diff --git a/packages/app/src/cli/models/app/app.test-data.ts b/packages/app/src/cli/models/app/app.test-data.ts
index 5d7eb1fb90e..a7c285dd3d5 100644
--- a/packages/app/src/cli/models/app/app.test-data.ts
+++ b/packages/app/src/cli/models/app/app.test-data.ts
@@ -82,7 +82,7 @@ import {Project} from '../project/project.js'
 import {Session} from '@shopify/cli-kit/node/session'
 import {vi} from 'vitest'
 import {joinPath} from '@shopify/cli-kit/node/path'
-import {PackageManager} from '@shopify/cli-kit/node/node-package-manager'
+import {ProjectPackageManager} from '@shopify/cli-kit/node/node-package-manager'
 
 export const DEFAULT_CONFIG = {
   application_url: 'https://myapp.com',
@@ -154,7 +154,7 @@ export function testAppWithConfig(options?: TestAppWithConfigOptions): AppLinked
 
 interface TestProjectOptions {
   directory?: string
-  packageManager?: PackageManager
+  packageManager?: ProjectPackageManager | 'unknown'
   nodeDependencies?: Record<string, string>
   usesWorkspaces?: boolean
 }
diff --git a/packages/app/src/cli/models/project/project-integration.test.ts b/packages/app/src/cli/models/project/project-integration.test.ts
index ef15d47636e..328df103ca4 100644
--- a/packages/app/src/cli/models/project/project-integration.test.ts
+++ b/packages/app/src/cli/models/project/project-integration.test.ts
@@ -1,5 +1,6 @@
 import {Project} from './project.js'
 import {resolveDotEnv, resolveHiddenConfig, extensionFilesForConfig, webFilesForConfig} from './config-selection.js'
+import {requireProjectPackageManagerForOperations} from '../../utilities/project-package-manager.js'
 import {loadApp, reloadApp} from '../app/loader.js'
 import {AppLinkedInterface} from '../app/app.js'
 import {loadLocalExtensionsSpecifications} from '../extensions/load-specifications.js'
@@ -212,6 +213,19 @@ describe('Project integration', () => {
     })
   })
 
+  test('requireProjectPackageManagerForOperations errors when the app root has no package.json', async () => {
+    await inTemporaryDirectory(async (dir) => {
+      await setupRealApp(dir)
+      await removeFile(joinPath(dir, 'package.json'))
+
+      const project = await Project.load(dir)
+
+      expect(() => requireProjectPackageManagerForOperations(project)).toThrow(
+        /Could not determine the project package manager/,
+      )
+    })
+  })
+
   test('multi-config project discovers all configs', async () => {
     await inTemporaryDirectory(async (dir) => {
       await setupRealApp(dir)
diff --git a/packages/app/src/cli/services/dependencies.test.ts b/packages/app/src/cli/services/dependencies.test.ts
index 363bbe98c25..1b893848e06 100644
--- a/packages/app/src/cli/services/dependencies.test.ts
+++ b/packages/app/src/cli/services/dependencies.test.ts
@@ -28,4 +28,16 @@ describe('installAppDependencies', () => {
       deep: 3,
     })
   })
+
+  test('errors before install when the project package manager is unknown', async () => {
+    const project = testProject({packageManager: 'unknown', directory: '/tmp/project'})
+
+    await installAppDependencies(project)
+
+    const tasks = vi.mocked(renderTasks).mock.calls[0]![0] as any
+    const task = tasks[0]
+
+    await expect(task.task()).rejects.toThrow(/Could not determine the project package manager/)
+    expect(installNPMDependenciesRecursively).not.toHaveBeenCalled()
+  })
 })
diff --git a/packages/app/src/cli/services/dependencies.ts b/packages/app/src/cli/services/dependencies.ts
index 5ef3b16f0b4..0d62365fdee 100644
--- a/packages/app/src/cli/services/dependencies.ts
+++ b/packages/app/src/cli/services/dependencies.ts
@@ -1,4 +1,5 @@
 import {Project} from '../models/project/project.js'
+import {requireProjectPackageManagerForOperations} from '../utilities/project-package-manager.js'
 import {installNPMDependenciesRecursively} from '@shopify/cli-kit/node/node-package-manager'
 import {renderTasks} from '@shopify/cli-kit/node/ui'
 
@@ -14,7 +15,7 @@ export async function installAppDependencies(project: Project) {
       title: 'Installing dependencies',
       task: async () => {
         await installNPMDependenciesRecursively({
-          packageManager: project.packageManager,
+          packageManager: requireProjectPackageManagerForOperations(project),
           directory: project.directory,
           deep: 3,
         })
diff --git a/packages/app/src/cli/services/generate/extension.test.ts b/packages/app/src/cli/services/generate/extension.test.ts
index 8c97436e93d..5d739faeaa9 100644
--- a/packages/app/src/cli/services/generate/extension.test.ts
+++ b/packages/app/src/cli/services/generate/extension.test.ts
@@ -11,6 +11,7 @@ import * as functionBuild from '../function/build.js'
 import {
   checkoutUITemplate,
   testDeveloperPlatformClient,
+  testProject,
   testRemoteExtensionTemplates,
 } from '../../models/app/app.test-data.js'
 import {ExtensionTemplate} from '../../models/app/template.js'
@@ -91,6 +92,29 @@ describe('initialize a extension', async () => {
     })
   })
 
