feat(core): implement silent fallback for Plan Mode model routing (#25317)

docs/cli/plan-mode.md

@@ -445,6 +445,10 @@ on the current phase of your task:
     switches to a high-speed **Flash** model. This provides a faster, more
     responsive experience during the implementation of the plan.
 
+If the high-reasoning model is unavailable or you don't have access to it,
+Gemini CLI automatically and silently falls back to a faster model to ensure
+your workflow isn't interrupted.
+
 This behavior is enabled by default to provide the best balance of quality and
 performance. You can disable this automatic switching in your settings:
 

packages/core/src/availability/policyCatalog.ts

@@ -41,7 +41,7 @@ const DEFAULT_ACTIONS: ModelPolicyActionMap = {
   unknown: 'prompt',
 };
 
-const SILENT_ACTIONS: ModelPolicyActionMap = {
+export const SILENT_ACTIONS: ModelPolicyActionMap = {
   terminal: 'silent',
   transient: 'silent',
   not_found: 'silent',

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

@@ -10,7 +10,7 @@ import {
   buildFallbackPolicyContext,
   applyModelSelection,
 } from './policyHelpers.js';
-import { createDefaultPolicy } from './policyCatalog.js';
+import { createDefaultPolicy, SILENT_ACTIONS } from './policyCatalog.js';
 import type { Config } from '../config/config.js';
 import {
   DEFAULT_GEMINI_FLASH_LITE_MODEL,
@@ -21,6 +21,7 @@ import {
 import { AuthType } from '../core/contentGenerator.js';
 import { ModelConfigService } from '../services/modelConfigService.js';
 import { DEFAULT_MODEL_CONFIGS } from '../config/defaultModelConfigs.js';
+import { ApprovalMode } from '../policy/types.js';
 
 const createMockConfig = (overrides: Partial<Config> = {}): Config => {
   const config = {
@@ -164,6 +165,18 @@ describe('policyHelpers', () => {
       expect(chain[0]?.model).toBe(PREVIEW_GEMINI_3_1_CUSTOM_TOOLS_MODEL);
       expect(chain[1]?.model).toBe('gemini-3-flash-preview');
     });
+
+    it('applies SILENT_ACTIONS when ApprovalMode is PLAN', () => {
+      const config = createMockConfig({
+        getApprovalMode: () => ApprovalMode.PLAN,
+        getModel: () => DEFAULT_GEMINI_MODEL_AUTO,
+      });
+      const chain = resolvePolicyChain(config);
+
+      expect(chain).toHaveLength(2);
+      expect(chain[0]?.actions).toEqual(SILENT_ACTIONS);
+      expect(chain[1]?.actions).toEqual(SILENT_ACTIONS);
+    });
   });
 
   describe('resolvePolicyChain behavior is identical between dynamic and legacy implementations', () => {

packages/core/src/availability/policyHelpers.ts

@@ -18,6 +18,7 @@ import {
   createSingleModelChain,
   getModelPolicyChain,
   getFlashLitePolicyChain,
+  SILENT_ACTIONS,
 } from './policyCatalog.js';
 import {
   DEFAULT_GEMINI_FLASH_LITE_MODEL,
@@ -29,6 +30,7 @@ import {
 } from '../config/models.js';
 import type { ModelSelectionResult } from './modelAvailabilityService.js';
 import type { ModelConfigKey } from '../services/modelConfigService.js';
+import { ApprovalMode } from '../policy/types.js';
 
 /**
  * Resolves the active policy chain for the given config, ensuring the
@@ -43,7 +45,7 @@ export function resolvePolicyChain(
     preferredModel ?? config.getActiveModel?.() ?? config.getModel();
   const configuredModel = config.getModel();
 
-  let chain;
+  let chain: ModelPolicyChain | undefined;
   const useGemini31 = config.getGemini31LaunchedSync?.() ?? false;
   const useGemini31FlashLite =
     config.getGemini31FlashLiteLaunchedSync?.() ?? false;
@@ -103,45 +105,55 @@ export function resolvePolicyChain(
       // No matching modelChains found, default to single model chain
       chain = createSingleModelChain(modelFromConfig);
     }
-    return applyDynamicSlicing(chain, resolvedModel, wrapsAround);
+    chain = applyDynamicSlicing(chain, resolvedModel, wrapsAround);
-  }
-
-  // --- LEGACY PATH ---
-
-  if (resolvedModel === DEFAULT_GEMINI_FLASH_LITE_MODEL) {
-    chain = getFlashLitePolicyChain();
-  } else if (
-    isGemini3Model(resolvedModel, config) ||
-    isAutoPreferred ||
-    isAutoConfigured
-  ) {
-    if (hasAccessToPreview) {
-      const previewEnabled =
-        isGemini3Model(resolvedModel, config) ||
-        preferredModel === PREVIEW_GEMINI_MODEL_AUTO ||
-        configuredModel === PREVIEW_GEMINI_MODEL_AUTO;
-      chain = getModelPolicyChain({
-        previewEnabled,
-        userTier: config.getUserTier(),
-        useGemini31,
-        useGemini31FlashLite,
-        useCustomToolModel,
-      });
-    } else {
-      // User requested Gemini 3 but has no access. Proactively downgrade
-      // to the stable Gemini 2.5 chain.
-      chain = getModelPolicyChain({
-        previewEnabled: false,
-        userTier: config.getUserTier(),
-        useGemini31,
-        useGemini31FlashLite,
-        useCustomToolModel,
-      });
-    }
   } else {
-    chain = createSingleModelChain(modelFromConfig);
+    // --- LEGACY PATH ---
+
+    if (resolvedModel === DEFAULT_GEMINI_FLASH_LITE_MODEL) {
+      chain = getFlashLitePolicyChain();
+    } else if (
+      isGemini3Model(resolvedModel, config) ||
+      isAutoPreferred ||
+      isAutoConfigured
+    ) {
+      if (hasAccessToPreview) {
+        const previewEnabled =
+          isGemini3Model(resolvedModel, config) ||
+          preferredModel === PREVIEW_GEMINI_MODEL_AUTO ||
+          configuredModel === PREVIEW_GEMINI_MODEL_AUTO;
+        chain = getModelPolicyChain({
+          previewEnabled,
+          userTier: config.getUserTier(),
+          useGemini31,
+          useGemini31FlashLite,
+          useCustomToolModel,
+        });
+      } else {
+        // User requested Gemini 3 but has no access. Proactively downgrade
+        // to the stable Gemini 2.5 chain.
+        chain = getModelPolicyChain({
+          previewEnabled: false,
+          userTier: config.getUserTier(),
+          useGemini31,
+          useGemini31FlashLite,
+          useCustomToolModel,
+        });
+      }
+    } else {
+      chain = createSingleModelChain(modelFromConfig);
+    }
+    chain = applyDynamicSlicing(chain, resolvedModel, wrapsAround);
   }
-  return applyDynamicSlicing(chain, resolvedModel, wrapsAround);
+
+  // Apply Unified Silent Injection for Plan Mode with defensive checks
+  if (config?.getApprovalMode?.() === ApprovalMode.PLAN) {
+    return chain.map((policy) => ({
+      ...policy,
+      actions: { ...SILENT_ACTIONS },
+    }));
+  }
+
+  return chain;
 }
 
 /**