diff --git a/packages/app/src/cli/models/extensions/extension-instance.test.ts b/packages/app/src/cli/models/extensions/extension-instance.test.ts
index 3fea6b4a05..9cebe16732 100644
--- a/packages/app/src/cli/models/extensions/extension-instance.test.ts
+++ b/packages/app/src/cli/models/extensions/extension-instance.test.ts
@@ -172,6 +172,27 @@ describe('build', async () => {
     })
   })
 
+  test('bundle() is a no-op when the spec declares no bundle lifecycle group', async () => {
+    await inTemporaryDirectory(async (tmpDir) => {
+      // Given — tax_calculation only declares a 'build' lifecycle group.
+      const extensionInstance = await testTaxCalculationExtension(tmpDir)
+      const options: ExtensionBuildOptions = {
+        stdout: new Writable({write: (_chunk, _enc, cb) => cb()}),
+        stderr: new Writable({write: (_chunk, _enc, cb) => cb()}),
+        app: testApp(),
+        environment: 'production',
+      }
+
+      const outputFilePath = joinPath(tmpDir, `dist/${extensionInstance.outputFileName}`)
+
+      // When — bundle() runs only the bundle steps; with none declared this is a no-op.
+      await extensionInstance.bundle(options)
+
+      // Then — no output file is produced (build() was not called).
+      await expect(readFile(outputFilePath)).rejects.toThrow()
+    })
+  })
+
   test('does not copy shopify.extension.toml file when bundling theme extensions', async () => {
     await inTemporaryDirectory(async (tmpDir) => {
       // Given
diff --git a/packages/app/src/cli/models/extensions/extension-instance.ts b/packages/app/src/cli/models/extensions/extension-instance.ts
index e266cb54c0..af6cc3e113 100644
--- a/packages/app/src/cli/models/extensions/extension-instance.ts
+++ b/packages/app/src/cli/models/extensions/extension-instance.ts
@@ -8,7 +8,7 @@ import {Identifiers} from '../app/identifiers.js'
 import {DeveloperPlatformClient} from '../../utilities/developer-platform-client.js'
 import {AppConfiguration} from '../app/app.js'
 import {ApplicationURLs} from '../../services/dev/urls.js'
-import {executeStep, BuildContext} from '../../services/build/client-steps.js'
+import {executeStep, BuildContext, LifecyclePhase} from '../../services/build/client-steps.js'
 import {ok} from '@shopify/cli-kit/node/result'
 import {constantize, slugify} from '@shopify/cli-kit/common/string'
 import {hashString, nonRandomUUID} from '@shopify/cli-kit/node/crypto'
@@ -114,7 +114,9 @@ export class ExtensionInstance<TConfiguration extends BaseConfigType = BaseConfi
 
   get hasDeploySteps(): boolean {
     return (
-      this.specification.clientSteps?.some((group) => group.lifecycle === 'deploy' && group.steps.length > 0) ?? false
+      this.specification.clientSteps?.some(
+        (group) => (group.lifecycle === 'build' || group.lifecycle === 'bundle') && group.steps.length > 0,
+      ) ?? false
     )
   }
 
@@ -316,7 +318,25 @@ export class ExtensionInstance<TConfiguration extends BaseConfigType = BaseConfi
   }
 
   async build(options: ExtensionBuildOptions): Promise<void> {
-    const {clientSteps = []} = this.specification
+    await this.runLifecyclePhase('build', options)
+  }
+
+  async bundle(options: ExtensionBuildOptions): Promise<void> {
+    await this.runLifecyclePhase('bundle', options)
+  }
+
+  async buildForBundle(options: ExtensionBuildOptions, bundleDirectory: string, outputId?: string) {
+    this.outputPath = this.getOutputPathForDirectory(bundleDirectory, outputId)
+    await this.build(options)
+    await this.bundle(options)
+
+    const bundleInputPath = joinPath(bundleDirectory, this.getOutputFolderId(outputId))
+    await this.keepBuiltSourcemapsLocally(bundleInputPath)
+  }
+
+  private async runLifecyclePhase(phase: LifecyclePhase, options: ExtensionBuildOptions): Promise<void> {
+    const steps = this.specification.clientSteps?.find((group) => group.lifecycle === phase)?.steps ?? []
+    if (steps.length === 0) return
 
