diff --git a/packages/core/src/availability/policyHelpers.test.ts b/packages/core/src/availability/policyHelpers.test.ts
index 2eb6129f61..23c6ef4fd4 100644
--- a/packages/core/src/availability/policyHelpers.test.ts
+++ b/packages/core/src/availability/policyHelpers.test.ts
@@ -20,14 +20,21 @@ import {
 } from '../config/models.js';
 import { AuthType } from '../core/contentGenerator.js';
 
-const createMockConfig = (overrides: Partial<Config> = {}): Config =>
-  ({
+const createMockConfig = (overrides: Partial<Config> = {}): Config => {
+  const config = {
     getUserTier: () => undefined,
     getModel: () => 'gemini-2.5-pro',
     getGemini31LaunchedSync: () => false,
+    getUseCustomToolModelSync: () => {
+      const useGemini31 = config.getGemini31LaunchedSync();
+      const authType = config.getContentGeneratorConfig().authType;
+      return useGemini31 && authType === AuthType.USE_GEMINI;
+    },
     getContentGeneratorConfig: () => ({ authType: undefined }),
     ...overrides,
-  }) as unknown as Config;
+  } as unknown as Config;
+  return config;
+};
 
 describe('policyHelpers', () => {
   describe('resolvePolicyChain', () => {
diff --git a/packages/core/src/availability/policyHelpers.ts b/packages/core/src/availability/policyHelpers.ts
index 47c465585c..406abde5e3 100644
--- a/packages/core/src/availability/policyHelpers.ts
+++ b/packages/core/src/availability/policyHelpers.ts
@@ -6,7 +6,6 @@
 
 import type { GenerateContentConfig } from '@google/genai';
 import type { Config } from '../config/config.js';
-import { AuthType } from '../core/contentGenerator.js';
 import type {
   FailureKind,
   FallbackAction,
@@ -46,9 +45,7 @@ export function resolvePolicyChain(
 
   let chain;
   const useGemini31 = config.getGemini31LaunchedSync?.() ?? false;
-  const useCustomToolModel =
-    useGemini31 &&
-    config.getContentGeneratorConfig?.()?.authType === AuthType.USE_GEMINI;
+  const useCustomToolModel = config.getUseCustomToolModelSync?.() ?? false;
   const hasAccessToPreview = config.getHasAccessToPreviewModel?.() ?? true;
 
   const resolvedModel = resolveModel(
diff --git a/packages/core/src/config/config.ts b/packages/core/src/config/config.ts
index ba8f5d508b..86cdf584b5 100644
--- a/packages/core/src/config/config.ts
+++ b/packages/core/src/config/config.ts
@@ -2529,6 +2529,26 @@ export class Config implements McpContext, AgentLoopContext {
     return this.getGemini31LaunchedSync();
   }
 
+  /**
+   * Returns whether the custom tool model should be used.
+   */
+  async getUseCustomToolModel(): Promise<boolean> {
+    const useGemini3_1 = await this.getGemini31Launched();
+    const authType = this.contentGeneratorConfig?.authType;
+    return useGemini3_1 && authType === AuthType.USE_GEMINI;
+  }
+
+  /**
+   * Returns whether the custom tool model should be used.
+   *
+   * Note: This method should only be called after startup, once experiments have been loaded.
+   */
+  getUseCustomToolModelSync(): boolean {
+    const useGemini3_1 = this.getGemini31LaunchedSync();
+    const authType = this.contentGeneratorConfig?.authType;
+    return useGemini3_1 && authType === AuthType.USE_GEMINI;
+  }
+
   /**
    * Returns whether Gemini 3.1 has been launched.
    *
diff --git a/packages/core/src/config/models.ts b/packages/core/src/config/models.ts
index 32014d5fbd..ffbf597793 100644
--- a/packages/core/src/config/models.ts
+++ b/packages/core/src/config/models.ts
@@ -168,7 +168,8 @@ export function isPreviewModel(model: string): boolean {
     model === PREVIEW_GEMINI_3_1_MODEL ||
     model === PREVIEW_GEMINI_3_1_CUSTOM_TOOLS_MODEL ||
     model === PREVIEW_GEMINI_FLASH_MODEL ||
-    model === PREVIEW_GEMINI_MODEL_AUTO
+    model === PREVIEW_GEMINI_MODEL_AUTO ||
+    model === GEMINI_MODEL_ALIAS_AUTO
   );
 }
 
