Respect backend definitions for 3.5 flash and Update auto mode to use 3.5 flash when the flag is enabled. (#27645)

packages/cli/src/acp/acpUtils.ts

@@ -265,6 +265,7 @@ export function buildAvailableModels(
   const preferredModel = config.getModel() || GEMINI_MODEL_ALIAS_AUTO;
   const shouldShowPreviewModels = config.getHasAccessToPreviewModel();
   const useGemini31 = config.getGemini31LaunchedSync?.() ?? false;
+  const useGemini3_5Flash = config.hasGemini35FlashGAAccess?.() ?? false;
   const selectedAuthType = settings.merged.security.auth.selectedType;
   const useCustomToolModel =
     useGemini31 && selectedAuthType === AuthType.USE_GEMINI;
@@ -276,6 +277,7 @@ export function buildAvailableModels(
   ) {
     const options = config.getModelConfigService().getAvailableModelOptions({
       useGemini3_1: useGemini31,
+      useGemini3_5Flash,
       useCustomTools: useCustomToolModel,
       hasAccessToPreview: shouldShowPreviewModels,
     });
@@ -294,6 +296,7 @@ export function buildAvailableModels(
       description: getAutoModelDescription(
         shouldShowPreviewModels,
         useGemini31,
+        useGemini3_5Flash,
       ),
     },
   ];