     const context: BuildContext = {
       extension: this,
@@ -324,8 +344,6 @@ export class ExtensionInstance<TConfiguration extends BaseConfigType = BaseConfi
       stepResults: new Map(),
     }
 
-    const steps = clientSteps.find((lifecycle) => lifecycle.lifecycle === 'deploy')?.steps ?? []
-
     for (const step of steps) {
       // eslint-disable-next-line no-await-in-loop
       const result = await executeStep(step, context)
@@ -333,14 +351,6 @@ export class ExtensionInstance<TConfiguration extends BaseConfigType = BaseConfi
     }
   }
 
-  async buildForBundle(options: ExtensionBuildOptions, bundleDirectory: string, outputId?: string) {
-    this.outputPath = this.getOutputPathForDirectory(bundleDirectory, outputId)
-    await this.build(options)
-
-    const bundleInputPath = joinPath(bundleDirectory, this.getOutputFolderId(outputId))
-    await this.keepBuiltSourcemapsLocally(bundleInputPath)
-  }
-
   async copyIntoBundle(options: ExtensionBuildOptions, bundleDirectory: string, extensionUuid?: string) {
     const defaultOutputPath = this.outputPath
 
diff --git a/packages/app/src/cli/models/extensions/specification.integration.test.ts b/packages/app/src/cli/models/extensions/specification.integration.test.ts
index f49d0712f3..18994e711a 100644
--- a/packages/app/src/cli/models/extensions/specification.integration.test.ts
+++ b/packages/app/src/cli/models/extensions/specification.integration.test.ts
@@ -37,7 +37,7 @@ describe('allLocalSpecs', () => {
 
 const testClientSteps: ClientSteps = [
   {
-    lifecycle: 'deploy',
+    lifecycle: 'build',
     steps: [
       {
         id: 'bundle-ui',
diff --git a/packages/app/src/cli/models/extensions/specifications/admin.ts b/packages/app/src/cli/models/extensions/specifications/admin.ts
index 7aad744d23..df8ae6042e 100644
--- a/packages/app/src/cli/models/extensions/specifications/admin.ts
+++ b/packages/app/src/cli/models/extensions/specifications/admin.ts
@@ -43,7 +43,7 @@ const adminSpecificationSpec = createExtensionSpecification<AdminConfigType>({
   },
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'bundle',
       steps: [
         {
           id: 'hosted_app_copy_files',
diff --git a/packages/app/src/cli/models/extensions/specifications/admin_link.test.ts b/packages/app/src/cli/models/extensions/specifications/admin_link.test.ts
index 0030fb7b26..b41872d7b5 100644
--- a/packages/app/src/cli/models/extensions/specifications/admin_link.test.ts
+++ b/packages/app/src/cli/models/extensions/specifications/admin_link.test.ts
@@ -52,7 +52,7 @@ describe('admin_link', async () => {
       const extension = await getTestAdminLink(tmpDir)
       const clientSteps = extension.specification.clientSteps!
       expect(clientSteps).toHaveLength(1)
-      expect(clientSteps[0]!.lifecycle).toBe('deploy')
+      expect(clientSteps[0]!.lifecycle).toBe('bundle')
 
