diff --git a/packages/schematics/angular/migrations/migrate-karma-to-vitest/migration.ts b/packages/schematics/angular/migrations/migrate-karma-to-vitest/migration.ts
index fc5b11fb5eb0..5a10f4a6f8a3 100644
--- a/packages/schematics/angular/migrations/migrate-karma-to-vitest/migration.ts
+++ b/packages/schematics/angular/migrations/migrate-karma-to-vitest/migration.ts
@@ -34,12 +34,16 @@ async function processTestTargetOptions(
     const configKey = configName || '';
 
     if (!customBuildOptions[configKey]) {
-      // Match Karma behavior where AOT was disabled by default
-      customBuildOptions[configKey] = {
-        aot: false,
-        optimization: false,
-        extractLicenses: false,
-      };
+      // Match Karma behavior where AOT was disabled by default. Configurations inherit these
+      // from the base options when present, so seeding them would override explicit base values.
+      customBuildOptions[configKey] =
+        configKey && testTarget.options
+          ? {}
+          : {
+              aot: false,
+              optimization: false,
+              extractLicenses: false,
+            };
     }
 
     // Collect custom build options
@@ -306,9 +310,16 @@ function updateProjects(tree: Tree, context: SchematicContext): Rule {
       for (const [configKey, configOptions] of Object.entries(customBuildOptions)) {
         const finalConfig: Record<string, json.JsonValue | undefined> = {};
 
+        // A test configuration inherits the base test options, so its build configuration must
+        // include the build options moved from them, with the configuration's own values on top.
+        const mergedOptions =
+          configKey && customBuildOptions['']
+            ? { ...customBuildOptions[''], ...configOptions }
+            : configOptions;
+
         // Omit options that already have the same value in the base build options.
         // Using isDeepStrictEqual for a deep comparison of arrays and objects.
-        for (const [key, value] of Object.entries(configOptions)) {
+        for (const [key, value] of Object.entries(mergedOptions)) {
           if (!isDeepStrictEqual(value, baseOptions[key])) {
             finalConfig[key] = value;
           }
diff --git a/packages/schematics/angular/migrations/migrate-karma-to-vitest/migration_spec.ts b/packages/schematics/angular/migrations/migrate-karma-to-vitest/migration_spec.ts
index fb6737b77d26..1989ec8777c7 100644
--- a/packages/schematics/angular/migrations/migrate-karma-to-vitest/migration_spec.ts
+++ b/packages/schematics/angular/migrations/migrate-karma-to-vitest/migration_spec.ts
@@ -128,6 +128,61 @@ describe('Migration from Karma to Vitest unit-test builder', () => {
     expect(newProjects.app.targets.test.configurations.ci.assets).toBeUndefined();
   });
 
+  it('should keep base custom build options in per configuration testing configurations', async () => {
+    const { projects } = tree.readJson('/angular.json') as any;
+    projects.app.targets.test.options.styles = ['src/test-styles.scss'];
+    projects.app.targets.test.configurations = {
+      ci: {
+        watch: false,
+      },
+    };
+    tree.overwrite('/angular.json', JSON.stringify({ version: 1, projects }));
+
+    const newTree = await schematicRunner.runSchematic('migrate-karma-to-vitest', {}, tree);
+    const { projects: newProjects } = newTree.readJson('/angular.json') as any;
+
+    expect(newProjects.app.targets.build.configurations.testing.styles).toEqual([
+      'src/test-styles.scss',
+    ]);
+    expect(newProjects.app.targets.test.configurations.ci.buildTarget).toBe(':build:testing-ci');
+    expect(newProjects.app.targets.build.configurations['testing-ci'].styles).toEqual([
+      'src/test-styles.scss',
+    ]);
+  });
+
+  it('should keep explicit base Karma build defaults in per configuration testing configurations', async () => {
+    const { projects } = tree.readJson('/angular.json') as any;
+    projects.app.targets.test.options.styles = ['src/test-styles.scss'];
+    projects.app.targets.test.options.aot = true;
+    projects.app.targets.test.configurations = {
+      ci: {
+        watch: false,
+      },
+    };
+    tree.overwrite('/angular.json', JSON.stringify({ version: 1, projects }));
+
+    const newTree = await schematicRunner.runSchematic('migrate-karma-to-vitest', {}, tree);
+    const { projects: newProjects } = newTree.readJson('/angular.json') as any;
+
+    expect(newProjects.app.targets.build.configurations.testing.aot).toBeTrue();
+    expect(newProjects.app.targets.build.configurations['testing-ci'].aot).toBeTrue();
+  });
+
+  it('should apply Karma build defaults to testing configurations without base test options', async () => {
+    const { projects } = tree.readJson('/angular.json') as any;
+    projects.app.targets.test.configurations = {
+      ci: projects.app.targets.test.options,
+    };
+    delete projects.app.targets.test.options;
+    tree.overwrite('/angular.json', JSON.stringify({ version: 1, projects }));
+
+    const newTree = await schematicRunner.runSchematic('migrate-karma-to-vitest', {}, tree);
+    const { projects: newProjects } = newTree.readJson('/angular.json') as any;
+
+    expect(newProjects.app.targets.test.configurations.ci.buildTarget).toBe(':build:testing-ci');
+    expect(newProjects.app.targets.build.configurations['testing-ci'].aot).toBeFalse();
+  });
+
   it('should omit testing configuration if options match base build options', async () => {
     const { projects } = tree.readJson('/angular.json') as any;
     projects.app.targets.build.options.aot = false;