diff --git a/packages/core/src/routing/strategies/approvalModeStrategy.test.ts b/packages/core/src/routing/strategies/approvalModeStrategy.test.ts
index 4a332ec77f..123a2329a6 100644
--- a/packages/core/src/routing/strategies/approvalModeStrategy.test.ts
+++ b/packages/core/src/routing/strategies/approvalModeStrategy.test.ts
@@ -15,7 +15,9 @@ import {
   PREVIEW_GEMINI_FLASH_MODEL,
   DEFAULT_GEMINI_MODEL_AUTO,
   PREVIEW_GEMINI_MODEL_AUTO,
+  GEMINI_MODEL_ALIAS_AUTO,
 } from '../../config/models.js';
+import { AuthType } from '../../core/contentGenerator.js';
 import { ApprovalMode } from '../../policy/types.js';
 import type { BaseLlmClient } from '../../core/baseLlmClient.js';
 
@@ -40,6 +42,15 @@ describe('ApprovalModeStrategy', () => {
       getApprovalMode: vi.fn().mockReturnValue(ApprovalMode.DEFAULT),
       getApprovedPlanPath: vi.fn().mockReturnValue(undefined),
       getPlanModeRoutingEnabled: vi.fn().mockResolvedValue(true),
+      getGemini31Launched: vi.fn().mockResolvedValue(false),
+      getUseCustomToolModel: vi.fn().mockImplementation(async () => {
+        const launched = await mockConfig.getGemini31Launched();
+        const authType = mockConfig.getContentGeneratorConfig?.()?.authType;
+        return launched && authType === AuthType.USE_GEMINI;
+      }),
+      getContentGeneratorConfig: vi.fn().mockReturnValue({
+        authType: AuthType.LOGIN_WITH_GOOGLE,
+      }),
     } as unknown as Config;
 
     mockBaseLlmClient = {} as BaseLlmClient;
@@ -184,4 +195,50 @@ describe('ApprovalModeStrategy', () => {
 
     expect(decision?.model).toBe(PREVIEW_GEMINI_MODEL);
   });
+
+  it('should route to Preview models when using "auto" alias', async () => {
+    vi.mocked(mockConfig.getModel).mockReturnValue(GEMINI_MODEL_ALIAS_AUTO);
+    vi.mocked(mockConfig.getApprovalMode).mockReturnValue(ApprovalMode.PLAN);
+
+    const decision = await strategy.route(
+      mockContext,
+      mockConfig,
+      mockBaseLlmClient,
+    );
+
+    expect(decision?.model).toBe(PREVIEW_GEMINI_MODEL);
+
+    vi.mocked(mockConfig.getApprovalMode).mockReturnValue(ApprovalMode.DEFAULT);
+    vi.mocked(mockConfig.getApprovedPlanPath).mockReturnValue(
+      '/path/to/plan.md',
+    );
+
+    const implementationDecision = await strategy.route(
+      mockContext,
+      mockConfig,
+      mockBaseLlmClient,
+    );
+
+    expect(implementationDecision?.model).toBe(PREVIEW_GEMINI_FLASH_MODEL);
+  });
+
+  it('should route to Preview Flash model when an approved plan exists and Gemini 3.1 is launched', async () => {
+    vi.mocked(mockConfig.getModel).mockReturnValue(GEMINI_MODEL_ALIAS_AUTO);
+    vi.mocked(mockConfig.getGemini31Launched).mockResolvedValue(true);
+
+    // Exit plan mode with approved plan
+    vi.mocked(mockConfig.getApprovalMode).mockReturnValue(ApprovalMode.DEFAULT);
+    vi.mocked(mockConfig.getApprovedPlanPath).mockReturnValue(
+      '/path/to/plan.md',
+    );
+
+    const decision = await strategy.route(
+      mockContext,
+      mockConfig,
+      mockBaseLlmClient,
+    );
+
+    // Should resolve to Preview Flash (3.0) because resolveClassifierModel uses preview variants for Gemini 3
+    expect(decision?.model).toBe(PREVIEW_GEMINI_FLASH_MODEL);
+  });
 });
diff --git a/packages/core/src/routing/strategies/approvalModeStrategy.ts b/packages/core/src/routing/strategies/approvalModeStrategy.ts
index 63b331f5a1..403a4c3176 100644
--- a/packages/core/src/routing/strategies/approvalModeStrategy.ts
+++ b/packages/core/src/routing/strategies/approvalModeStrategy.ts
@@ -6,12 +6,10 @@
 