       const steps = clientSteps[0]!.steps
       expect(steps).toHaveLength(1)
diff --git a/packages/app/src/cli/models/extensions/specifications/admin_link.ts b/packages/app/src/cli/models/extensions/specifications/admin_link.ts
index a375e12f55..34fed0219a 100644
--- a/packages/app/src/cli/models/extensions/specifications/admin_link.ts
+++ b/packages/app/src/cli/models/extensions/specifications/admin_link.ts
@@ -6,7 +6,7 @@ const adminLinkSpec = createContractBasedModuleSpecification({
   experience: 'extension',
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'bundle',
       steps: [
         {
           id: 'include-admin-link-assets',
diff --git a/packages/app/src/cli/models/extensions/specifications/channel.ts b/packages/app/src/cli/models/extensions/specifications/channel.ts
index 3973fa91d2..c38f24d478 100644
--- a/packages/app/src/cli/models/extensions/specifications/channel.ts
+++ b/packages/app/src/cli/models/extensions/specifications/channel.ts
@@ -9,7 +9,7 @@ const channelSpecificationSpec = createContractBasedModuleSpecification({
   experience: 'extension',
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'bundle',
       steps: [
         {
           id: 'copy-files',
diff --git a/packages/app/src/cli/models/extensions/specifications/checkout_post_purchase.ts b/packages/app/src/cli/models/extensions/specifications/checkout_post_purchase.ts
index fad34726ed..d8d3faaa47 100644
--- a/packages/app/src/cli/models/extensions/specifications/checkout_post_purchase.ts
+++ b/packages/app/src/cli/models/extensions/specifications/checkout_post_purchase.ts
@@ -20,7 +20,7 @@ const checkoutPostPurchaseSpec = createExtensionSpecification({
     `dist/${extension.handle}.js`,
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'build',
       steps: [{id: 'bundle-ui', name: 'Bundle UI Extension', type: 'bundle_ui', config: {}}],
     },
   ],
diff --git a/packages/app/src/cli/models/extensions/specifications/checkout_ui_extension.ts b/packages/app/src/cli/models/extensions/specifications/checkout_ui_extension.ts
index 239f164c9b..992975b163 100644
--- a/packages/app/src/cli/models/extensions/specifications/checkout_ui_extension.ts
+++ b/packages/app/src/cli/models/extensions/specifications/checkout_ui_extension.ts
@@ -26,7 +26,7 @@ const checkoutSpec = createExtensionSpecification({
   getOutputRelativePath: (extension: ExtensionInstance<CheckoutConfigType>) => `dist/${extension.handle}.js`,
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'build',
       steps: [{id: 'bundle-ui', name: 'Bundle UI Extension', type: 'bundle_ui', config: {}}],
     },
   ],
diff --git a/packages/app/src/cli/models/extensions/specifications/flow_template.ts b/packages/app/src/cli/models/extensions/specifications/flow_template.ts
index 805f953c98..7c19bf27f1 100644
--- a/packages/app/src/cli/models/extensions/specifications/flow_template.ts
+++ b/packages/app/src/cli/models/extensions/specifications/flow_template.ts
@@ -51,7 +51,7 @@ const flowTemplateSpec = createExtensionSpecification({
   appModuleFeatures: (_) => ['ui_preview'],
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'bundle',
       steps: [
         {
           id: 'copy-files',
diff --git a/packages/app/src/cli/models/extensions/specifications/function.ts b/packages/app/src/cli/models/extensions/specifications/function.ts
index 2d73e819d0..7c96f15c7f 100644
--- a/packages/app/src/cli/models/extensions/specifications/function.ts
+++ b/packages/app/src/cli/models/extensions/specifications/function.ts
@@ -103,7 +103,7 @@ const functionSpec = createExtensionSpecification({
   },
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'build',
