fix(core): handle non-compliant mcpbridge responses from Xcode 26.3 (#18376)

packages/core/src/tools/mcp-client.ts

@@ -42,6 +42,7 @@ import { AuthProviderType } from '../config/config.js';
 import { GoogleCredentialProvider } from '../mcp/google-auth-provider.js';
 import { ServiceAccountImpersonationProvider } from '../mcp/sa-impersonation-provider.js';
 import { DiscoveredMCPTool } from './mcp-tool.js';
+import { XcodeMcpBridgeFixTransport } from './xcode-mcp-fix-transport.js';
 
 import type { CallableTool, FunctionCall, Part, Tool } from '@google/genai';
 import { basename } from 'node:path';
@@ -1905,7 +1906,7 @@ export async function createTransport(
   }
 
   if (mcpServerConfig.command) {
-    const transport = new StdioClientTransport({
+    let transport: Transport = new StdioClientTransport({
       command: mcpServerConfig.command,
       args: mcpServerConfig.args || [],
       env: sanitizeEnvironment(
@@ -1928,14 +1929,38 @@ export async function createTransport(
       cwd: mcpServerConfig.cwd,
       stderr: 'pipe',
     });
+
+    // Fix for Xcode 26.3 mcpbridge non-compliant responses
+    // It returns JSON in `content` instead of `structuredContent`
+    if (
+      mcpServerConfig.command === 'xcrun' &&
+      mcpServerConfig.args?.includes('mcpbridge')
+    ) {
+      transport = new XcodeMcpBridgeFixTransport(transport);
+    }
+
     if (debugMode) {
-      transport.stderr!.on('data', (data) => {
+      // The `XcodeMcpBridgeFixTransport` wrapper hides the underlying `StdioClientTransport`,
-        const stderrStr = data.toString().trim();
+      // which exposes `stderr` for debug logging. We need to unwrap it to attach the listener.
-        debugLogger.debug(
+
-          `[DEBUG] [MCP STDERR (${mcpServerName})]: `,
+      const underlyingTransport =
-          stderrStr,
+        transport instanceof XcodeMcpBridgeFixTransport
-        );
+          ? // eslint-disable-next-line @typescript-eslint/no-explicit-any
-      });
+            (transport as any).transport
+          : transport;
+
+      if (
+        underlyingTransport instanceof StdioClientTransport &&
+        underlyingTransport.stderr
+      ) {
+        underlyingTransport.stderr.on('data', (data) => {
+          const stderrStr = data.toString().trim();
+          debugLogger.debug(
+            `[DEBUG] [MCP STDERR (${mcpServerName})]: `,
+            stderrStr,
+          );
+        });
+      }
     }
     return transport;
   }

packages/core/src/tools/xcode-mcp-fix-transport.test.ts

@@ -0,0 +1,120 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+import { describe, it, expect } from 'vitest';
+import { EventEmitter } from 'node:events';
+import { XcodeMcpBridgeFixTransport } from './xcode-mcp-fix-transport.js';
+import type { Transport } from '@modelcontextprotocol/sdk/shared/transport.js';
+import type { JSONRPCMessage } from '@modelcontextprotocol/sdk/types.js';
+
+// Mock Transport that simulates the mcpbridge behavior
+class MockBadMcpBridgeTransport extends EventEmitter implements Transport {
+  onclose?: () => void;
+  onerror?: (error: Error) => void;
+  onmessage?: (message: JSONRPCMessage) => void;
+
+  async start() {}
+  async close() {}
+  async send(_message: JSONRPCMessage) {}
+
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  emitMessage(msg: any) {
+    this.onmessage?.(msg);
+  }
+}
+
+describe('Xcode MCP Bridge Fix', () => {
+  it('intercepts and fixes the non-compliant mcpbridge response', async () => {
+    const mockTransport = new MockBadMcpBridgeTransport();
+    const fixTransport = new XcodeMcpBridgeFixTransport(mockTransport);
+
+    // We need to capture what the fixTransport emits to its listeners
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    const messages: any[] = [];
+    fixTransport.onmessage = (msg) => {
+      messages.push(msg);
+    };
+
+    await fixTransport.start();
+
+    // SCENARIO 1: Bad response from Xcode
+    // It has `content` stringified JSON, but misses `structuredContent`
+    const badPayload = {
+      jsonrpc: '2.0',
+      id: 1,
+      result: {
+        content: [
+          {
+            type: 'text',
+            text: JSON.stringify({
+              windows: [{ title: 'HelloWorld', path: '/path/to/project' }],
+            }),
+          },
+        ],
+        // Missing: structuredContent
+      },
+    };
+
+    mockTransport.emitMessage(badPayload);
+
+    // Verify the message received by the client (listener of fixTransport)
+    const fixedMsg = messages.find((m) => m.id === 1);
+    expect(fixedMsg).toBeDefined();
+    expect(fixedMsg.result.structuredContent).toBeDefined();
+    expect(fixedMsg.result.structuredContent.windows[0].title).toBe(
+      'HelloWorld',
+    );
+
+    // SCENARIO 2: Good response (should be untouched)
+    const goodPayload = {
+      jsonrpc: '2.0',
+      id: 2,
+      result: {
+        content: [{ type: 'text', text: 'normal text' }],
+        structuredContent: { some: 'data' },
+      },
+    };
+    mockTransport.emitMessage(goodPayload);
+
+    const goodMsg = messages.find((m) => m.id === 2);
+    expect(goodMsg).toBeDefined();
+    expect(goodMsg.result.structuredContent).toEqual({ some: 'data' });
+  });
+
+  it('ignores responses that cannot be parsed as JSON', async () => {
+    const mockTransport = new MockBadMcpBridgeTransport();
+    const fixTransport = new XcodeMcpBridgeFixTransport(mockTransport);
+
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    const messages: any[] = [];
+    fixTransport.onmessage = (msg) => {
+      messages.push(msg);
+    };
+
+    await fixTransport.start();
+
+    const nonJsonPayload = {
+      jsonrpc: '2.0',
+      id: 3,
+      result: {
+        content: [
+          {
+            type: 'text',
+            text: "Just some plain text that isn't JSON",
+          },
+        ],
+      },
+    };
+
+    mockTransport.emitMessage(nonJsonPayload);
+
+    const msg = messages.find((m) => m.id === 3);
+    expect(msg).toBeDefined();
+    expect(msg.result.structuredContent).toBeUndefined();
+    expect(msg.result.content[0].text).toBe(
+      "Just some plain text that isn't JSON",
+    );
+  });
+});

