fix: getBundlerRuntime should respect buildOutputPath

.changeset/bundler-runtime-build-output-path.md

@@ -0,0 +1,14 @@
+---
+"@opennextjs/aws": patch
+---
+
+Fix `getBundlerRuntime` ignoring `buildOutputPath`
+
+`getBundlerRuntime` looked for the Next.js server output under `appPath`, which
+is always the project root and does not follow the `buildOutputPath` config.
+Every other consumer of the Next.js build output (`getBuildId`,
+`copyEnvFile`, `createServerBundle`, …) resolves it from `appBuildOutputPath`.
+
+As a result, setting `buildOutputPath` to anything other than `.` failed the
+build with `Unable to determine Next.js runtime (webpack or turbopack)` as soon
+as the project root no longer contained a stale `.next` directory.

packages/open-next/src/build/helper.ts

@@ -511,7 +511,10 @@ export function getPackagePath(options: BuildOptions) {
 export function getBundlerRuntime(
   options: BuildOptions,
 ): "webpack" | "turbopack" {
-  const dotNextServerPath = path.join(options.appPath, ".next/server");
+  const dotNextServerPath = path.join(
+    options.appBuildOutputPath,
+    ".next/server",
+  );
   if (fs.existsSync(path.join(dotNextServerPath, "webpack-runtime.js"))) {
     return "webpack";
   }