If a handler throws McpError, use its values for the RPC error

Handlers should be able to throw RPC errors and `McpError` is the
right exception for that.

Improve `DefaultMcpStatelessServerHandler` error handler to check if the
exception is `McpError` and, if so, use it to build the RPC error
result instead of re-writing as `INTERNAL_ERROR`.

Author: Randgalt

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

@@ -35,9 +35,17 @@ public Mono<McpSchema.JSONRPCResponse> handleRequest(McpTransportContext transpo
 		}
 		return requestHandler.handle(transportContext, request.params())
 			.map(result -> new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(), result, null))
-			.onErrorResume(t -> Mono.just(new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(), null,
-					new McpSchema.JSONRPCResponse.JSONRPCError(McpSchema.ErrorCodes.INTERNAL_ERROR, t.getMessage(),
-							null))));
+			.onErrorResume(t -> {
+				McpSchema.JSONRPCResponse.JSONRPCError error;
+				if (t instanceof McpError mcpError && mcpError.getJsonRpcError() != null) {
+					error = mcpError.getJsonRpcError();
+				}
+				else {
+					error = new McpSchema.JSONRPCResponse.JSONRPCError(McpSchema.ErrorCodes.INTERNAL_ERROR,
+							t.getMessage(), null);
+				}
+				return Mono.just(new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION, request.id(), null, error));
+			});
 	}
 
 	@Override

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

@@ -4,33 +4,13 @@
 
 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.awaitility.Awaitility.await;
-
-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 org.apache.catalina.LifecycleException;
-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.params.ParameterizedTest;
-import org.junit.jupiter.params.provider.ValueSource;
-import org.springframework.web.client.RestClient;
-
 import com.fasterxml.jackson.databind.ObjectMapper;
-
 import io.modelcontextprotocol.client.McpClient;
 import io.modelcontextprotocol.client.transport.HttpClientStreamableHttpTransport;
 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;
@@ -41,7 +21,33 @@
 import io.modelcontextprotocol.spec.McpSchema.PromptReference;
 import io.modelcontextprotocol.spec.McpSchema.ServerCapabilities;
 import io.modelcontextprotocol.spec.McpSchema.Tool;
+import io.modelcontextprotocol.spec.ProtocolVersions;
 import net.javacrumbs.jsonunit.core.Option;
+import org.apache.catalina.LifecycleException;
+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.Test;
+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;
+import static net.javacrumbs.jsonunit.assertj.JsonAssertions.json;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.awaitility.Awaitility.await;
 
 class HttpServletStatelessIntegrationTests {
 
@@ -460,6 +466,49 @@ void testStructuredOutputRuntimeToolAddition(String clientType) {
 		mcpServer.close();
 	}
 
+	@Test
+	void testThrownMcpError() throws Exception {
+		var mcpServer = McpServer.sync(mcpStatelessServerTransport)
+			.serverInfo("test-server", "1.0.0")
+			.capabilities(ServerCapabilities.builder().tools(true).build())
+			.build();
+
+		Tool testTool = Tool.builder().name("test").description("test").build();
+
+		McpStatelessServerFeatures.SyncToolSpecification toolSpec = new McpStatelessServerFeatures.SyncToolSpecification(
+				testTool, (transportContext, request) -> {
+					throw new McpError(new McpSchema.JSONRPCResponse.JSONRPCError(12345, "testing", Map.of("a", "b")));
+				});
+
+		mcpServer.addTool(toolSpec);
+
+		McpSchema.CallToolRequest callToolRequest = new McpSchema.CallToolRequest("test", Map.of());
+		McpSchema.JSONRPCRequest jsonrpcRequest = new McpSchema.JSONRPCRequest(McpSchema.JSONRPC_VERSION,
+				McpSchema.METHOD_TOOLS_CALL, "test", callToolRequest);
+
+		MockHttpServletRequest request = new MockHttpServletRequest("POST", CUSTOM_MESSAGE_ENDPOINT);
+		MockHttpServletResponse response = new MockHttpServletResponse();
+
+		byte[] content = new ObjectMapper().writeValueAsBytes(jsonrpcRequest);
+		request.setContent(content);
+		request.addHeader("Content-Type", "application/json");
+		request.addHeader("Content-Length", Integer.toString(content.length));
+		request.addHeader("Content-Length", Integer.toString(content.length));
+		request.addHeader("Accept", APPLICATION_JSON + ", " + TEXT_EVENT_STREAM);
+		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())
+			.isEqualTo(new McpSchema.JSONRPCResponse.JSONRPCError(12345, "testing", Map.of("a", "b")));
+
+		mcpServer.close();
+	}
+
 	private double evaluateExpression(String expression) {
 		// Simple expression evaluator for testing
 		return switch (expression) {