do not persist the fallback model (#15483)

packages/core/src/config/config.test.ts

@@ -1641,6 +1641,18 @@ describe('Config getHooks', () => {
 
       expect(onModelChange).toHaveBeenCalledWith(DEFAULT_GEMINI_MODEL);
     });
+
+    it('should NOT call onModelChange when a new model is set as a fallback', () => {
+      const onModelChange = vi.fn();
+      const config = new Config({
+        ...baseParams,
+        onModelChange,
+      });
+
+      config.setModel(DEFAULT_GEMINI_MODEL, true);
+
+      expect(onModelChange).not.toHaveBeenCalled();
+    });
   });
 });
 

packages/core/src/config/config.ts

@@ -877,13 +877,13 @@ export class Config {
     return this.model;
   }
 
-  setModel(newModel: string): void {
+  setModel(newModel: string, isFallbackModel: boolean = false): void {
     if (this.model !== newModel || this._activeModel !== newModel) {
       this.model = newModel;
       // When the user explicitly sets a model, that becomes the active model.
       this._activeModel = newModel;
       coreEvents.emitModelChanged(newModel);
-      if (this.onModelChange) {
+      if (this.onModelChange && !isFallbackModel) {
         this.onModelChange(newModel);
       }
     }