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

@@ -4,6 +4,10 @@
 
 package io.modelcontextprotocol.server;
 
+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.List;
 import java.util.Map;
@@ -308,7 +312,7 @@ void testStructuredOutputOfObjectArrayValidationSuccess(String clientType) {
 				"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()
@@ -639,6 +643,42 @@ void testThrownMcpErrorAndJsonRpcError() throws Exception {
 		mcpServer.close();
 	}
 
+	@ParameterizedTest
+	@ValueSource(strings = { "tools/list", "resources/list", "prompts/list" })
+	void testMissingHandlerReturnsMethodNotFoundError(String method) throws Exception {
+		var mcpServer = McpServer.sync(mcpStatelessServerTransport)
+			.serverInfo("test-server", "1.0.0")
+			.capabilities(ServerCapabilities.builder().build())
+			.build();
+
+		HttpResponse<String> response;
+
+		try {
+			HttpRequest request = HttpRequest.newBuilder()
+				.uri(URI.create("http://localhost:" + PORT + CUSTOM_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);
+	}
+
 	private double evaluateExpression(String expression) {
 		// Simple expression evaluator for testing
 		return switch (expression) {