 import io.modelcontextprotocol.spec.McpSchema.CompleteResult;
+import io.modelcontextprotocol.spec.McpSchema.ErrorCodes;
 import io.modelcontextprotocol.spec.McpSchema.InitializeResult;
 import io.modelcontextprotocol.spec.McpSchema.Prompt;
 import io.modelcontextprotocol.spec.McpSchema.PromptArgument;
 import io.modelcontextprotocol.spec.McpSchema.PromptReference;
 import io.modelcontextprotocol.spec.McpSchema.ServerCapabilities;
+import io.modelcontextprotocol.spec.McpSchema.TextContent;
 import io.modelcontextprotocol.spec.McpSchema.Tool;
 import io.modelcontextprotocol.spec.ProtocolVersions;
 import net.javacrumbs.jsonunit.core.Option;
@@ -33,17 +41,11 @@ import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.Timeout;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.ValueSource;
+
 import org.springframework.mock.web.MockHttpServletRequest;
 import org.springframework.mock.web.MockHttpServletResponse;
 import org.springframework.web.client.RestClient;
 
-import java.time.Duration;
-import java.util.List;
-import java.util.Map;
-import java.util.concurrent.ConcurrentHashMap;
-import java.util.concurrent.atomic.AtomicReference;
-import java.util.function.BiFunction;
-
 import static io.modelcontextprotocol.server.transport.HttpServletStatelessServerTransport.APPLICATION_JSON;
 import static io.modelcontextprotocol.server.transport.HttpServletStatelessServerTransport.TEXT_EVENT_STREAM;
 import static net.javacrumbs.jsonunit.assertj.JsonAssertions.assertThatJson;
@@ -298,6 +300,65 @@ class HttpServletStatelessIntegrationTests {
 		}
 	}
 
+	@ParameterizedTest(name = "{0} : {displayName} ")
+	@ValueSource(strings = { "httpclient" })
+	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 returns an error result
+		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 = McpServer.sync(mcpStatelessServerTransport)
+			.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) {
@@ -477,7 +538,7 @@ class HttpServletStatelessIntegrationTests {
 	}
 
 	@Test
-	void testThrownMcpError() throws Exception {
+	void testThrownMcpErrorAndJsonRpcError() throws Exception {
 		var mcpServer = McpServer.sync(mcpStatelessServerTransport)
 			.serverInfo("test-server", "1.0.0")
 			.capabilities(ServerCapabilities.builder().tools(true).build())
@@ -487,7 +548,7 @@ class HttpServletStatelessIntegrationTests {
 
 		McpStatelessServerFeatures.SyncToolSpecification toolSpec = new McpStatelessServerFeatures.SyncToolSpecification(
 				testTool, (transportContext, request) -> {
-					throw new McpError(new McpSchema.JSONRPCResponse.JSONRPCError(12345, "testing", Map.of("a", "b")));
+					throw new RuntimeException("testing");
 				});
 
 		mcpServer.addTool(toolSpec);
@@ -508,13 +569,16 @@ class HttpServletStatelessIntegrationTests {
 		request.addHeader("Content-Type", APPLICATION_JSON);
 		request.addHeader("Cache-Control", "no-cache");
 		request.addHeader(HttpHeaders.PROTOCOL_VERSION, ProtocolVersions.MCP_2025_03_26);
+
 		mcpStatelessServerTransport.service(request, response);
 
 		McpSchema.JSONRPCResponse jsonrpcResponse = new ObjectMapper().readValue(response.getContentAsByteArray(),
 				McpSchema.JSONRPCResponse.class);
 
-		assertThat(jsonrpcResponse.error())
+		assertThat(jsonrpcResponse).isNotNull();
-			.isEqualTo(new McpSchema.JSONRPCResponse.JSONRPCError(12345, "testing", Map.of("a", "b")));
+		assertThat(jsonrpcResponse.error()).isNotNull();
+		assertThat(jsonrpcResponse.error().code()).isEqualTo(ErrorCodes.INTERNAL_ERROR);
+		assertThat(jsonrpcResponse.error().message()).isEqualTo("testing");
 
 		mcpServer.close();
 	}

mcp/src/test/java/io/modelcontextprotocol/server/HttpServletStreamableIntegrationTests.java

@@ -4,12 +4,18 @@
 
 package io.modelcontextprotocol.server;
 
-import static org.assertj.core.api.Assertions.assertThat;
-
-import io.modelcontextprotocol.common.McpTransportContext;
 import java.time.Duration;
 import java.util.Map;
 
+import com.fasterxml.jackson.databind.ObjectMapper;
+import io.modelcontextprotocol.client.McpClient;
+import io.modelcontextprotocol.client.transport.HttpClientStreamableHttpTransport;
+import io.modelcontextprotocol.common.McpTransportContext;
+import io.modelcontextprotocol.server.McpServer.AsyncSpecification;
+import io.modelcontextprotocol.server.McpServer.SyncSpecification;
+import io.modelcontextprotocol.server.transport.HttpServletStreamableServerTransportProvider;
+import io.modelcontextprotocol.server.transport.TomcatTestUtil;
+import jakarta.servlet.http.HttpServletRequest;
 import org.apache.catalina.LifecycleException;
 import org.apache.catalina.LifecycleState;
 import org.apache.catalina.startup.Tomcat;
@@ -17,15 +23,7 @@ import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Timeout;
 
-import com.fasterxml.jackson.databind.ObjectMapper;
+import static org.assertj.core.api.Assertions.assertThat;
-
-import io.modelcontextprotocol.client.McpClient;
-import io.modelcontextprotocol.client.transport.HttpClientStreamableHttpTransport;
-import io.modelcontextprotocol.server.McpServer.AsyncSpecification;
-import io.modelcontextprotocol.server.McpServer.SyncSpecification;
-import io.modelcontextprotocol.server.transport.HttpServletStreamableServerTransportProvider;
-import io.modelcontextprotocol.server.transport.TomcatTestUtil;
-import jakarta.servlet.http.HttpServletRequest;
 
 @Timeout(15)
 class HttpServletStreamableIntegrationTests extends AbstractMcpClientServerIntegrationTests {

pom.xml

@@ -6,7 +6,7 @@
 
 	<groupId>io.modelcontextprotocol.sdk</groupId>
 	<artifactId>mcp-parent</artifactId>
-	<version>0.12.0-SNAPSHOT</version>
+	<version>0.12.1</version>
 
 	<packaging>pom</packaging>
 	<url>https://github.com/modelcontextprotocol/java-sdk</url>