       steps: [{id: 'build-function', name: 'Build Function', type: 'build_function', config: {}}],
     },
   ],
diff --git a/packages/app/src/cli/models/extensions/specifications/order_attribution_config.ts b/packages/app/src/cli/models/extensions/specifications/order_attribution_config.ts
index d2af493e27..2141fa197e 100644
--- a/packages/app/src/cli/models/extensions/specifications/order_attribution_config.ts
+++ b/packages/app/src/cli/models/extensions/specifications/order_attribution_config.ts
@@ -9,7 +9,7 @@ const orderAttributionConfigSpec = createContractBasedModuleSpecification({
   experience: 'extension',
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'bundle',
       steps: [
         {
           id: 'copy-files',
diff --git a/packages/app/src/cli/models/extensions/specifications/pos_ui_extension.ts b/packages/app/src/cli/models/extensions/specifications/pos_ui_extension.ts
index 3835d873ec..668ed6cd85 100644
--- a/packages/app/src/cli/models/extensions/specifications/pos_ui_extension.ts
+++ b/packages/app/src/cli/models/extensions/specifications/pos_ui_extension.ts
@@ -18,7 +18,7 @@ const posUISpec = createExtensionSpecification({
   getOutputRelativePath: (extension: ExtensionInstance<PosUIConfigType>) => `dist/${extension.handle}.js`,
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'build',
       steps: [{id: 'bundle-ui', name: 'Bundle UI Extension', type: 'bundle_ui', config: {}}],
     },
   ],
diff --git a/packages/app/src/cli/models/extensions/specifications/product_subscription.ts b/packages/app/src/cli/models/extensions/specifications/product_subscription.ts
index 77c95fde10..14edab2f64 100644
--- a/packages/app/src/cli/models/extensions/specifications/product_subscription.ts
+++ b/packages/app/src/cli/models/extensions/specifications/product_subscription.ts
@@ -19,7 +19,7 @@ const productSubscriptionSpec = createExtensionSpecification({
   getOutputRelativePath: (extension: ExtensionInstance<ProductSubscriptionConfigType>) => `dist/${extension.handle}.js`,
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'build',
       steps: [{id: 'bundle-ui', name: 'Bundle UI Extension', type: 'bundle_ui', config: {}}],
     },
   ],
diff --git a/packages/app/src/cli/models/extensions/specifications/tax_calculation.ts b/packages/app/src/cli/models/extensions/specifications/tax_calculation.ts
index 1cc2caa351..3b75d35b9b 100644
--- a/packages/app/src/cli/models/extensions/specifications/tax_calculation.ts
+++ b/packages/app/src/cli/models/extensions/specifications/tax_calculation.ts
@@ -35,7 +35,7 @@ const spec = createExtensionSpecification({
     joinPath('dist', `${extension.handle}.js`),
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'build',
       steps: [{id: 'create-tax-stub', name: 'Create Tax Stub', type: 'create_tax_stub', config: {}}],
     },
   ],
diff --git a/packages/app/src/cli/models/extensions/specifications/theme.ts b/packages/app/src/cli/models/extensions/specifications/theme.ts
index 8e6d27d27e..2412d4a6ea 100644
--- a/packages/app/src/cli/models/extensions/specifications/theme.ts
+++ b/packages/app/src/cli/models/extensions/specifications/theme.ts
@@ -14,7 +14,7 @@ const themeSpec = createExtensionSpecification({
   graphQLType: 'theme_app_extension',
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'build',
       steps: [
         {id: 'build-theme', name: 'Build Theme Extension', type: 'build_theme', config: {}},
         {id: 'bundle-theme', name: 'Bundle Theme Extension', type: 'bundle_theme', config: {}},
diff --git a/packages/app/src/cli/models/extensions/specifications/ui_extension.ts b/packages/app/src/cli/models/extensions/specifications/ui_extension.ts
