diff --git a/.github/workflows/publish.yaml b/.github/workflows/publish.yaml
index 92c4360..dc74266 100644
--- a/.github/workflows/publish.yaml
+++ b/.github/workflows/publish.yaml
@@ -28,7 +28,7 @@ jobs:
         run: curl https://curl.se/ca/cacert.pem -o build/public-certs.pem
 
       - name: Zip Lambda
-        run: zip -r -qq proxy-lambda.zip build node_modules
+        run: bun run package
 
       - name: Upload Release Assets
         uses: softprops/action-gh-release@v3
diff --git a/.github/workflows/test.yaml b/.github/workflows/test.yaml
index 20e5099..f893afc 100644
--- a/.github/workflows/test.yaml
+++ b/.github/workflows/test.yaml
@@ -29,6 +29,23 @@ jobs:
       - name: Build
         run: bun run build
 
+      - name: Setup Node
+        uses: actions/setup-node@v7
+        with:
+          # Keep in sync with the Lambda runtime in lambda.tf
+          node-version: 20
+
+      - name: Smoke Test Lambda Bundle
+        # Load the handler from the packaged zip the way Lambda does: as CommonJS, with
+        # require(esm) and module syntax detection off, so an ESM bundle fails here too.
+        run: |
+          bun run package
+          unzip -q proxy-lambda.zip -d "$RUNNER_TEMP/lambda"
+          cd "$RUNNER_TEMP/lambda"
+          node --no-experimental-require-module --no-experimental-detect-module -e "
+            if (typeof require('./build/proxy.js').handler !== 'function') throw new Error('build/proxy.js does not export handler');
+          "
+
       - name: Test
         run: bun run test
 
diff --git a/package.json b/package.json
index 516f27f..40dedf2 100644
--- a/package.json
+++ b/package.json
@@ -4,7 +4,8 @@
   "main": "lambdas/proxy/index.js",
   "type": "module",
   "scripts": {
-    "build": "bun build lambda/proxy.ts --outdir build/ --target node",
+    "build": "bun build lambda/proxy.ts --outdir build/ --target node --format cjs",
+    "package": "zip -r -qq proxy-lambda.zip build node_modules",
     "format": "prettier --write .",
     "format-check": "prettier --check .",
     "test": "bun test"