feat: Add flash lite utility fallback chain (#17056)

docs/cli/model-routing.md

@@ -17,6 +17,11 @@ policies.
     may prompt you to switch to a fallback model (by default always prompts
     you).
 
+    Some internal utility calls (such as prompt completion and classification)
+    use a silent fallback chain for `gemini-2.5-flash-lite` and will fall back
+    to `gemini-2.5-flash` and `gemini-2.5-pro` without prompting or changing the
+    configured model.
+
 3.  **Model switch:** If approved, or if the policy allows for silent fallback,
     the CLI will use an available fallback model for the current turn or the
     remainder of the session.

docs/core/index.md

@@ -68,6 +68,10 @@ If you are using the default "pro" model and the CLI detects that you are being
 rate-limited, it automatically switches to the "flash" model for the current
 session. This allows you to continue working without interruption.
 
+Internal utility calls that use `gemini-2.5-flash-lite` (for example, prompt
+completion and classification) silently fall back to `gemini-2.5-flash` and
+`gemini-2.5-pro` when quota is exhausted, without changing the configured model.
+
 ## File discovery service
 
 The file discovery service is responsible for finding files in the project that

packages/core/src/availability/policyCatalog.ts

@@ -11,6 +11,7 @@ import type {
   ModelPolicyStateMap,
 } from './modelPolicy.js';
 import {
+  DEFAULT_GEMINI_FLASH_LITE_MODEL,
   DEFAULT_GEMINI_FLASH_MODEL,
   DEFAULT_GEMINI_MODEL,
   PREVIEW_GEMINI_FLASH_MODEL,
@@ -36,6 +37,13 @@ const DEFAULT_ACTIONS: ModelPolicyActionMap = {
   unknown: 'prompt',
 };
 
+const SILENT_ACTIONS: ModelPolicyActionMap = {
+  terminal: 'silent',
+  transient: 'silent',
+  not_found: 'silent',
+  unknown: 'silent',
+};
+
 const DEFAULT_STATE: ModelPolicyStateMap = {
   terminal: 'terminal',
   transient: 'terminal',
@@ -53,6 +61,22 @@ const PREVIEW_CHAIN: ModelPolicyChain = [
   definePolicy({ model: PREVIEW_GEMINI_FLASH_MODEL, isLastResort: true }),
 ];
 
+const FLASH_LITE_CHAIN: ModelPolicyChain = [
+  definePolicy({
+    model: DEFAULT_GEMINI_FLASH_LITE_MODEL,
+    actions: SILENT_ACTIONS,
+  }),
+  definePolicy({
+    model: DEFAULT_GEMINI_FLASH_MODEL,
+    actions: SILENT_ACTIONS,
+  }),
+  definePolicy({
+    model: DEFAULT_GEMINI_MODEL,
+    isLastResort: true,
+    actions: SILENT_ACTIONS,
+  }),
+];
+
 /**
  * Returns the default ordered model policy chain for the user.
  */
@@ -70,6 +94,10 @@ export function createSingleModelChain(model: string): ModelPolicyChain {
   return [definePolicy({ model, isLastResort: true })];
 }
 
+export function getFlashLitePolicyChain(): ModelPolicyChain {
+  return cloneChain(FLASH_LITE_CHAIN);
+}
+
 /**
  * Provides a default policy scaffold for models not present in the catalog.
  */

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

@@ -12,7 +12,10 @@ import {
 } from './policyHelpers.js';
 import { createDefaultPolicy } from './policyCatalog.js';
 import type { Config } from '../config/config.js';
-import { DEFAULT_GEMINI_MODEL_AUTO } from '../config/models.js';
+import {
+  DEFAULT_GEMINI_FLASH_LITE_MODEL,
+  DEFAULT_GEMINI_MODEL_AUTO,
+} from '../config/models.js';
 
 const createMockConfig = (overrides: Partial<Config> = {}): Config =>
   ({
@@ -53,6 +56,26 @@ describe('policyHelpers', () => {
       expect(chain[1]?.model).toBe('gemini-2.5-flash');
     });
 
+    it('uses auto chain when preferred model is auto', () => {
+      const config = createMockConfig({
+        getModel: () => 'gemini-2.5-pro',
+      });
+      const chain = resolvePolicyChain(config, DEFAULT_GEMINI_MODEL_AUTO);
+      expect(chain).toHaveLength(2);
+      expect(chain[0]?.model).toBe('gemini-2.5-pro');
+      expect(chain[1]?.model).toBe('gemini-2.5-flash');
+    });
+
+    it('uses auto chain when configured model is auto even if preferred is concrete', () => {
+      const config = createMockConfig({
+        getModel: () => DEFAULT_GEMINI_MODEL_AUTO,
+      });
+      const chain = resolvePolicyChain(config, 'gemini-2.5-pro');
+      expect(chain).toHaveLength(2);
+      expect(chain[0]?.model).toBe('gemini-2.5-pro');
+      expect(chain[1]?.model).toBe('gemini-2.5-flash');
+    });
+
     it('starts chain from preferredModel when model is "auto"', () => {
       const config = createMockConfig({
         getModel: () => DEFAULT_GEMINI_MODEL_AUTO,
@@ -62,6 +85,28 @@ describe('policyHelpers', () => {
       expect(chain[0]?.model).toBe('gemini-2.5-flash');
     });
 