index e2a33fda99..f7feeb66e4 100644
--- a/packages/app/src/cli/models/extensions/specifications/ui_extension.ts
+++ b/packages/app/src/cli/models/extensions/specifications/ui_extension.ts
@@ -87,13 +87,24 @@ const uiExtensionSpec = createExtensionSpecification({
   getOutputRelativePath: (extension: ExtensionInstance<UIExtensionConfigType>) => `${extension.handle}.js`,
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'build',
       steps: [
         {
           id: 'bundle-ui',
           name: 'Bundle UI Extension',
           type: 'bundle_ui',
-          config: {generatesAssetsManifest: true, bundleFolder: 'dist/'},
+          config: {bundleFolder: 'dist/'},
+        },
+      ],
+    },
+    {
+      lifecycle: 'bundle',
+      steps: [
+        {
+          id: 'generate-ui-assets-manifest',
+          name: 'Generate UI Assets Manifest',
+          type: 'generate_ui_assets_manifest',
+          config: {bundleFolder: 'dist/'},
         },
         {
           id: 'include-ui-extension-assets',
diff --git a/packages/app/src/cli/models/extensions/specifications/web_pixel_extension.ts b/packages/app/src/cli/models/extensions/specifications/web_pixel_extension.ts
index 500db61748..53b5ea9309 100644
--- a/packages/app/src/cli/models/extensions/specifications/web_pixel_extension.ts
+++ b/packages/app/src/cli/models/extensions/specifications/web_pixel_extension.ts
@@ -36,7 +36,7 @@ const webPixelSpec = createExtensionSpecification({
   getOutputRelativePath: (extension: ExtensionInstance<WebPixelConfigType>) => `dist/${extension.handle}.js`,
   clientSteps: [
     {
-      lifecycle: 'deploy',
+      lifecycle: 'build',
       steps: [{id: 'bundle-ui', name: 'Bundle UI Extension', type: 'bundle_ui', config: {}}],
     },
   ],
diff --git a/packages/app/src/cli/services/build/client-steps.ts b/packages/app/src/cli/services/build/client-steps.ts
index a5092f0bae..a446637f58 100644
--- a/packages/app/src/cli/services/build/client-steps.ts
+++ b/packages/app/src/cli/services/build/client-steps.ts
@@ -25,7 +25,14 @@ interface IncludeAssetsStep extends BaseStep {
 export interface BundleUIStep extends BaseStep {
   readonly type: 'bundle_ui'
   readonly config?: {
-    readonly generatesAssetsManifest?: boolean
+    readonly bundleFolder?: string
+  }
+}
+
+/** Step with typed config specific to generate_ui_assets_manifest. */
+export interface GenerateUIAssetsManifestStep extends BaseStep {
+  readonly type: 'generate_ui_assets_manifest'
+  readonly config?: {
     readonly bundleFolder?: string
   }
 }
@@ -43,14 +50,31 @@ interface NoConfigStep extends BaseStep {
  * This is a discriminated union on `type`: each step type carries its own
  * typed `config`, so TypeScript catches config typos at compile time.
  */
-export type LifecycleStep = IncludeAssetsStep | BundleUIStep | NoConfigStep
+export type LifecycleStep = IncludeAssetsStep | BundleUIStep | GenerateUIAssetsManifestStep | NoConfigStep
+
+/**
+ * Lifecycle phases supported by the client-step pipeline.
+ *
+ * Phases compose additively. A spec lists only the steps that are *unique* to a
+ * phase; the runtime combines them per command:
+ *
+ * - `'build'` — local build (e.g. `shopify app build`). Steps required to
+ *   produce the on-disk artifact.
+ * - `'bundle'` — extra steps needed when bundling for upload (driven by `dev`
+ *   and `deploy`). Run *after* the build steps; never on their own.
+ *
+ * The `build` command runs `'build'` steps. The `dev`/`deploy` pipeline runs
+ * `'build'` then `'bundle'`. A spec only declares a `'bundle'` group when the
+ * bundle phase needs to do something the local build doesn't.
+ */
+export type LifecyclePhase = 'build' | 'bundle'
 
