diff --git a/packages/angular/build/src/builders/karma/application_builder.ts b/packages/angular/build/src/builders/karma/application_builder.ts
index b9a28e1bf0b0..d0adb6c3cf0e 100644
--- a/packages/angular/build/src/builders/karma/application_builder.ts
+++ b/packages/angular/build/src/builders/karma/application_builder.ts
@@ -11,7 +11,6 @@ import type { Config, ConfigOptions, FilePattern, InlinePluginDef, Server } from
 import { randomUUID } from 'node:crypto';
 import { rmSync } from 'node:fs';
 import * as fs from 'node:fs/promises';
-import { createRequire } from 'node:module';
 import path from 'node:path';
 import { ReadableStream } from 'node:stream/web';
 import { createVirtualModulePlugin } from '../../tools/esbuild/virtual-module-plugin';
@@ -66,8 +65,14 @@ export function execute(
         init = await initializeApplication(normalizedOptions, context, karmaOptions, transforms);
       } catch (err) {
         if (err instanceof ApplicationBuildError) {
-          controller.enqueue({ success: false, message: err.message });
-          controller.close();
+          if (controller.desiredSize !== null) {
+            try {
+              controller.enqueue({ success: false, message: err.message });
+              controller.close();
+            } catch {
+              // Stream controller may already be closed or cancelled
+            }
+          }
 
           return;
         }
@@ -85,8 +90,14 @@ export function execute(
 
       // Close the stream once the Karma server returns.
       karmaServer = new karma.Server(karmaConfig as Config, (exitCode) => {
-        controller.enqueue({ success: exitCode === 0 });
-        controller.close();
+        if (controller.desiredSize !== null) {
+          try {
+            controller.enqueue({ success: exitCode === 0 });
+            controller.close();
+          } catch {
+            // Stream controller may already be closed or cancelled
+          }
+        }
       });
 
       await karmaServer.start();
diff --git a/packages/angular/build/src/builders/karma/progress-reporter.ts b/packages/angular/build/src/builders/karma/progress-reporter.ts
index 16824badd095..6fc92b6c74c8 100644
--- a/packages/angular/build/src/builders/karma/progress-reporter.ts
+++ b/packages/angular/build/src/builders/karma/progress-reporter.ts
@@ -59,8 +59,18 @@ export function injectKarmaReporter(
             break;
           }
 
+          if (controller.desiredSize === null) {
+            break;
+          }
+
           if (buildOutput.kind === ResultKind.Failure) {
-            controller.enqueue({ success: false, message: 'Build failed' });
+            if (controller.desiredSize !== null) {
+              try {
+                controller.enqueue({ success: false, message: 'Build failed' });
+              } catch {
+                // Stream controller may already be closed or cancelled
+              }
+            }
           } else if (
             buildOutput.kind === ResultKind.Incremental ||
             buildOutput.kind === ResultKind.Full
@@ -81,10 +91,12 @@ export function injectKarmaReporter(
     }
 
     onRunComplete = function (_browsers: unknown, results: RunCompleteInfo): void {
-      if (results.exitCode === 0) {
-        controller.enqueue({ success: true });
-      } else {
-        controller.enqueue({ success: false });
+      if (controller.desiredSize !== null) {
+        try {
+          controller.enqueue({ success: results.exitCode === 0 });
+        } catch {
+          // Stream controller may already be closed or cancelled
+        }
       }
     };
   }
diff --git a/packages/angular/build/src/builders/karma/tests/behavior/errors_spec.ts b/packages/angular/build/src/builders/karma/tests/behavior/errors_spec.ts
index 366cc2aa20a1..a69698b8d743 100644
--- a/packages/angular/build/src/builders/karma/tests/behavior/errors_spec.ts
+++ b/packages/angular/build/src/builders/karma/tests/behavior/errors_spec.ts
@@ -28,5 +28,18 @@ describeKarmaBuilder(execute, KARMA_BUILDER_INFO, (harness, setupTarget) => {
 
       expect(result?.success).toBeFalse();
     });
+
+    it('handles stream cancellation gracefully in watch mode', async () => {
+      harness.useTarget('test', {
+        ...BASE_OPTIONS,
+        watch: true,
+      });
+
+      const { result } = await harness.executeOnce({
+        outputLogsOnFailure: false,
+      });
+
+      expect(result?.success).toBeTrue();
+    });
   });
 });