feat(core): add policy chain support for Gemini 3.1 (#19991)

packages/core/src/availability/policyHelpers.ts

@@ -6,6 +6,7 @@
 
 import type { GenerateContentConfig } from '@google/genai';
 import type { Config } from '../config/config.js';
+import { AuthType } from '../core/contentGenerator.js';
 import type {
   FailureKind,
   FallbackAction,
@@ -44,9 +45,15 @@ export function resolvePolicyChain(
   const configuredModel = config.getModel();
 
   let chain;
+  const useGemini31 = config.getGemini31LaunchedSync?.() ?? false;
+  const useCustomToolModel =
+    useGemini31 &&
+    config.getContentGeneratorConfig?.()?.authType === AuthType.USE_GEMINI;
+
   const resolvedModel = resolveModel(
     modelFromConfig,
-    config.getGemini31LaunchedSync?.() ?? false,
+    useGemini31,
+    useCustomToolModel,
   );
   const isAutoPreferred = preferredModel ? isAutoModel(preferredModel) : false;
   const isAutoConfigured = isAutoModel(configuredModel);
@@ -67,6 +74,8 @@ export function resolvePolicyChain(
       chain = getModelPolicyChain({
         previewEnabled,
         userTier: config.getUserTier(),
+        useGemini31,
+        useCustomToolModel,
       });
     } else {
       // User requested Gemini 3 but has no access. Proactively downgrade
@@ -74,6 +83,8 @@ export function resolvePolicyChain(
       return getModelPolicyChain({
         previewEnabled: false,
         userTier: config.getUserTier(),
+        useGemini31,
+        useCustomToolModel,
       });
     }
   } else {