 /**
  * A group of steps scoped to a specific lifecycle phase.
- * Allows executing only the steps relevant to a given lifecycle (e.g. 'deploy').
+ * Allows executing only the steps relevant to a given lifecycle.
  */
 interface ClientLifecycleGroup {
-  readonly lifecycle: 'deploy'
+  readonly lifecycle: LifecyclePhase
   readonly steps: ReadonlyArray<LifecycleStep>
 }
 
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
index 5f73580949..da95f3d295 100644
--- 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
@@ -32,7 +32,7 @@ describe('executeBundleUIStep', () => {
     id: 'bundle-ui',
     name: 'Bundle UI Extension',
     type: 'bundle_ui',
-    config: {generatesAssetsManifest: false},
+    config: {},
   }
 
   test('copies when local and bundle output directories differ', async () => {
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 dd65b01ce7..312f227c2a 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
@@ -1,25 +1,16 @@
-import {createOrUpdateManifestFile} from './include-assets/generate-manifest.js'
 import {buildUIExtension} from '../extension.js'
-import {BuildManifest} from '../../../models/extensions/specifications/ui_extension.js'
 import {copyFile} from '@shopify/cli-kit/node/fs'
 import {dirname, joinPath, resolvePath} from '@shopify/cli-kit/node/path'
 import type {BundleUIStep, BuildContext} from '../client-steps.js'
 