+    it('returns flash-lite chain when preferred model is flash-lite', () => {
+      const config = createMockConfig({
+        getModel: () => DEFAULT_GEMINI_MODEL_AUTO,
+      });
+      const chain = resolvePolicyChain(config, DEFAULT_GEMINI_FLASH_LITE_MODEL);
+      expect(chain).toHaveLength(3);
+      expect(chain[0]?.model).toBe('gemini-2.5-flash-lite');
+      expect(chain[1]?.model).toBe('gemini-2.5-flash');
+      expect(chain[2]?.model).toBe('gemini-2.5-pro');
+    });
+
+    it('returns flash-lite chain when configured model is flash-lite', () => {
+      const config = createMockConfig({
+        getModel: () => DEFAULT_GEMINI_FLASH_LITE_MODEL,
+      });
+      const chain = resolvePolicyChain(config);
+      expect(chain).toHaveLength(3);
+      expect(chain[0]?.model).toBe('gemini-2.5-flash-lite');
+      expect(chain[1]?.model).toBe('gemini-2.5-flash');
+      expect(chain[2]?.model).toBe('gemini-2.5-pro');
+    });
+
     it('wraps around the chain when wrapsAround is true', () => {
       const config = createMockConfig({
         getModel: () => DEFAULT_GEMINI_MODEL_AUTO,

packages/core/src/availability/policyHelpers.ts

@@ -17,11 +17,13 @@ import {
   createDefaultPolicy,
   createSingleModelChain,
   getModelPolicyChain,
+  getFlashLitePolicyChain,
 } from './policyCatalog.js';
 import {
+  DEFAULT_GEMINI_FLASH_LITE_MODEL,
   DEFAULT_GEMINI_MODEL,
-  DEFAULT_GEMINI_MODEL_AUTO,
   PREVIEW_GEMINI_MODEL_AUTO,
+  isAutoModel,
   resolveModel,
 } from '../config/models.js';
 import type { ModelSelectionResult } from './modelAvailabilityService.js';
@@ -38,24 +40,30 @@ export function resolvePolicyChain(
 ): ModelPolicyChain {
   const modelFromConfig =
     preferredModel ?? config.getActiveModel?.() ?? config.getModel();
+  const configuredModel = config.getModel();
 
   let chain;
+  const resolvedModel = resolveModel(modelFromConfig);
+  const isAutoPreferred = preferredModel ? isAutoModel(preferredModel) : false;
+  const isAutoConfigured = isAutoModel(configuredModel);
 
-  if (
-    config.getModel() === PREVIEW_GEMINI_MODEL_AUTO ||
-    config.getModel() === DEFAULT_GEMINI_MODEL_AUTO
-  ) {
+  if (resolvedModel === DEFAULT_GEMINI_FLASH_LITE_MODEL) {
+    chain = getFlashLitePolicyChain();
+  } else if (isAutoPreferred || isAutoConfigured) {
+    const previewEnabled =
+      preferredModel === PREVIEW_GEMINI_MODEL_AUTO ||
+      configuredModel === PREVIEW_GEMINI_MODEL_AUTO;
     chain = getModelPolicyChain({
-      previewEnabled: config.getModel() === PREVIEW_GEMINI_MODEL_AUTO,
+      previewEnabled,
       userTier: config.getUserTier(),
     });
   } else {
     chain = createSingleModelChain(modelFromConfig);
   }
 
-  const activeModel = resolveModel(modelFromConfig);
-
-  const activeIndex = chain.findIndex((policy) => policy.model === activeModel);
+  const activeIndex = chain.findIndex(
+    (policy) => policy.model === resolvedModel,
+  );
   if (activeIndex !== -1) {
     return wrapsAround
       ? [...chain.slice(activeIndex), ...chain.slice(0, activeIndex)]
@@ -64,7 +72,7 @@ export function resolvePolicyChain(
 
   // If the user specified a model not in the default chain, we assume they want
   // *only* that model. We do not fallback to the default chain.
-  return [createDefaultPolicy(activeModel, { isLastResort: true })];
+  return [createDefaultPolicy(resolvedModel, { isLastResort: true })];
 }
 
 /**