diff --git a/test/unit/package-manager-detection.test.ts b/test/unit/package-manager-detection.test.ts
new file mode 100644
index 0000000000..e175f4d09d
--- /dev/null
+++ b/test/unit/package-manager-detection.test.ts
@@ -0,0 +1,90 @@
+import { mkdtempSync, rmSync, writeFileSync } from 'fs'
+import { tmpdir } from 'os'
+import { join } from 'path'
+
+import { getPkgManager as getCreateNextAppPkgManager } from '../../packages/create-next-app/helpers/get-pkg-manager'
+import { getPkgManager as getCodemodPkgManager } from '../../packages/next-codemod/lib/handle-package'
+import { getPkgManager as getNextPkgManager } from '../../packages/next/src/lib/helpers/get-pkg-manager'
+
+const originalUserAgent = process.env.npm_config_user_agent
+const temporaryDirectories: string[] = []
+
+function projectWithLockfiles(...lockfiles: string[]) {
+  const directory = mkdtempSync(join(tmpdir(), 'next-pkg-manager-'))
+  temporaryDirectories.push(directory)
+  for (const lockfile of lockfiles) {
+    writeFileSync(join(directory, lockfile), '')
+  }
+  return directory
+}
+
+afterEach(() => {
+  if (originalUserAgent === undefined) {
+    delete process.env.npm_config_user_agent
+  } else {
+    process.env.npm_config_user_agent = originalUserAgent
+  }
+
+  for (const directory of temporaryDirectories.splice(0)) {
+    rmSync(directory, { recursive: true, force: true })
+  }
+})
+
+describe('package manager detection', () => {
+  test.each([
+    ['yarn', 'package-lock.json'],
+    ['pnpm', 'package-lock.json'],
+    ['bun', 'package-lock.json'],
+    ['npm', 'yarn.lock'],
+  ] as const)(
+    'the codemod prefers a %s user agent over a conflicting lockfile',
+    (packageManager, conflictingLockfile) => {
+      process.env.npm_config_user_agent = `${packageManager}/1.2.3 node/v22`
+      const project = projectWithLockfiles(conflictingLockfile)
+
+      expect(getCodemodPkgManager(project)).toBe(packageManager)
+    }
+  )
+
+  test('the codemod uses the shared package manager order when lockfiles coexist', () => {
+    delete process.env.npm_config_user_agent
+    const project = projectWithLockfiles(
+      'package-lock.json',
+      'bun.lock',
+      'pnpm-lock.yaml',
+      'yarn.lock'
+    )
+
+    expect(getCodemodPkgManager(project)).toBe('yarn')
+  })
+
+  test.each([
+    ['yarn', 'package-lock.json'],
+    ['pnpm', 'yarn.lock'],
+    ['npm', 'yarn.lock'],
+  ] as const)(
+    'Next.js prefers a %s user agent over a conflicting lockfile',
+    (packageManager, conflictingLockfile) => {
+      process.env.npm_config_user_agent = `${packageManager}/1.2.3 node/v22`
+      const project = projectWithLockfiles(conflictingLockfile)
+
+      expect(getNextPkgManager(project)).toBe(packageManager)
+    }
+  )
+
+  test('Next.js falls back to lockfile detection for a bun user agent', () => {
+    process.env.npm_config_user_agent = 'bun/1.2.3 node/v22'
+    const project = projectWithLockfiles('pnpm-lock.yaml')
+
+    expect(getNextPkgManager(project)).toBe('pnpm')
+  })
+
+  test.each(['yarn', 'pnpm', 'bun', 'npm'] as const)(
+    'create-next-app recognizes a %s user agent',
+    (packageManager) => {
+      process.env.npm_config_user_agent = `${packageManager}/1.2.3 node/v22`
+
+      expect(getCreateNextAppPkgManager()).toBe(packageManager)
+    }
+  )
+})