-interface ExtensionPointWithBuildManifest {
-  target: string
-  build_manifest: BuildManifest
-}
-
 /**
  * Executes a bundle_ui build step.
  *
  * Bundles the UI extension using esbuild into the extension's local directory
- * and copies the output to the bundle. When `generatesAssetsManifest` is true,
- * writes built asset entries (from build_manifest) into manifest.json so
- * downstream steps can merge on top.
+ * and copies the output to the bundle. Manifest emission lives in the separate
+ * `generate_ui_assets_manifest` step so it only runs in lifecycles that need it.
  */
 export async function executeBundleUIStep(step: BundleUIStep, context: BuildContext): Promise<void> {
-  const config = context.extension.configuration
   context.options.buildDirectory = step.config?.bundleFolder ?? undefined
   const localOutputPath = await buildUIExtension(context.extension, context.options)
   const localOutputDir = dirname(localOutputPath)
@@ -29,39 +20,4 @@ export async function executeBundleUIStep(step: BundleUIStep, context: BuildCont
   if (resolvePath(localOutputDir) !== resolvePath(bundleOutputDir)) {
     await copyFile(localOutputDir, bundleOutputDir)
   }
-
-  if (!step.config?.generatesAssetsManifest) return
-
-  if (!Array.isArray(config.extension_points)) return
-
-  const pointsWithManifest = config.extension_points.filter(
-    (ep): ep is ExtensionPointWithBuildManifest => typeof ep === 'object' && ep.build_manifest,
-  )
-
-  const entries = extractBuiltAssetEntries(pointsWithManifest, step.config?.bundleFolder)
-  if (Object.keys(entries).length > 0) {
-    await createOrUpdateManifestFile(context, entries)
-  }
-}
-
-/**
- * Extracts built asset filepaths from `build_manifest` on each extension point,
- * grouped by target. Returns a map of target → `{assetName: filepath}`.
- */
-function extractBuiltAssetEntries(
-  extensionPoints: {target: string; build_manifest: BuildManifest}[],
-  bundleFolder?: string,
-): {
-  [target: string]: {[assetName: string]: string}
-} {
-  const entries: {[target: string]: {[assetName: string]: string}} = {}
-  for (const {target, build_manifest: buildManifest} of extensionPoints) {
-    if (!buildManifest?.assets) continue
-    const assets: {[name: string]: string} = {}
-    for (const [name, asset] of Object.entries(buildManifest.assets)) {
-      if (asset?.filepath) assets[name] = bundleFolder ? joinPath(bundleFolder, asset.filepath) : asset.filepath
-    }
-    if (Object.keys(assets).length > 0) entries[target] = assets
-  }
-  return entries
 }
diff --git a/packages/app/src/cli/services/build/steps/generate-ui-assets-manifest-step.test.ts b/packages/app/src/cli/services/build/steps/generate-ui-assets-manifest-step.test.ts
new file mode 100644
index 0000000000..e0b47f09f0
--- /dev/null
+++ b/packages/app/src/cli/services/build/steps/generate-ui-assets-manifest-step.test.ts
@@ -0,0 +1,107 @@
+import {executeGenerateUIAssetsManifestStep} from './generate-ui-assets-manifest-step.js'
+import * as generateManifest from './include-assets/generate-manifest.js'
+import {GenerateUIAssetsManifestStep, BuildContext} from '../client-steps.js'
+import {ExtensionInstance} from '../../../models/extensions/extension-instance.js'
+import {describe, expect, test, vi, beforeEach} from 'vitest'
+
+vi.mock('./include-assets/generate-manifest.js')
+
+const step: GenerateUIAssetsManifestStep = {
+  id: 'generate-ui-assets-manifest',
+  name: 'Generate UI Assets Manifest',
+  type: 'generate_ui_assets_manifest',
+  config: {bundleFolder: 'dist/'},
+}
+
+describe('executeGenerateUIAssetsManifestStep', () => {
+  let mockContext: BuildContext
+
+  beforeEach(() => {
+    vi.mocked(generateManifest.createOrUpdateManifestFile).mockResolvedValue()
+    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(),
+    }
+  })
+
+  test('writes manifest entries derived from build_manifest assets, prefixed with bundleFolder', async () => {
+    // Given
+    mockContext.extension.configuration = {
+      extension_points: [
+        {
+          target: 'admin.product-details.block.render',
+          build_manifest: {
+            assets: {
+              main: {filepath: 'main.js'},
+              styles: {filepath: 'main.css'},
+            },
+          },
+        },
+      ],
+    } as any
+
+    // When
+    await executeGenerateUIAssetsManifestStep(step, mockContext)
+
+    // Then
+    expect(generateManifest.createOrUpdateManifestFile).toHaveBeenCalledWith(mockContext, {
+      'admin.product-details.block.render': {
+        main: 'dist/main.js',
+        styles: 'dist/main.css',
+      },
+    })
+  })
+
+  test('omits the bundleFolder prefix when not configured', async () => {
+    // Given
+    const stepWithoutFolder: GenerateUIAssetsManifestStep = {
+      id: 'generate-ui-assets-manifest',
+      name: 'Generate UI Assets Manifest',
+      type: 'generate_ui_assets_manifest',
+    }
+    mockContext.extension.configuration = {
