Fix error behavior on unregistered handlers in stateless server handler

mcp-core/src/main/java/io/modelcontextprotocol/server/DefaultMcpStatelessServerHandler.java

@@ -32,9 +32,9 @@ public Mono<McpSchema.JSONRPCResponse> handleRequest(McpTransportContext transpo
 			McpSchema.JSONRPCRequest request) {
 		McpStatelessRequestHandler<?> requestHandler = this.requestHandlers.get(request.method());
 		if (requestHandler == null) {
-			return Mono.error(McpError.builder(McpSchema.ErrorCodes.METHOD_NOT_FOUND)
-				.message("Missing handler for request type: " + request.method())
-				.build());
+			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)));
 		}
 		return requestHandler.handle(transportContext, request.params())
 			.map(result -> McpSchema.JSONRPCResponse.result(request.id(), result))

mcp-core/src/test/java/io/modelcontextprotocol/server/DefaultMcpStatelessServerHandlerTests.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2026-2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.server;
+
+import io.modelcontextprotocol.common.McpTransportContext;
+import io.modelcontextprotocol.spec.McpSchema;
+import org.junit.jupiter.api.Test;
+import reactor.test.StepVerifier;
+
+import java.util.Collections;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class DefaultMcpStatelessServerHandlerTests {
+
+	@Test
+	void testHandleRequestWithUnregisteredMethod() {
+		// no request/initialization handlers
+		DefaultMcpStatelessServerHandler handler = new DefaultMcpStatelessServerHandler(Collections.emptyMap(),
+				Collections.emptyMap());
+
+		// unregistered method
+		McpSchema.JSONRPCRequest request = new McpSchema.JSONRPCRequest(McpSchema.JSONRPC_VERSION, "resources/list",
+				"test-id-123", null);
+
+		StepVerifier.create(handler.handleRequest(McpTransportContext.EMPTY, request)).assertNext(response -> {
+			assertThat(response).isNotNull();
+			assertThat(response.jsonrpc()).isEqualTo(McpSchema.JSONRPC_VERSION);
+			assertThat(response.id()).isEqualTo("test-id-123");
+			assertThat(response.result()).isNull();
+
+			assertThat(response.error()).isNotNull();
+			assertThat(response.error().code()).isEqualTo(McpSchema.ErrorCodes.METHOD_NOT_FOUND);
+			assertThat(response.error().message()).isEqualTo("Method not found: resources/list");
+		}).verifyComplete();
+	}
+
+}

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

@@ -9,6 +9,7 @@
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.BiFunction;
+import java.util.function.Function;
 
 import io.modelcontextprotocol.client.McpClient;
 import io.modelcontextprotocol.client.transport.HttpClientStreamableHttpTransport;
@@ -45,6 +46,8 @@
 import org.springframework.mock.web.MockHttpServletRequest;
 import org.springframework.mock.web.MockHttpServletResponse;
 import org.springframework.web.client.RestClient;
+import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
 
 import static io.modelcontextprotocol.server.transport.HttpServletStatelessServerTransport.APPLICATION_JSON;
 import static io.modelcontextprotocol.server.transport.HttpServletStatelessServerTransport.TEXT_EVENT_STREAM;
@@ -448,7 +451,7 @@ void testStructuredOutputOfObjectArrayValidationSuccess() {
 				"type", "object",
 				"properties", Map.of(
 					"name", Map.of("type", "string"),
-					"age", Map.of("type", "number")),					
+					"age", Map.of("type", "number")),
 				"required", List.of("name", "age"))); // @formatter:on
 
 		Tool calculatorTool = Tool.builder("getMembers")
@@ -765,6 +768,48 @@ void testThrownMcpErrorAndJsonRpcError() throws Exception {
 		mcpServer.close();
 	}
 
+	@Test
+	void testMissingHandlerReturnsMethodNotFoundError() {
+		var mcpServer = McpServer.sync(mcpStatelessServerTransport)
+			.serverInfo("test-server", "1.0.0")
+			.capabilities(ServerCapabilities.builder().build())
+			.build();
+		var clientTransport = HttpClientStreamableHttpTransport.builder("http://localhost:" + PORT)
+			.endpoint(CUSTOM_MESSAGE_ENDPOINT)
+			.build();
+
+		try (var mcpClient = McpClient.sync(clientTransport).build()) {
+			// Create a session using an MCP client
+			McpSchema.InitializeResult initResult = mcpClient.initialize();
+			assertThat(initResult).isNotNull();
+
+			// Override the response handler in the client to capture responses
+			AtomicReference<McpSchema.JSONRPCResponse> response = new AtomicReference<>();
+			var handler = (Function<Mono<McpSchema.JSONRPCMessage>, Mono<McpSchema.JSONRPCMessage>>) (
+					message) -> message.doOnNext(r -> {
+						if (r instanceof McpSchema.JSONRPCResponse resp) {
+							response.set(resp);
+						}
+					});
+			StepVerifier.create(clientTransport.connect(handler)).verifyComplete();
+
+			// Send a request for a non-existent method through the transport, bypassing
+			// the client's capability checks
+			StepVerifier
+				.create(clientTransport.sendMessage(new McpSchema.JSONRPCRequest("foo/bar", "test-request-123")))
+				.verifyComplete();
+
+			// Wait until we've received the response
+			await().atMost(Duration.ofSeconds(1)).until(() -> response.get() != null);
+
+			assertThat(response.get().error().code()).isEqualTo(McpSchema.ErrorCodes.METHOD_NOT_FOUND);
+			assertThat(response.get().error().message()).isEqualTo("Method not found: foo/bar");
+		}
+		finally {
+			mcpServer.closeGracefully();
+		}
+	}
+
 	private double evaluateExpression(String expression) {
 		// Simple expression evaluator for testing
 		return switch (expression) {