Fix issue where Gemini CLI creates tests in a new file

.gemini/commands/fix-behavioral-eval.toml

@@ -25,7 +25,7 @@ You are an expert at fixing behavioral evaluations.
        the same scenario. We don't want to lose test fidelity by making the prompts too
        direct (i.e.: easy).
      - Your primary mechanism for improving the agent's behavior is to make changes to
-       tool instructions, prompt.ts, and/or modules that contribute to the prompt.
+       tool instructions, system prompt (snippets.ts), and/or modules that contribute to the prompt.
      - If prompt and description changes are unsuccessful, use logs and debugging to
        confirm that everything is working as expected.
     - If unable to fix the test, you can make recommendations for architecture changes

evals/edit-locations-eval.eval.ts

@@ -0,0 +1,110 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, expect } from 'vitest';
+import { evalTest } from './test-helper.js';
+
+describe('Edits location eval', () => {
+  /**
+   * Ensure that Gemini CLI always updates existing test files, if present,
+   * instead of creating a new one.
+   */
+  evalTest('USUALLY_PASSES', {
+    name: 'should update existing test file instead of creating a new one',
+    files: {
+      'package.json': JSON.stringify(
+        {
+          name: 'test-location-repro',
+          version: '1.0.0',
+          scripts: {
+            test: 'vitest run',
+          },
+          devDependencies: {
+            vitest: '^1.0.0',
+            typescript: '^5.0.0',
+          },
+        },
+        null,
+        2,
+      ),
+      'src/math.ts': `
+export function add(a: number, b: number): number {
+  return a + b;
+}
+
+export function subtract(a: number, b: number): number {
+  return a - b;
+}
+
+export function multiply(a: number, b: number): number {
+  return a + b;
+}
+`,
+      'src/math.test.ts': `
+import { expect, test } from 'vitest';
+import { add, subtract } from './math';
+
+test('add adds two numbers', () => {
+  expect(add(2, 3)).toBe(5);
+});
+
+test('subtract subtracts two numbers', () => {
+  expect(subtract(5, 3)).toBe(2);
+});
+`,
+      'src/utils.ts': `
+export function capitalize(s: string): string {
+  return s.charAt(0).toUpperCase() + s.slice(1);
+}
+`,
+      'src/utils.test.ts': `
+import { expect, test } from 'vitest';
+import { capitalize } from './utils';
+
+test('capitalize capitalizes the first letter', () => {
+  expect(capitalize('hello')).toBe('Hello');
+});
+`,
+    },
+    prompt: 'Fix the bug in src/math.ts. Do not run the code.',
+    timeout: 180000,
+    assert: async (rig) => {
+      const toolLogs = rig.readToolLogs();
+      const replaceCalls = toolLogs.filter(
+        (t) => t.toolRequest.name === 'replace',
+      );
+      const writeFileCalls = toolLogs.filter(
+        (t) => t.toolRequest.name === 'write_file',
+      );
+
+      expect(replaceCalls.length).toBeGreaterThan(0);
+      expect(
+        writeFileCalls.some((file) =>
+          file.toolRequest.args.includes('.test.ts'),
+        ),
+      ).toBe(false);
+
+      const targetFiles = replaceCalls.map((t) => {
+        try {
+          return JSON.parse(t.toolRequest.args).file_path;
+        } catch {
+          return null;
+        }
+      });
+
+      console.log('DEBUG: targetFiles', targetFiles);
+
+      expect(
+        new Set(targetFiles).size,
+        'Expected only two files changed',
+      ).greaterThanOrEqual(2);
+      expect(targetFiles.some((f) => f?.endsWith('src/math.ts'))).toBe(true);
+      expect(targetFiles.some((f) => f?.endsWith('src/math.test.ts'))).toBe(
+        true,
+      );
+    },
+  });
+});