KevinAndrewDong · Nov 10, 2026
diff --git a/src/app/components/chat/chat.component.i18n.ts b/src/app/components/chat/chat.component.i18n.ts
@@ -39,6 +39,8 @@ export const CHAT_MESSAGES = {
   cancelUserIdEditTooltip: 'Cancel editing user ID',
   invalidUserIdMessage: 'User ID cannot be empty',
   loadingSessionLabel: 'Loading session...',
+  sessionNotFoundMessage:
+      'This session is no longer available. Start a new session to continue.',
   tokenStreamingLabel: 'Token Streaming',
   moreOptionsTooltip: 'More options',
   createNewSessionTooltip: 'Create a new Session',

diff --git a/src/app/components/chat/chat.component.spec.ts b/src/app/components/chat/chat.component.spec.ts
@@ -28,6 +28,7 @@ import {ActivatedRoute, NavigationEnd, Router, UrlTree} from '@angular/router';
 // 1p-ONLY-IMPORTS: import {beforeEach, describe, expect, it}
 import {BehaviorSubject, NEVER, of, ReplaySubject, Subject, throwError} from 'rxjs';
 
+import {AgentRunRequest} from '../../core/models/AgentRunRequest';
 import {EvalCase} from '../../core/models/Eval';
 import {OPERATION_GENERATE_CONTENT} from '../../core/models/Trace';
 import {Session} from '../../core/models/Session';
@@ -85,6 +86,7 @@ import {TelemetryConsentDialogComponent} from '../telemetry-consent-dialog/telem
 import {computed, signal} from '@angular/core';
 
 import {ChatComponent, HIDE_SIDE_PANEL_QUERY_PARAM, INITIAL_USER_INPUT_QUERY_PARAM} from './chat.component';
+import {CHAT_MESSAGES} from './chat.component.i18n';
 
 // Mock EvalTabComponent to satisfy the required viewChild in ChatComponent
 @Component({
@@ -1173,6 +1175,58 @@ describe('ChatComponent', () => {
     });
   });
 
+  describe('Agent run errors', () => {
+    const request: AgentRunRequest = {
+      appName: TEST_APP_1_NAME,
+      userId: USER_ID,
+      sessionId: SESSION_1_ID,
+      newMessage: {role: 'user', parts: [{text: TEST_MESSAGE}]},
+    };
+
+    beforeEach(() => {
+      mockAgentService.listAppsResponse.next([TEST_APP_1_NAME]);
+      component.sessionId = SESSION_1_ID;
+      component.submitAgentRunRequest(request);
+    });
+
+    it('should explain how to recover from a missing session', () => {
+      const events = component.uiEvents();
+      mockAgentService.runSseResponse.error(
+          new HttpErrorResponse({status: 404, statusText: 'Not Found'}));
+
+      expect(mockSnackBar.open).toHaveBeenCalledWith(
+          CHAT_MESSAGES.sessionNotFoundMessage, OK_BUTTON_TEXT);
+      expect(component.sessionId).toBe(SESSION_1_ID);
+      expect(component.uiEvents()).toBe(events);
+      expect(mockSessionService.createSession).not.toHaveBeenCalled();
+      expect(mockAgentService.runSse).toHaveBeenCalledTimes(1);
+    });
+
+    it('should display other HTTP errors as readable text', () => {
+      const error = new HttpErrorResponse(
+          {status: 500, statusText: 'Internal Server Error'});
+      mockAgentService.runSseResponse.error(error);
+
+      expect(mockSnackBar.open)
+          .toHaveBeenCalledWith(error.message, OK_BUTTON_TEXT);
+    });
+
+    it('should display network errors as readable text', () => {
+      const error = new Error('Network request failed');
+      mockAgentService.runSseResponse.error(error);
+
+      expect(mockSnackBar.open)
+          .toHaveBeenCalledWith(String(error), OK_BUTTON_TEXT);
+    });
+
+    it('should keep cancelled requests silent', () => {
+      mockAgentService.runSseResponse.error(
+          new DOMException('The operation was aborted.', 'AbortError'));
+
+      expect(mockSnackBar.open).not.toHaveBeenCalled();
+    });
+  });
+
   describe('ChatPanel integration', () => {
     beforeEach(() => {
       mockAgentService.listAppsResponse.next(

