fix(core): ensure correct flash model steering in plan mode implementation phase (#21871)

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.
    *

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
   );
 }