diff --git a/src/findDependencies.spec.ts b/src/findDependencies.spec.ts
index 176c0be..e655d77 100644
--- a/src/findDependencies.spec.ts
+++ b/src/findDependencies.spec.ts
@@ -7,6 +7,21 @@ import { findDependencies } from './findDependencies.js'
 const __dirname = new URL('.', import.meta.url).pathname
 
 void describe('findDependencies()', () => {
+	void it('should return a list of external dependencies', () => {
+		const { packages } = findDependencies({
+			sourceFilePath: path.join(__dirname, '..', 'cdk', 'lambda.ts'),
+		})
+		assert.equal(
+			packages.has('aws-lambda'),
+			true,
+			"Should include the 'aws-lambda' package",
+		)
+		assert.equal(
+			packages.has('id128'),
+			true,
+			"Should include the 'id128' package",
+		)
+	})
 	void it('should honor tsconfig.json paths', () => {
 		const { dependencies } = findDependencies({
 			sourceFilePath: path.join(
diff --git a/src/findDependencies.ts b/src/findDependencies.ts
index 4b77162..81f1ff2 100644
--- a/src/findDependencies.ts
+++ b/src/findDependencies.ts
@@ -16,6 +16,7 @@ export const findDependencies = (args: {
 	sourceFilePath: string
 	imports?: string[]
 	visited?: string[]
+	packages?: Set<string>
 	tsConfigFilePath?: string
 	importsSubpathPatterns?: Record<string, string>
 }): {
@@ -25,13 +26,18 @@ export const findDependencies = (args: {
 	 * @see https://nodejs.org/api/packages.html#subpath-patterns
 	 */
 	importsSubpathPatterns: Record<string, string>
+	/**
+	 * The external packages that the source file depends on
+	 */
+	packages: Set<string>
 } => {
 	const sourceFilePath = args.sourceFilePath
 	const visited = args.visited ?? []
 	const dependencies = args.imports ?? []
+	const packages = args.packages ?? new Set<string>()
 	let importsSubpathPatterns = args.importsSubpathPatterns ?? {}
 	if (visited.includes(sourceFilePath))
-		return { dependencies, importsSubpathPatterns }
+		return { dependencies, importsSubpathPatterns, packages }
 	const tsConfigFilePath = args.tsConfigFilePath
 	const tsConfig =
 		tsConfigFilePath !== undefined
@@ -71,6 +77,7 @@ export const findDependencies = (args: {
 		} catch {
 			// Module or file not found
 			visited.push(file)
+			packages.add(moduleSpecifier)
 		}
 	}
 	ts.forEachChild(fileNode, parseChild)
@@ -83,10 +90,11 @@ export const findDependencies = (args: {
 			visited,
 			tsConfigFilePath,
 			importsSubpathPatterns,
+			packages,
 		})
 	}
 
-	return { dependencies, importsSubpathPatterns }
+	return { dependencies, importsSubpathPatterns, packages }
 }
 
 const resolve = ({
diff --git a/src/packLambda.ts b/src/packLambda.ts
index e7d15a1..11445e4 100644
--- a/src/packLambda.ts
+++ b/src/packLambda.ts
@@ -49,10 +49,15 @@ export const packLambda = async ({
 	debug?: (label: string, info: string) => void
 	progress?: (label: string, info: string) => void
 }): Promise<{ handler: string; hash: string }> => {
-	const { dependencies: deps, importsSubpathPatterns } = findDependencies({
+	const {
+		dependencies: deps,
+		importsSubpathPatterns,
+		packages,
+	} = findDependencies({
 		sourceFilePath,
 		tsConfigFilePath,
 	})
+	debug?.(`dependencies`, [...packages].join(', '))
 	const lambdaFiles = [sourceFilePath, ...deps]
 
 	const zipfile = new yazl.ZipFile()
@@ -104,6 +109,9 @@ export const packLambda = async ({
 			JSON.stringify({
 				type: 'module',
 				imports: importsSubpathPatterns,
+				dependencies: Object.fromEntries(
+					packages.values().map((pkg) => [pkg, '*']),
+				),
 			}),
 			'utf-8',
 		),