SEA: embedder main cannot dynamically import non-builtin modules on Node 25.5+

Version

v25.9.0 (also reproduces on v25.7.0, v25.8.x). Does not reproduce on v24.x.

Platform

Linux 6.17.0-19-generic x86_64

Subsystem

sea, esm, embedding

What steps will reproduce the bug?

Minimal repro, no external tools beyond postject:

mkdir -p /tmp/sea-repro && cd /tmp/sea-repro

cat > user.mjs <<'JS'
console.log('hello from user module');
JS

cat > bootstrap.js <<'JS'
import('file:///tmp/sea-repro/user.mjs').catch(err => {
  console.error(err);
  process.exit(1);
});
JS

cat > sea-config.json <<'JSON'
{
  "main": "bootstrap.js",
  "output": "sea-prep.blob",
  "disableExperimentalSEAWarning": true
}
JSON

node --experimental-sea-config sea-config.json
cp "$(command -v node)" ./app
npx postject ./app NODE_SEA_BLOB sea-prep.blob \
  --sentinel-fuse NODE_SEA_FUSE_fce680ab2cc467b6e072b8b5df1996b2
./app

Same failure occurs when:

• mainFormat is set to "module" and the bootstrap uses await import('file:///...')
• a CJS bootstrap calls require('/abs/path.js') on a non-builtin path

How often does it reproduce? Is there a required condition?

100% on Node 25.5+. The identical repro prints hello from user module on Node 24.14.0, so this is a regression introduced somewhere in the 25.5+ window (around the --build-sea landing in #61167 and the mainFormat: "module" support in #61813).

What is the expected behavior?

Dynamic import() (and require() of absolute paths) from the SEA main should resolve through the normal module loader. This is how SEA worked from v20 through v24 and is what enables the common pattern of a small bootstrap baked into the SEA that stages setup (VFS overlays, monkey-patches, diagnostics, ...) and then hands control off to a real user entrypoint on disk.

What do you see instead?

Error [ERR_UNKNOWN_BUILTIN_MODULE]: No such built-in module: file:///tmp/sea-repro/user.mjs
    at loadBuiltinModuleForEmbedder (node:internal/modules/helpers:165:9)
    at getBuiltinModuleWrapForEmbedder (node:internal/modules/esm/utils:237:10)
    at importModuleDynamicallyForEmbedder (node:internal/modules/esm/utils:250:10)
    at importModuleDynamicallyCallback (node:internal/modules/esm/utils:282:12)
    at bootstrap.js:1:1
    at embedderRunCjs (node:internal/main/embedding:93:10)
    at embedderRunEntryPoint (node:internal/main/embedding:128:12) {
  code: 'ERR_UNKNOWN_BUILTIN_MODULE'
}

Root cause

From lib/internal/modules/esm/utils.js in v25.9.0:

// For embedder entry point ESM, only allow built-in modules.
if (referrerSymbol === embedder_module_hdo) {
  return importModuleDynamicallyForEmbedder(specifier, phase, attributes, referrerName);
}

function importModuleDynamicallyForEmbedder(specifier, phase, attributes, referrerName) {
  // Ignore phase and attributes for embedder ESM for now, because this only supports loading builtins.
  return getBuiltinModuleWrapForEmbedder(specifier).getNamespace();
}

The CJS path has the same limitation: embedderRequire in lib/internal/main/embedding.js routes through loadBuiltinModuleForEmbedder, so require('/abs/path') from a CJS SEA main on Node 25.5+ also throws ERR_UNKNOWN_BUILTIN_MODULE.

This means the SEA main — whether mainFormat: "commonjs" or mainFormat: "module" — can only load builtin modules via its own require/import(), and cannot hand off to a user script.

Additional information

Workaround (for anyone hitting this in the wild): obtain Module via require('module') (which succeeds because module is a builtin), set process.argv[1] to the real entrypoint, then call Module.runMain(). Module.runMain uses the real CJS loader and, on Node 22.12+, transparently handles ESM entries via require(esm). Caveat: user entrypoints that use top-level await cannot go through this path — require(esm) rejects them — so there is currently no way to load a TLA-using ESM user entrypoint from an SEA main on Node 25.5+.

Request: route importModuleDynamicallyForEmbedder and embedderRequire through the default loaders (the same path source_text_module_default_hdo / vm_dynamic_import_default_internal use), so embedders — including SEA — can keep using dynamic import() / require() to hand off to a user entrypoint after setup.

Context: I'm the maintainer of yao-pkg/pkg (the maintained fork of vercel/pkg); pkg's enhanced SEA mode builds a small bootstrap that mounts a VFS and then hands off to the user entrypoint, which is exactly the pattern this regression breaks.

提供された bootstrap.js、sea-config.json、および postject コマンドを使って SEA の失敗を再現し、その後 lib/internal/modules/esm/utils.js と lib/internal/main/embedding.js を調査します。特に importModuleDynamicallyForEmbedder と embedderRequire を確認します。それらの動作をデフォルトのローダーのパスと比較し、SEA main が組み込みモジュールの読み込みを維持したまま、絶対パスの ESM および CommonJS エントリポイントに処理を引き渡せることを検証します。