data in error response

Author: mattzcarey

src/examples/server/honoWebStandardStreamableHttp.ts

@@ -56,7 +56,7 @@ app.use(
 // Health check endpoint
 app.get('/health', c => c.json({ status: 'ok' }));
 
-// MCP endpoint - pass requests directly to the transport
+// MCP endpoint
 app.all('/mcp', c => transport.handleRequest(c.req.raw));
 
 // Start the server

src/server/webStandardStreamableHttp.ts

@@ -255,18 +255,27 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
     /**
      * Helper to create a JSON error response
      */
-    private createJsonErrorResponse(status: number, code: number, message: string, headers?: Record<string, string>): Response {
+    private createJsonErrorResponse(
+        status: number,
+        code: number,
+        message: string,
+        options?: { headers?: Record<string, string>; data?: string }
+    ): Response {
+        const error: { code: number; message: string; data?: string } = { code, message };
+        if (options?.data !== undefined) {
+            error.data = options.data;
+        }
         return new Response(
             JSON.stringify({
                 jsonrpc: '2.0',
-                error: { code, message },
+                error,
                 id: null
             }),
             {
                 status,
                 headers: {
                     'Content-Type': 'application/json',
-                    ...headers
+                    ...options?.headers
                 }
             }
         );
@@ -767,7 +776,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
         } catch (error) {
             // return JSON-RPC formatted error
             this.onerror?.(error as Error);
-            return this.createJsonErrorResponse(400, -32700, 'Parse error');
+            return this.createJsonErrorResponse(400, -32700, 'Parse error', { data: String(error) });
         }
     }
 

test/server/streamableHttp.test.ts

@@ -119,14 +119,22 @@ async function sendPostRequest(
     });
 }
 
-function expectErrorResponse(data: unknown, expectedCode: number, expectedMessagePattern: RegExp): void {
+function expectErrorResponse(
+    data: unknown,
+    expectedCode: number,
+    expectedMessagePattern: RegExp,
+    options?: { expectData?: boolean }
+): void {
     expect(data).toMatchObject({
         jsonrpc: '2.0',
         error: expect.objectContaining({
             code: expectedCode,
             message: expect.stringMatching(expectedMessagePattern)
         })
     });
+    if (options?.expectData) {
+        expect((data as { error: { data?: string } }).error.data).toBeDefined();
+    }
 }
 describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
     /**
@@ -679,6 +687,28 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
             expectErrorResponse(errorData, -32700, /Parse error/);
         });
 
+        it('should include error data in parse error response for unexpected errors', async () => {
+            sessionId = await initializeServer();
+
+            // We can't easily trigger the catch-all error handler, but we can verify
+            // that the JSON parse error includes useful information
+            const response = await fetch(baseUrl, {
+                method: 'POST',
+                headers: {
+                    'Content-Type': 'application/json',
+                    Accept: 'application/json, text/event-stream',
+                    'mcp-session-id': sessionId
+                },
+                body: '{ invalid json }'
+            });
+
+            expect(response.status).toBe(400);
+            const errorData = await response.json();
+            expectErrorResponse(errorData, -32700, /Parse error/);
+            // The error message should contain details about what went wrong
+            expect(errorData.error.message).toContain('Invalid JSON');
+        });
+
         it('should return 400 error for invalid JSON-RPC messages', async () => {
             sessionId = await initializeServer();