diff --git a/packages/app/src/cli/services/build/steps/bundle-ui-step.test.ts b/packages/app/src/cli/services/build/steps/bundle-ui-step.test.ts
new file mode 100644
index 0000000000..18aa974214
--- /dev/null
+++ b/packages/app/src/cli/services/build/steps/bundle-ui-step.test.ts
@@ -0,0 +1,60 @@
+import {executeBundleUIStep} from './bundle-ui-step.js'
+import * as buildExtension from '../extension.js'
+import {BundleUIStep, BuildContext} from '../client-steps.js'
+import {ExtensionInstance} from '../../../models/extensions/extension-instance.js'
+import {describe, expect, test, vi, beforeEach} from 'vitest'
+import * as fs from '@shopify/cli-kit/node/fs'
+
+vi.mock('@shopify/cli-kit/node/fs')
+vi.mock('../extension.js')
+
+describe('executeBundleUIStep', () => {
+  let mockContext: BuildContext
+
+  beforeEach(() => {
+    mockContext = {
+      extension: {
+        directory: '/test/extension',
+        outputPath: '/test/extension/dist/handle.js',
+        configuration: {},
+      } as ExtensionInstance,
+      options: {
+        stdout: {write: vi.fn()} as any,
+        stderr: {write: vi.fn()} as any,
+        app: {} as any,
+        environment: 'production',
+      },
+      stepResults: new Map(),
+    }
+  })
+
+  const step: BundleUIStep = {
+    id: 'bundle-ui',
+    name: 'Bundle UI Extension',
+    type: 'bundle_ui',
+    config: {generatesAssetsManifest: false},
+  }
+
+  test('skips the copy when local and bundle output directories are identical', async () => {
+    // Given
+    vi.mocked(buildExtension.buildUIExtension).mockResolvedValue('/test/extension/dist/handle.js')
+
+    // When
+    await executeBundleUIStep(step, mockContext)
+
+    // Then — fs-extra would throw "Source and destination must not be the same"
+    expect(fs.copyFile).not.toHaveBeenCalled()
+  })
+
+  test('copies when local and bundle output directories differ', async () => {
+    // Given
+    mockContext.extension.outputPath = '/bundle/handle/handle.js'
+    vi.mocked(buildExtension.buildUIExtension).mockResolvedValue('/test/extension/dist/handle.js')
+
+    // When
+    await executeBundleUIStep(step, mockContext)
+
+    // Then
+    expect(fs.copyFile).toHaveBeenCalledWith('/test/extension/dist', '/bundle/handle')
+  })
+})
diff --git a/packages/app/src/cli/services/build/steps/bundle-ui-step.ts b/packages/app/src/cli/services/build/steps/bundle-ui-step.ts
index 81174b8a4d..0c71029ced 100644
--- a/packages/app/src/cli/services/build/steps/bundle-ui-step.ts
+++ b/packages/app/src/cli/services/build/steps/bundle-ui-step.ts
@@ -21,8 +21,12 @@ interface ExtensionPointWithBuildManifest {
 export async function executeBundleUIStep(step: BundleUIStep, context: BuildContext): Promise<void> {
   const config = context.extension.configuration
   const localOutputPath = await buildUIExtension(context.extension, context.options)
-  // Copy the locally built files into the bundle
-  await copyFile(dirname(localOutputPath), dirname(context.extension.outputPath))
+  // When invoked outside a bundle directory (e.g. `shopify app build`), localOutputPath and outputPath collapse onto the same directory; fs-extra rejects same-path copies.
+  const localOutputDir = dirname(localOutputPath)
+  const bundleOutputDir = dirname(context.extension.outputPath)
+  if (localOutputDir !== bundleOutputDir) {
+    await copyFile(localOutputDir, bundleOutputDir)
+  }
 
   if (!step.config?.generatesAssetsManifest) return