ci(evals): only run evals in CI if prompts or tools changed (#20898)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

scripts/changed_prompt.js

@@ -0,0 +1,49 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+import { execSync } from 'node:child_process';
+
+const EVALS_FILE_PREFIXES = [
+  'packages/core/src/prompts/',
+  'packages/core/src/tools/',
+  'evals/',
+];
+
+function main() {
+  const targetBranch = process.env.GITHUB_BASE_REF || 'main';
+  try {
+    // Fetch target branch from origin.
+    execSync(`git fetch origin ${targetBranch}`, {
+      stdio: 'ignore',
+    });
+
+    // Find the merge base with the target branch.
+    const mergeBase = execSync('git merge-base HEAD FETCH_HEAD', {
+      encoding: 'utf-8',
+    }).trim();
+
+    // Get changed files
+    const changedFiles = execSync(`git diff --name-only ${mergeBase} HEAD`, {
+      encoding: 'utf-8',
+    })
+      .split('\n')
+      .filter(Boolean);
+
+    const shouldRun = changedFiles.some((file) =>
+      EVALS_FILE_PREFIXES.some((prefix) => file.startsWith(prefix)),
+    );
+
+    console.log(shouldRun ? 'true' : 'false');
+  } catch (error) {
+    // If anything fails (e.g., no git history), run evals to be safe
+    console.warn(
+      'Warning: Failed to determine if evals should run. Defaulting to true.',
+    );
+    console.error(error);
+    console.log('true');
+  }
+}
+
+main();