fix(ui): ensure model changes update the UI immediately (#12412)

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

@@ -147,6 +147,7 @@ vi.mock('../agents/subagent-tool-wrapper.js', () => ({
 
 const mockCoreEvents = vi.hoisted(() => ({
   emitFeedback: vi.fn(),
+  emitModelChanged: vi.fn(),
 }));
 
 const mockSetGlobalProxy = vi.hoisted(() => vi.fn());

packages/core/src/config/config.ts

@@ -41,6 +41,7 @@ import {
   DEFAULT_OTLP_ENDPOINT,
   uiTelemetryService,
 } from '../telemetry/index.js';
+import { coreEvents } from '../utils/events.js';
 import { tokenLimit } from '../core/tokenLimits.js';
 import {
   DEFAULT_GEMINI_EMBEDDING_MODEL,
@@ -76,7 +77,6 @@ import type { UserTierId } from '../code_assist/types.js';
 import { AgentRegistry } from '../agents/registry.js';
 import { setGlobalProxy } from '../utils/fetch.js';
 import { SubagentToolWrapper } from '../agents/subagent-tool-wrapper.js';
-import { coreEvents } from '../utils/events.js';
 
 export enum ApprovalMode {
   DEFAULT = 'default',
@@ -711,7 +711,10 @@ export class Config {
       return;
     }
 
-    this.model = newModel;
+    if (this.model !== newModel) {
+      this.model = newModel;
+      coreEvents.emitModelChanged(newModel);
+    }
   }
 
   isInFallbackMode(): boolean {

packages/core/src/utils/events.test.ts

@@ -156,4 +156,17 @@ describe('CoreEventEmitter', () => {
     });
     expect(listener.mock.calls[2][0]).toMatchObject({ message: 'Buffered 2' });
   });
+
+  describe('ModelChanged Event', () => {
+    it('should emit ModelChanged event with correct payload', () => {
+      const listener = vi.fn();
+      events.on(CoreEvent.ModelChanged, listener);
+
+      const newModel = 'gemini-2.5-pro';
+      events.emitModelChanged(newModel);
+
+      expect(listener).toHaveBeenCalledTimes(1);
+      expect(listener).toHaveBeenCalledWith({ model: newModel });
+    });
+  });
 });

packages/core/src/utils/events.ts

@@ -43,9 +43,20 @@ export interface FallbackModeChangedPayload {
   isInFallbackMode: boolean;
 }
 
+/**
+ * Payload for the 'model-changed' event.
+ */
+export interface ModelChangedPayload {
+  /**
+   * The new model that was set.
+   */
+  model: string;
+}
+
 export enum CoreEvent {
   UserFeedback = 'user-feedback',
   FallbackModeChanged = 'fallback-mode-changed',
+  ModelChanged = 'model-changed',
 }
 
 export class CoreEventEmitter extends EventEmitter {
@@ -86,6 +97,14 @@ export class CoreEventEmitter extends EventEmitter {
     this.emit(CoreEvent.FallbackModeChanged, payload);
   }
 
+  /**
+   * Notifies subscribers that the model has changed.
+   */
+  emitModelChanged(model: string): void {
+    const payload: ModelChangedPayload = { model };
+    this.emit(CoreEvent.ModelChanged, payload);
+  }
+
   /**
    * Flushes buffered messages. Call this immediately after primary UI listener
    * subscribes.
@@ -106,6 +125,10 @@ export class CoreEventEmitter extends EventEmitter {
     event: CoreEvent.FallbackModeChanged,
     listener: (payload: FallbackModeChangedPayload) => void,
   ): this;
+  override on(
+    event: CoreEvent.ModelChanged,
+    listener: (payload: ModelChangedPayload) => void,
+  ): this;
   override on(
     event: string | symbol,
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
@@ -122,6 +145,10 @@ export class CoreEventEmitter extends EventEmitter {
     event: CoreEvent.FallbackModeChanged,
     listener: (payload: FallbackModeChangedPayload) => void,
   ): this;
+  override off(
+    event: CoreEvent.ModelChanged,
+    listener: (payload: ModelChangedPayload) => void,
+  ): this;
   override off(
     event: string | symbol,
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
@@ -138,6 +165,10 @@ export class CoreEventEmitter extends EventEmitter {
     event: CoreEvent.FallbackModeChanged,
     payload: FallbackModeChangedPayload,
   ): boolean;
+  override emit(
+    event: CoreEvent.ModelChanged,
+    payload: ModelChangedPayload,
+  ): boolean;
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   override emit(event: string | symbol, ...args: any[]): boolean {
     return super.emit(event, ...args);