+      extension_points: [
+        {
+          target: 'admin.x',
+          build_manifest: {assets: {main: {filepath: 'main.js'}}},
+        },
+      ],
+    } as any
+
+    // When
+    await executeGenerateUIAssetsManifestStep(stepWithoutFolder, mockContext)
+
+    // Then
+    expect(generateManifest.createOrUpdateManifestFile).toHaveBeenCalledWith(mockContext, {
+      'admin.x': {main: 'main.js'},
+    })
+  })
+
+  test('does nothing when extension has no extension_points', async () => {
+    mockContext.extension.configuration = {} as any
+
+    await executeGenerateUIAssetsManifestStep(step, mockContext)
+
+    expect(generateManifest.createOrUpdateManifestFile).not.toHaveBeenCalled()
+  })
+
+  test('does nothing when no extension_point has a build_manifest with assets', async () => {
+    mockContext.extension.configuration = {
+      extension_points: [{target: 'admin.x'}],
+    } as any
+
+    await executeGenerateUIAssetsManifestStep(step, mockContext)
+
+    expect(generateManifest.createOrUpdateManifestFile).not.toHaveBeenCalled()
+  })
+})
diff --git a/packages/app/src/cli/services/build/steps/generate-ui-assets-manifest-step.ts b/packages/app/src/cli/services/build/steps/generate-ui-assets-manifest-step.ts
new file mode 100644
index 0000000000..427a889e9b
--- /dev/null
+++ b/packages/app/src/cli/services/build/steps/generate-ui-assets-manifest-step.ts
@@ -0,0 +1,57 @@
+import {createOrUpdateManifestFile} from './include-assets/generate-manifest.js'
+import {BuildManifest} from '../../../models/extensions/specifications/ui_extension.js'
+import {joinPath} from '@shopify/cli-kit/node/path'
+import type {GenerateUIAssetsManifestStep, BuildContext} from '../client-steps.js'
+
+interface ExtensionPointWithBuildManifest {
+  target: string
+  build_manifest: BuildManifest
+}
+
+/**
+ * Executes a generate_ui_assets_manifest build step.
+ *
+ * Reads `extension_points[].build_manifest.assets` from the extension
+ * configuration and writes the resulting target → asset map into manifest.json
+ * in the bundle output directory. Intended to run after a `bundle_ui` step in
+ * lifecycles that need a deployable manifest (deploy, dev) and to be omitted
+ * from the build-only lifecycle.
+ */
+export async function executeGenerateUIAssetsManifestStep(
+  step: GenerateUIAssetsManifestStep,
+  context: BuildContext,
+): Promise<void> {
+  const config = context.extension.configuration
+  if (!Array.isArray(config.extension_points)) return
+
+  const pointsWithManifest = config.extension_points.filter(
+    (ep): ep is ExtensionPointWithBuildManifest => typeof ep === 'object' && ep.build_manifest,
+  )
+
+  const entries = extractBuiltAssetEntries(pointsWithManifest, step.config?.bundleFolder)
+  if (Object.keys(entries).length > 0) {
+    await createOrUpdateManifestFile(context, entries)
+  }
+}
+
+/**
+ * Extracts built asset filepaths from `build_manifest` on each extension point,
+ * grouped by target. Returns a map of target → `{assetName: filepath}`.
+ */
+function extractBuiltAssetEntries(
+  extensionPoints: {target: string; build_manifest: BuildManifest}[],
+  bundleFolder?: string,
+): {
+  [target: string]: {[assetName: string]: string}
+} {
+  const entries: {[target: string]: {[assetName: string]: string}} = {}
+  for (const {target, build_manifest: buildManifest} of extensionPoints) {
+    if (!buildManifest?.assets) continue
+    const assets: {[name: string]: string} = {}
+    for (const [name, asset] of Object.entries(buildManifest.assets)) {
+      if (asset?.filepath) assets[name] = bundleFolder ? joinPath(bundleFolder, asset.filepath) : asset.filepath
+    }
+    if (Object.keys(assets).length > 0) entries[target] = assets
+  }
+  return entries
+}
diff --git a/packages/app/src/cli/services/build/steps/index.ts b/packages/app/src/cli/services/build/steps/index.ts
index bf0a52f4f3..25241a2e7a 100644
--- a/packages/app/src/cli/services/build/steps/index.ts
+++ b/packages/app/src/cli/services/build/steps/index.ts
@@ -2,6 +2,7 @@ import {executeIncludeAssetsStep} from './include-assets-step.js'
 import {executeBuildThemeStep} from './build-theme-step.js'
 import {executeBundleThemeStep} from './bundle-theme-step.js'
 import {executeBundleUIStep} from './bundle-ui-step.js'
+import {executeGenerateUIAssetsManifestStep} from './generate-ui-assets-manifest-step.js'
 import {executeBuildFunctionStep} from './build-function-step.js'
 import {executeCreateTaxStubStep} from './create-tax-stub-step.js'
 import type {LifecycleStep, BuildContext} from '../client-steps.js'
@@ -29,6 +30,9 @@ export async function executeStepByType(step: LifecycleStep, context: BuildConte
     case 'bundle_ui':
       return executeBundleUIStep(step, context)
 
+    case 'generate_ui_assets_manifest':
+      return executeGenerateUIAssetsManifestStep(step, context)
+
     case 'build_function':
       return executeBuildFunctionStep(step, context)