chore: strengthen validation guidance in system prompt (#18544)

evals/validation_fidelity_pre_existing_errors.eval.ts

@@ -0,0 +1,79 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, expect } from 'vitest';
+import { evalTest } from './test-helper.js';
+
+describe('validation_fidelity_pre_existing_errors', () => {
+  evalTest('ALWAYS_PASSES', {
+    name: 'should handle pre-existing project errors gracefully during validation',
+    files: {
+      'src/math.ts': `
+export function add(a: number, b: number): number {
+  return a + b;
+}
+`,
+      'src/index.ts': `
+import { add } from './math.js';
+console.log(add(1, 2));
+`,
+      'src/utils.ts': `
+export function multiply(a: number, b: number): number {
+  return a * c; // 'c' is not defined - PRE-EXISTING ERROR
+}
+`,
+      'package.json': JSON.stringify({
+        name: 'test-project',
+        type: 'module',
+        scripts: {
+          test: 'vitest run',
+          build: 'tsc --noEmit',
+        },
+      }),
+      'tsconfig.json': JSON.stringify({
+        compilerOptions: {
+          target: 'ESNext',
+          module: 'ESNext',
+          moduleResolution: 'node',
+          strict: true,
+          esModuleInterop: true,
+          skipLibCheck: true,
+          forceConsistentCasingInFileNames: true,
+        },
+      }),
+    },
+    prompt: "In src/math.ts, rename the 'add' function to 'sum'.",
+    timeout: 600000,
+    assert: async (rig) => {
+      const toolLogs = rig.readToolLogs();
+      const replaceCalls = toolLogs.filter(
+        (log) => log.toolRequest.name === 'replace',
+      );
+
+      // Verify it did the work in math.ts
+      const mathRefactor = replaceCalls.some((log) => {
+        const args = JSON.parse(log.toolRequest.args);
+        return (
+          args.file_path.endsWith('src/math.ts') &&
+          args.new_string.includes('sum')
+        );
+      });
+      expect(mathRefactor, 'Agent should have refactored math.ts').toBe(true);
+
+      const shellCalls = toolLogs.filter(
+        (log) => log.toolRequest.name === 'run_shell_command',
+      );
+      const ranValidation = shellCalls.some((log) => {
+        const cmd = JSON.parse(log.toolRequest.args).command.toLowerCase();
+        return cmd.includes('build') || cmd.includes('tsc');
+      });
+
+      expect(ranValidation, 'Agent should have attempted validation').toBe(
+        true,
+      );
+    },
+  });
+});