feat(core): implement towards policy-driven model fallback mechanism (#13781)

packages/core/src/availability/policyHelpers.test.ts

@@ -0,0 +1,59 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect } from 'vitest';
+import {
+  resolvePolicyChain,
+  buildFallbackPolicyContext,
+} from './policyHelpers.js';
+import { createDefaultPolicy } from './policyCatalog.js';
+import type { Config } from '../config/config.js';
+
+describe('policyHelpers', () => {
+  describe('resolvePolicyChain', () => {
+    it('inserts the active model when missing from the catalog', () => {
+      const config = {
+        getPreviewFeatures: () => false,
+        getUserTier: () => undefined,
+        getModel: () => 'custom-model',
+        isInFallbackMode: () => false,
+      } as unknown as Config;
+      const chain = resolvePolicyChain(config);
+      expect(chain[0]?.model).toBe('custom-model');
+    });
+
+    it('leaves catalog order untouched when active model already present', () => {
+      const config = {
+        getPreviewFeatures: () => false,
+        getUserTier: () => undefined,
+        getModel: () => 'gemini-2.5-pro',
+        isInFallbackMode: () => false,
+      } as unknown as Config;
+      const chain = resolvePolicyChain(config);
+      expect(chain[0]?.model).toBe('gemini-2.5-pro');
+    });
+  });
+
+  describe('buildFallbackPolicyContext', () => {
+    it('returns remaining candidates after the failed model', () => {
+      const chain = [
+        createDefaultPolicy('a'),
+        createDefaultPolicy('b'),
+        createDefaultPolicy('c'),
+      ];
+      const context = buildFallbackPolicyContext(chain, 'b');
+      expect(context.failedPolicy?.model).toBe('b');
+      expect(context.candidates.map((p) => p.model)).toEqual(['c']);
+    });
+
+    it('returns full chain when model is not in policy list', () => {
+      const chain = [createDefaultPolicy('a'), createDefaultPolicy('b')];
+      const context = buildFallbackPolicyContext(chain, 'x');
+      expect(context.failedPolicy).toBeUndefined();
+      expect(context.candidates).toEqual(chain);
+    });
+  });
+});