Add option for immediate execution in McpSyncServer

- The McpSyncServer wraps an async server. By default, reactive
  operations are scheduled on a bounded-elastic scheduler, to offload
  blocking work and prevent accidental blocking of non-blocking
  operations.
- With the default behavior, there will be thead ops, even in a blocking
  context, which means thread-locals from the request thread will be
  lost. This is inconenvient for frameworks that store state in
  thread-locals.
- This commit adds the ability to avoid offloading, when the user is
  sure they are executing code in a blocking environment. Work happens
  in the calling thread, and thread-locals are available throughout the
  execution.

mcp/src/test/java/io/modelcontextprotocol/server/transport/HttpServletSseServerTransportProviderIntegrationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2024 - 2024 the original author or authors.
+ * Copyright 2024 - 2025 the original author or authors.
  */
 package io.modelcontextprotocol.server.transport;
 
@@ -37,7 +37,6 @@ import org.apache.catalina.LifecycleState;
 import org.apache.catalina.startup.Tomcat;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
-import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
@@ -46,6 +45,7 @@ import org.springframework.web.client.RestClient;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.InstanceOfAssertFactories.type;
 import static org.awaitility.Awaitility.await;
 import static org.mockito.Mockito.mock;
 
@@ -728,6 +728,9 @@ class HttpServletSseServerTransportProviderIntegrationTests {
 		var callResponse = new McpSchema.CallToolResult(List.of(new McpSchema.TextContent("CALL RESPONSE")), null);
 		McpServerFeatures.SyncToolSpecification tool1 = new McpServerFeatures.SyncToolSpecification(
 				new McpSchema.Tool("tool1", "tool1 description", emptyJsonSchema), (exchange, request) -> {
+					assertThat(McpTestServletFilter.getThreadLocalValue())
+						.as("blocking code exectuion should be offloaded")
+						.isNull();
 					// perform a blocking call to a remote service
 					String response = RestClient.create()
 						.get()
@@ -758,6 +761,37 @@ class HttpServletSseServerTransportProviderIntegrationTests {
 		mcpServer.close();
 	}
 
+	@Test
+	void testToolCallImmediateExecution() {
+		McpServerFeatures.SyncToolSpecification tool1 = new McpServerFeatures.SyncToolSpecification(
+				new McpSchema.Tool("tool1", "tool1 description", emptyJsonSchema), (exchange, request) -> {
+					var threadLocalValue = McpTestServletFilter.getThreadLocalValue();
+					return CallToolResult.builder()
+						.addTextContent(threadLocalValue != null ? threadLocalValue : "<unset>")
+						.build();
+				});
+
+		var mcpServer = McpServer.sync(mcpServerTransportProvider)
+			.capabilities(ServerCapabilities.builder().tools(true).build())
+			.tools(tool1)
+			.immediateExecution(true)
+			.build();
+
+		try (var mcpClient = clientBuilder.build()) {
+			mcpClient.initialize();
+
+			CallToolResult response = mcpClient.callTool(new McpSchema.CallToolRequest("tool1", Map.of()));
+
+			assertThat(response).isNotNull();
+			assertThat(response.content()).first()
+				.asInstanceOf(type(McpSchema.TextContent.class))
+				.extracting(McpSchema.TextContent::text)
+				.isEqualTo(McpTestServletFilter.THREAD_LOCAL_VALUE);
+		}
+
+		mcpServer.close();
+	}
+
 	@Test
 	void testToolListChangeHandlingSuccess() {
 

mcp/src/test/java/io/modelcontextprotocol/server/transport/McpTestServletFilter.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2025 - 2025 the original author or authors.
+ */
+
+package io.modelcontextprotocol.server.transport;
+
+import java.io.IOException;
+
+import jakarta.servlet.Filter;
+import jakarta.servlet.FilterChain;
+import jakarta.servlet.ServletException;
+import jakarta.servlet.ServletRequest;
+import jakarta.servlet.ServletResponse;
+
+/**
+ * Simple {@link Filter} which sets a value in a thread local. Used to verify whether MCP
+ * executions happen on the thread processing the request or are offloaded.
+ *
+ * @author Daniel Garnier-Moiroux
+ */
+public class McpTestServletFilter implements Filter {
+
+	public static final String THREAD_LOCAL_VALUE = McpTestServletFilter.class.getName();
+
+	private static final ThreadLocal<String> holder = new ThreadLocal<>();
+
+	@Override
+	public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain)
+			throws IOException, ServletException {
+		holder.set(THREAD_LOCAL_VALUE);
+		try {
+			filterChain.doFilter(servletRequest, servletResponse);
+		}
+		finally {
+			holder.remove();
+		}
+	}
+
+	public static String getThreadLocalValue() {
+		return holder.get();
+	}
+
+}

mcp/src/test/java/io/modelcontextprotocol/server/transport/TomcatTestUtil.java

@@ -10,9 +10,12 @@ import java.net.ServerSocket;
 import jakarta.servlet.Servlet;
 import org.apache.catalina.Context;
 import org.apache.catalina.startup.Tomcat;
+import org.apache.tomcat.util.descriptor.web.FilterDef;
+import org.apache.tomcat.util.descriptor.web.FilterMap;
 
 /**
  * @author Christian Tzolov
+ * @author Daniel Garnier-Moiroux
  */
 public class TomcatTestUtil {
 
@@ -39,6 +42,16 @@ public class TomcatTestUtil {
 		context.addChild(wrapper);
 		context.addServletMappingDecoded("/*", "mcpServlet");
 
+		var filterDef = new FilterDef();
+		filterDef.setFilterClass(McpTestServletFilter.class.getName());
+		filterDef.setFilterName(McpTestServletFilter.class.getSimpleName());
+		context.addFilterDef(filterDef);
+
+		var filterMap = new FilterMap();
+		filterMap.setFilterName(McpTestServletFilter.class.getSimpleName());
+		filterMap.addURLPattern("/*");
+		context.addFilterMap(filterMap);
+
 		var connector = tomcat.getConnector();
 		connector.setAsyncTimeout(3000);