packages/core/src/tools/xcode-mcp-fix-transport.ts

@@ -0,0 +1,101 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type { Transport } from '@modelcontextprotocol/sdk/shared/transport.js';
+import type {
+  JSONRPCMessage,
+  JSONRPCResponse,
+} from '@modelcontextprotocol/sdk/types.js';
+import { EventEmitter } from 'node:events';
+
+/**
+ * A wrapper transport that intercepts messages from Xcode's mcpbridge and fixes
+ * non-compliant responses.
+ *
+ * Issue: Xcode 26.3's mcpbridge returns tool results in `content` but misses
+ * `structuredContent` when the tool has an output schema.
+ *
+ * Fix: Parse the text content as JSON and populate `structuredContent`.
+ */
+export class XcodeMcpBridgeFixTransport
+  extends EventEmitter
+  implements Transport
+{
+  constructor(private readonly transport: Transport) {
+    super();
+
+    // Forward messages from the underlying transport
+    this.transport.onmessage = (message) => {
+      this.handleMessage(message);
+    };
+
+    this.transport.onclose = () => {
+      this.onclose?.();
+    };
+
+    this.transport.onerror = (error) => {
+      this.onerror?.(error);
+    };
+  }
+
+  // Transport interface implementation
+  onclose?: () => void;
+  onerror?: (error: Error) => void;
+  onmessage?: (message: JSONRPCMessage) => void;
+
+  async start(): Promise<void> {
+    await this.transport.start();
+  }
+
+  async close(): Promise<void> {
+    await this.transport.close();
+  }
+
+  async send(message: JSONRPCMessage): Promise<void> {
+    await this.transport.send(message);
+  }
+
+  private handleMessage(message: JSONRPCMessage) {
+    if (this.isJsonResponse(message)) {
+      this.fixStructuredContent(message);
+    }
+    this.onmessage?.(message);
+  }
+
+  private isJsonResponse(message: JSONRPCMessage): message is JSONRPCResponse {
+    return 'result' in message || 'error' in message;
+  }
+
+  private fixStructuredContent(response: JSONRPCResponse) {
+    if (!('result' in response)) return;
+
+    // We can cast because we verified 'result' is in response,
+    // but TS might still be picky if the type is a strict union.
+    // Let's treat it safely.
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    const result = response.result as any;
+
+    // Check if we have content but missing structuredContent
+    if (
+      result.content &&
+      Array.isArray(result.content) &&
+      result.content.length > 0 &&
+      !result.structuredContent
+    ) {
+      const firstItem = result.content[0];
+      if (firstItem.type === 'text' && typeof firstItem.text === 'string') {
+        try {
+          // Attempt to parse the text as JSON
+          const parsed = JSON.parse(firstItem.text);
+          // If successful, populate structuredContent
+          result.structuredContent = parsed;
+        } catch (_) {
+          // Ignored: Content is likely plain text, not JSON.
+        }
+      }
+    }
+  }
+}