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

packages/core/src/availability/policyHelpers.ts

@@ -0,0 +1,66 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type { Config } from '../config/config.js';
+import type {
+  FailureKind,
+  FallbackAction,
+  ModelPolicy,
+  ModelPolicyChain,
+} from './modelPolicy.js';
+import { createDefaultPolicy, getModelPolicyChain } from './policyCatalog.js';
+import { getEffectiveModel } from '../config/models.js';
+
+/**
+ * Resolves the active policy chain for the given config, ensuring the
+ * user-selected active model is represented.
+ */
+export function resolvePolicyChain(config: Config): ModelPolicyChain {
+  const chain = getModelPolicyChain({
+    previewEnabled: !!config.getPreviewFeatures(),
+    userTier: config.getUserTier(),
+  });
+  // TODO: This will be replaced when we get rid of Fallback Modes
+  const activeModel = getEffectiveModel(
+    config.isInFallbackMode(),
+    config.getModel(),
+    config.getPreviewFeatures(),
+  );
+
+  if (chain.some((policy) => policy.model === activeModel)) {
+    return chain;
+  }
+
+  return [createDefaultPolicy(activeModel), ...chain];
+}
+
+/**
+ * Produces the failed policy (if it exists in the chain) and the list of
+ * fallback candidates that follow it.
+ */
+export function buildFallbackPolicyContext(
+  chain: ModelPolicyChain,
+  failedModel: string,
+): {
+  failedPolicy?: ModelPolicy;
+  candidates: ModelPolicy[];
+} {
+  const index = chain.findIndex((policy) => policy.model === failedModel);
+  if (index === -1) {
+    return { failedPolicy: undefined, candidates: chain };
+  }
+  return {
+    failedPolicy: chain[index],
+    candidates: chain.slice(index + 1),
+  };
+}
+
+export function resolvePolicyAction(
+  failureKind: FailureKind,
+  policy: ModelPolicy,
+): FallbackAction {
+  return policy.actions?.[failureKind] ?? 'prompt';
+}