Fix error behavior on unregistered handlers in stateless server handler

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

@@ -32,7 +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(new McpError("Missing handler for request type: " + 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)));
 		}
 		return requestHandler.handle(transportContext, request.params())
 			.map(result -> new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(), result, null))

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

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2024-2025 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-spring/mcp-spring-webmvc/src/test/java/io/modelcontextprotocol/server/WebMvcStatelessIntegrationTests.java

@@ -3,32 +3,39 @@
  */
 package io.modelcontextprotocol.server;
 
+import static net.javacrumbs.jsonunit.assertj.JsonAssertions.assertThatJson;
 import static org.assertj.core.api.Assertions.assertThat;
 
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
 import java.time.Duration;
 import java.util.stream.Stream;
 
+import io.modelcontextprotocol.AbstractStatelessIntegrationTests;
+import io.modelcontextprotocol.client.McpClient;
+import io.modelcontextprotocol.client.transport.HttpClientStreamableHttpTransport;
+import io.modelcontextprotocol.client.transport.WebClientStreamableHttpTransport;
+import io.modelcontextprotocol.server.McpServer.StatelessAsyncSpecification;
+import io.modelcontextprotocol.server.McpServer.StatelessSyncSpecification;
+import io.modelcontextprotocol.server.transport.WebMvcStatelessServerTransport;
+import io.modelcontextprotocol.spec.McpSchema;
 import org.apache.catalina.LifecycleException;
 import org.apache.catalina.LifecycleState;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Timeout;
+import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 
+import org.junit.jupiter.params.provider.ValueSource;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.web.reactive.function.client.WebClient;
 import org.springframework.web.servlet.config.annotation.EnableWebMvc;
 import org.springframework.web.servlet.function.RouterFunction;
 import org.springframework.web.servlet.function.ServerResponse;
-
-import io.modelcontextprotocol.AbstractStatelessIntegrationTests;
-import io.modelcontextprotocol.client.McpClient;
-import io.modelcontextprotocol.client.transport.HttpClientStreamableHttpTransport;
-import io.modelcontextprotocol.client.transport.WebClientStreamableHttpTransport;
-import io.modelcontextprotocol.server.McpServer.StatelessAsyncSpecification;
-import io.modelcontextprotocol.server.McpServer.StatelessSyncSpecification;
-import io.modelcontextprotocol.server.transport.WebMvcStatelessServerTransport;
 import reactor.core.scheduler.Schedulers;
 
 @Timeout(15)
@@ -131,4 +138,37 @@ public void after() {
 		}
 	}
 
+	@ParameterizedTest
+	@ValueSource(strings = { "tools/list", "resources/list", "prompts/list" })
+	void testMissingHandlerReturnsMethodNotFoundError(String method) throws Exception {
+		var mcpServer = prepareSyncServerBuilder().build();
+
+		HttpResponse<String> response;
+
+		try {
+			HttpRequest request = HttpRequest.newBuilder()
+				.uri(URI.create("http://localhost:" + PORT + MESSAGE_ENDPOINT))
+				.header("Content-Type", "application/json")
+				.header("Accept", "application/json, text/event-stream")
+				.POST(HttpRequest.BodyPublishers.ofString("""
+						{
+							"jsonrpc": "2.0",
+							"method": "%s",
+							"id": "test-request-123",
+							"params": {}
+						}
+						""".formatted(method)))
+				.build();
+
+			response = HttpClient.newHttpClient().send(request, HttpResponse.BodyHandlers.ofString());
+		}
+		finally {
+			mcpServer.closeGracefully();
+		}
+
+		final var responseBody = response.body();
+		assertThatJson(responseBody).inPath("error.code").isEqualTo(McpSchema.ErrorCodes.METHOD_NOT_FOUND);
+		assertThatJson(responseBody).inPath("error.message").isEqualTo("Method not found: " + method);
+	}
+
 }