 import type { Config } from '../../config/config.js';
 import {
-  DEFAULT_GEMINI_MODEL,
-  DEFAULT_GEMINI_FLASH_MODEL,
-  PREVIEW_GEMINI_MODEL,
-  PREVIEW_GEMINI_FLASH_MODEL,
   isAutoModel,
-  isPreviewModel,
+  resolveClassifierModel,
+  GEMINI_MODEL_ALIAS_FLASH,
+  GEMINI_MODEL_ALIAS_PRO,
 } from '../../config/models.js';
 import type { BaseLlmClient } from '../../core/baseLlmClient.js';
 import { ApprovalMode } from '../../policy/types.js';
@@ -50,11 +48,19 @@ export class ApprovalModeStrategy implements RoutingStrategy {
     const approvalMode = config.getApprovalMode();
     const approvedPlanPath = config.getApprovedPlanPath();
 
-    const isPreview = isPreviewModel(model);
+    const [useGemini3_1, useCustomToolModel] = await Promise.all([
+      config.getGemini31Launched(),
+      config.getUseCustomToolModel(),
+    ]);
 
     // 1. Planning Phase: If ApprovalMode === PLAN, explicitly route to the Pro model.
     if (approvalMode === ApprovalMode.PLAN) {
-      const proModel = isPreview ? PREVIEW_GEMINI_MODEL : DEFAULT_GEMINI_MODEL;
+      const proModel = resolveClassifierModel(
+        model,
+        GEMINI_MODEL_ALIAS_PRO,
+        useGemini3_1,
+        useCustomToolModel,
+      );
       return {
         model: proModel,
         metadata: {
@@ -65,9 +71,12 @@ export class ApprovalModeStrategy implements RoutingStrategy {
       };
     } else if (approvedPlanPath) {
       // 2. Implementation Phase: If ApprovalMode !== PLAN AND an approved plan path is set, prefer the Flash model.
-      const flashModel = isPreview
-        ? PREVIEW_GEMINI_FLASH_MODEL
-        : DEFAULT_GEMINI_FLASH_MODEL;
+      const flashModel = resolveClassifierModel(
+        model,
+        GEMINI_MODEL_ALIAS_FLASH,
+        useGemini3_1,
+        useCustomToolModel,
+      );
       return {
         model: flashModel,
         metadata: {
diff --git a/packages/core/src/routing/strategies/classifierStrategy.test.ts b/packages/core/src/routing/strategies/classifierStrategy.test.ts
index 701e7de932..58908a7d3b 100644
--- a/packages/core/src/routing/strategies/classifierStrategy.test.ts
+++ b/packages/core/src/routing/strategies/classifierStrategy.test.ts
@@ -59,6 +59,11 @@ describe('ClassifierStrategy', () => {
       getModel: vi.fn().mockReturnValue(DEFAULT_GEMINI_MODEL_AUTO),
       getNumericalRoutingEnabled: vi.fn().mockResolvedValue(false),
       getGemini31Launched: vi.fn().mockResolvedValue(false),
+      getUseCustomToolModel: vi.fn().mockImplementation(async () => {
+        const launched = await mockConfig.getGemini31Launched();
+        const authType = mockConfig.getContentGeneratorConfig().authType;
+        return launched && authType === AuthType.USE_GEMINI;
+      }),
       getContentGeneratorConfig: vi.fn().mockReturnValue({
         authType: AuthType.LOGIN_WITH_GOOGLE,
       }),
diff --git a/packages/core/src/routing/strategies/classifierStrategy.ts b/packages/core/src/routing/strategies/classifierStrategy.ts
index 5fd6208b15..2040e7eccd 100644
--- a/packages/core/src/routing/strategies/classifierStrategy.ts
+++ b/packages/core/src/routing/strategies/classifierStrategy.ts
@@ -22,7 +22,6 @@ import {
 import { debugLogger } from '../../utils/debugLogger.js';
 import type { LocalLiteRtLmClient } from '../../core/localLiteRtLmClient.js';
 import { LlmRole } from '../../telemetry/types.js';
-import { AuthType } from '../../core/contentGenerator.js';
 
