Steer outer agent to use expert subagents when present (#16763)

evals/subagents.eval.ts

@@ -0,0 +1,64 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe } from 'vitest';
+import { evalTest } from './test-helper.js';
+
+const AGENT_DEFINITION = `---
+name: docs-agent
+description: An agent with expertise in updating documentation.
+tools:
+  - read_file
+  - write_file
+---
+
+You are the docs agent. Update the documentation.
+`;
+
+const INDEX_TS = 'export const add = (a: number, b: number) => a + b;';
+
+describe('subagent eval test cases', () => {
+  /**
+   * Checks whether the outer agent reliably utilizes an expert subagent to
+   * accomplish a task when one is available.
+   *
+   * Note that the test is intentionally crafted to avoid the word "document"
+   * or "docs". We want to see the outer agent make the connection even when
+   * the prompt indirectly implies need of expertise.
+   *
+   * This tests the system prompt's subagent specific clauses.
+   */
+  evalTest('ALWAYS_PASSES', {
+    name: 'should delegate to user provided agent with relevant expertise',
+    params: {
+      settings: {
+        experimental: {
+          enableAgents: true,
+        },
+      },
+    },
+    prompt: 'Please update README.md with a description of this library.',
+    files: {
+      '.gemini/agents/test-agent.md': AGENT_DEFINITION,
+      'index.ts': INDEX_TS,
+      'README.md': 'TODO: update the README.',
+    },
+    assert: async (rig, _result) => {
+      await rig.expectToolCallSuccess(
+        ['delegate_to_agent'],
+        undefined,
+        (args) => {
+          try {
+            const parsed = JSON.parse(args);
+            return parsed.agent_name === 'docs-agent';
+          } catch {
+            return false;
+          }
+        },
+      );
+    },
+  });
+});