+  test('errors before installing extension dependencies when the project package manager is unknown', async () => {
+    await withTemporaryApp(async (tmpDir) => {
+      const app = (await loadApp({
+        directory: tmpDir,
+        specifications,
+        userProvidedConfigName: undefined,
+      })) as AppLinkedInterface
+
+      const result = generateExtensionTemplate({
+        extensionTemplate: checkoutUITemplate,
+        app,
+        project: testProject({directory: tmpDir, packageManager: 'unknown'}),
+        extensionChoices: {name: 'extension-name', flavor: 'vanilla-js'},
+        developerPlatformClient: testDeveloperPlatformClient(),
+        onGetTemplateRepository,
+      })
+
+      await expect(result).rejects.toThrow(/Could not determine the project package manager/)
+      expect(vi.mocked(installNodeModules)).not.toHaveBeenCalled()
+      expect(vi.mocked(addNPMDependenciesIfNeeded)).not.toHaveBeenCalled()
+    })
+  })
+
   test('successfully generates the extension when another extension exists', async () => {
     await withTemporaryApp(async (tmpDir) => {
       const name1 = 'my-ext-1'
diff --git a/packages/app/src/cli/services/generate/extension.ts b/packages/app/src/cli/services/generate/extension.ts
index ab50ff65835..71800423fb3 100644
--- a/packages/app/src/cli/services/generate/extension.ts
+++ b/packages/app/src/cli/services/generate/extension.ts
@@ -5,6 +5,7 @@ import {buildGraphqlTypes, PREFERRED_FUNCTION_NPM_PACKAGE_MAJOR_VERSION} from '.
 import {GenerateExtensionContentOutput} from '../../prompts/generate/extension.js'
 import {ExtensionFlavor, ExtensionTemplate} from '../../models/app/template.js'
 import {ensureDownloadedExtensionFlavorExists, ensureExtensionDirectoryExists} from '../extensions/common.js'
+import {requireProjectPackageManagerForOperations} from '../../utilities/project-package-manager.js'
 import {DeveloperPlatformClient} from '../../utilities/developer-platform-client.js'
 import {reloadApp} from '../../models/app/loader.js'
 import {
@@ -190,14 +191,16 @@ async function functionExtensionInit({
     taskList.push({
       title: 'Installing additional dependencies',
       task: async () => {
+        const packageManager = requireProjectPackageManagerForOperations(project)
+
         // We need to run install once to setup the workspace correctly
         if (project.usesWorkspaces) {
-          await installNodeModules({packageManager: project.packageManager, directory: project.directory})
+          await installNodeModules({packageManager, directory: project.directory})
         }
 
         const requiredDependencies = getFunctionRuntimeDependencies(templateLanguage)
         await addNPMDependenciesIfNeeded(requiredDependencies, {
-          packageManager: project.packageManager,
+          packageManager,
           type: 'prod',
           directory: project.usesWorkspaces ? directory : project.directory,
         })
@@ -258,7 +261,7 @@ async function uiExtensionInit({
     {
       title: 'Installing dependencies',
       task: async () => {
-        const packageManager = project.packageManager
+        const packageManager = requireProjectPackageManagerForOperations(project)
         if (project.usesWorkspaces) {
           // Only install dependencies if the extension is javascript
           if (getTemplateLanguage(extensionFlavor?.value) === 'javascript') {
diff --git a/packages/app/src/cli/utilities/project-package-manager.test.ts b/packages/app/src/cli/utilities/project-package-manager.test.ts
new file mode 100644
index 00000000000..2ecca3409d3
--- /dev/null
+++ b/packages/app/src/cli/utilities/project-package-manager.test.ts
@@ -0,0 +1,22 @@
+import {requireProjectPackageManagerForOperations} from './project-package-manager.js'
+import {describe, expect, test} from 'vitest'
+
+describe('requireProjectPackageManagerForOperations', () => {
+  test.each(['npm', 'pnpm', 'yarn', 'bun'])('returns %s for supported project package managers', (packageManager) => {
+    const result = requireProjectPackageManagerForOperations({
+      packageManager,
+      directory: '/tmp/project',
+    } as any)
+
+    expect(result).toBe(packageManager)
+  })
+
+  test('throws when the project package manager is unknown', () => {
+    expect(() =>
+      requireProjectPackageManagerForOperations({
+        packageManager: 'unknown',
+        directory: '/tmp/project',
+      } as any),
+    ).toThrow(/Could not determine the project package manager for \/tmp\/project/)
+  })
+})
diff --git a/packages/app/src/cli/utilities/project-package-manager.ts b/packages/app/src/cli/utilities/project-package-manager.ts
new file mode 100644
index 00000000000..1070ecae122
--- /dev/null
+++ b/packages/app/src/cli/utilities/project-package-manager.ts
@@ -0,0 +1,18 @@
+import {Project} from '../models/project/project.js'
+import {ProjectPackageManager} from '@shopify/cli-kit/node/node-package-manager'
+import {AbortError} from '@shopify/cli-kit/node/error'
+
+/**
+ * Narrows project package-manager metadata for install/mutation operations.
+ */
+export function requireProjectPackageManagerForOperations(
+  project: Pick<Project, 'packageManager' | 'directory'>,
+): ProjectPackageManager {
+  if (project.packageManager === 'unknown') {
+    throw new AbortError(
+      `Could not determine the project package manager for ${project.directory}. Add a package.json to the app root before running dependency operations.`,
+    )
+  }
+
+  return project.packageManager
+}