refactor: Centralize and improve model fallback handling (#7634)

packages/a2a-server/src/agent/task.ts

@@ -88,12 +88,11 @@ export class Task {
     this.eventBus = eventBus;
     this.completedToolCalls = [];
     this._resetToolCompletionPromise();
-    this.config.setFlashFallbackHandler(
-      async (currentModel: string, fallbackModel: string): Promise<boolean> => {
-        config.setModel(fallbackModel); // gemini-cli-core sets to DEFAULT_GEMINI_FLASH_MODEL
-        // Switch model for future use but return false to stop current retry
-        return false;
-      },
+    this.config.setFallbackModelHandler(
+      // For a2a-server, we want to automatically switch to the fallback model
+      // for future requests without retrying the current one. The 'stop'
+      // intent achieves this.
+      async () => 'stop',
     );
   }
 

packages/a2a-server/src/utils/testing_utils.ts

@@ -43,7 +43,7 @@ export function createMockConfig(
     getContentGeneratorConfig: vi.fn().mockReturnValue({ model: 'gemini-pro' }),
     getModel: vi.fn().mockReturnValue('gemini-pro'),
     getUsageStatisticsEnabled: vi.fn().mockReturnValue(false),
-    setFlashFallbackHandler: vi.fn(),
+    setFallbackModelHandler: vi.fn(),
     initialize: vi.fn().mockResolvedValue(undefined),
     getProxy: vi.fn().mockReturnValue(undefined),
     getHistory: vi.fn().mockReturnValue([]),