feat(evals): centralize test agents into test-utils for reuse (#23616)

Co-authored-by: Samee Zahid <sameez@google.com>

packages/test-utils/src/fixtures/agents.ts

@@ -0,0 +1,72 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * Represents a test agent used in evaluations and tests.
+ */
+export interface TestAgent {
+  /** The unique name of the agent. */
+  readonly name: string;
+  /** The full YAML/Markdown definition of the agent. */
+  readonly definition: string;
+  /** The standard path where this agent should be saved in a test project. */
+  readonly path: string;
+  /** A helper to spread this agent directly into a 'files' object for evalTest. */
+  readonly asFile: () => Record<string, string>;
+}
+
+/**
+ * Helper to create a TestAgent with consistent formatting and pathing.
+ */
+function createAgent(options: {
+  name: string;
+  description: string;
+  tools: string[];
+  body: string;
+}): TestAgent {
+  const definition = `---
+name: ${options.name}
+description: ${options.description}
+tools:
+${options.tools.map((t) => `  - ${t}`).join('\n')}
+---
+${options.body}
+`;
+
+  const path = `.gemini/agents/${options.name}.md`;
+
+  return {
+    name: options.name,
+    definition,
+    path,
+    asFile: () => ({ [path]: definition }),
+  };
+}
+
+/**
+ * A collection of predefined test agents for use in evaluations and tests.
+ */
+export const TEST_AGENTS = {
+  /**
+   * An agent with expertise in updating documentation.
+   */
+  DOCS_AGENT: createAgent({
+    name: 'docs-agent',
+    description: 'An agent with expertise in updating documentation.',
+    tools: ['read_file', 'write_file'],
+    body: 'You are the docs agent. Update documentation clearly and accurately.',
+  }),
+
+  /**
+   * An agent with expertise in writing and updating tests.
+   */
+  TESTING_AGENT: createAgent({
+    name: 'testing-agent',
+    description: 'An agent with expertise in writing and updating tests.',
+    tools: ['read_file', 'write_file'],
+    body: 'You are the test agent. Add or update tests.',
+  }),
+} as const;

packages/test-utils/src/index.ts

@@ -5,6 +5,7 @@
  */
 
 export * from './file-system-test-helpers.js';
-export * from './test-rig.js';
+export * from './fixtures/agents.js';
 export * from './mock-utils.js';
 export * from './test-mcp-server.js';
+export * from './test-rig.js';