diff --git a/src/app/components/chat/chat.component.ts b/src/app/components/chat/chat.component.ts
@@ -1409,7 +1409,13 @@ export class ChatComponent implements OnInit, AfterViewInit, OnDestroy {
         if (errString.includes('aborted') || errString.includes('AbortError')) {
           return;
         }
-        this.openSnackBar(err, 'OK');
+        let errorMessage = errString;
+        if (err instanceof HttpErrorResponse) {
+          errorMessage = err.status === 404
+            ? this.i18n.sessionNotFoundMessage
+            : err.message;
+        }
+        this.openSnackBar(errorMessage, 'OK');
       },
       complete: () => {
         this.activeSseSubscription = undefined;

diff --git a/src/app/core/services/agent.service.spec.ts b/src/app/core/services/agent.service.spec.ts
@@ -15,6 +15,7 @@
  * limitations under the License.
  */
 
+import {HttpErrorResponse} from '@angular/common/http';
 import {HttpClientTestingModule, HttpTestingController,} from '@angular/common/http/testing';
 import {TestBed} from '@angular/core/testing';
 // 1p-ONLY-IMPORTS: import {beforeEach, describe, expect, it}
@@ -128,6 +129,40 @@ describe('AgentService', () => {
   });
 
   describe('runSse', () => {
+    for (const {status, statusText, body} of [
+      {
+        status: 404,
+        statusText: 'Not Found',
+        body: JSON.stringify({detail: 'Session not found'}),
+      },
+      {status: 401, statusText: 'Unauthorized', body: 'Unauthorized'},
+      {
+        status: 500,
+        statusText: 'Internal Server Error',
+        body: '<html>Server error</html>',
+      },
+    ]) {
+      it(`should reject HTTP ${status} without reading it as SSE`, async () => {
+        const response = new Response(body, {status, statusText});
+        const getReader = spyOn(response.body!, 'getReader').and.callThrough();
+        spyOn(window, 'fetch').and.resolveTo(response);
+
+        const result = firstValueFrom(
+            service.runSse(RUN_SSE_PAYLOAD).pipe(toArray()),
+        );
+        await expectAsync(result)
+            .toBeRejectedWith(jasmine.any(HttpErrorResponse));
+        await expectAsync(result).toBeRejectedWith(jasmine.objectContaining({
+          name: 'HttpErrorResponse',
+          status,
+          statusText,
+          url: API_SERVER_BASE_URL + RUN_SSE_PATH,
+        }));
+        expect(getReader).not.toHaveBeenCalled();
+        expect(service.getLoadingState().value).toBeFalse();
+      });
+    }
+
     it('should set loading state to true when called', async () => {
       spyOn(window, 'fetch').and.resolveTo(new Response());
       service.runSse(RUN_SSE_PAYLOAD).subscribe();

diff --git a/src/app/core/services/agent.service.ts b/src/app/core/services/agent.service.ts
@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-import { HttpClient } from '@angular/common/http';
+import { HttpClient, HttpErrorResponse } from '@angular/common/http';
 import { Injectable, NgZone } from '@angular/core';
 import { BehaviorSubject, Observable, of } from 'rxjs';
 import { URLUtil } from '../../../utils/url-util';
@@ -68,6 +68,13 @@ export class AgentService implements AgentServiceInterface {
         signal,
       })
         .then((response) => {
+          if (!response.ok) {
+            throw new HttpErrorResponse({
+              status: response.status,
+              statusText: response.statusText,
+              url: response.url || url,
+            });
+          }
           reader = response.body?.getReader();
           const decoder = new TextDecoder('utf-8');
           let lastData = '';