 // The number of recent history turns to provide to the router for context.
 const HISTORY_TURNS_FOR_CONTEXT = 4;
@@ -172,10 +171,10 @@ export class ClassifierStrategy implements RoutingStrategy {
 
       const reasoning = routerResponse.reasoning;
       const latencyMs = Date.now() - startTime;
-      const useGemini3_1 = (await config.getGemini31Launched?.()) ?? false;
-      const useCustomToolModel =
-        useGemini3_1 &&
-        config.getContentGeneratorConfig().authType === AuthType.USE_GEMINI;
+      const [useGemini3_1, useCustomToolModel] = await Promise.all([
+        config.getGemini31Launched(),
+        config.getUseCustomToolModel(),
+      ]);
       const selectedModel = resolveClassifierModel(
         model,
         routerResponse.model_choice,
diff --git a/packages/core/src/routing/strategies/numericalClassifierStrategy.test.ts b/packages/core/src/routing/strategies/numericalClassifierStrategy.test.ts
index 77fc69a218..7a0439bd19 100644
--- a/packages/core/src/routing/strategies/numericalClassifierStrategy.test.ts
+++ b/packages/core/src/routing/strategies/numericalClassifierStrategy.test.ts
@@ -58,6 +58,11 @@ describe('NumericalClassifierStrategy', () => {
       getNumericalRoutingEnabled: vi.fn().mockResolvedValue(true),
       getClassifierThreshold: vi.fn().mockResolvedValue(undefined),
       getGemini31Launched: vi.fn().mockResolvedValue(false),
+      getUseCustomToolModel: vi.fn().mockImplementation(async () => {
+        const launched = await mockConfig.getGemini31Launched();
+        const authType = mockConfig.getContentGeneratorConfig().authType;
+        return launched && authType === AuthType.USE_GEMINI;
+      }),
       getContentGeneratorConfig: vi.fn().mockReturnValue({
         authType: AuthType.LOGIN_WITH_GOOGLE,
       }),
diff --git a/packages/core/src/routing/strategies/numericalClassifierStrategy.ts b/packages/core/src/routing/strategies/numericalClassifierStrategy.ts
index 39805fb43c..1b5b67aac4 100644
--- a/packages/core/src/routing/strategies/numericalClassifierStrategy.ts
+++ b/packages/core/src/routing/strategies/numericalClassifierStrategy.ts
@@ -18,7 +18,6 @@ import type { Config } from '../../config/config.js';
 import { debugLogger } from '../../utils/debugLogger.js';
 import type { LocalLiteRtLmClient } from '../../core/localLiteRtLmClient.js';
 import { LlmRole } from '../../telemetry/types.js';
-import { AuthType } from '../../core/contentGenerator.js';
 
 // The number of recent history turns to provide to the router for context.
 const HISTORY_TURNS_FOR_CONTEXT = 8;
@@ -185,10 +184,10 @@ export class NumericalClassifierStrategy implements RoutingStrategy {
           config,
           config.getSessionId() || 'unknown-session',
         );
-      const useGemini3_1 = (await config.getGemini31Launched?.()) ?? false;
-      const useCustomToolModel =
-        useGemini3_1 &&
-        config.getContentGeneratorConfig().authType === AuthType.USE_GEMINI;
+      const [useGemini3_1, useCustomToolModel] = await Promise.all([
+        config.getGemini31Launched(),
+        config.getUseCustomToolModel(),
+      ]);
       const selectedModel = resolveClassifierModel(
         model,
         modelAlias,