Fixes #3110.

What was broken

createLogger({ file }) never wrote the file from an ESM entrypoint. openFileStream loaded fs with a lazy require() to keep fs out of the browser bundle graph (the entry is deliberately browser-safe — @objectstack/client bundles it). esbuild rewrites that to its __require shim in the ESM output, which throws Dynamic require of "fs" is not supported, and the surrounding bare catch {} swallowed it.

The workspace is type: module, so this hit every Node ESM consumer — os serve, os dev — while the CJS build kept working. File logging has effectively never run in production.

The fix

• Load the builtin via process.getBuiltinModule — a plain method call, so bundlers can't see it, working in both module systems, still behind the existing typeof process guard. require stays as a fallback for Node < 20.16, which predates it (engines allows >=18, so CJS on old Node must not regress).
• A file destination that can't be opened now says so on stderr. The silence is precisely what kept this hidden, per the issue. It deliberately bypasses level filtering: this reports that the logger cannot honour its own config, which isn't an application log event.

Three faults this un-hid

Opening the destination for the first time made latent code reachable. Each is verified load-bearing by reverting it alone and watching the matching test go red:

The third is the reason this isn't a one-line change: shipping only the require fix would have turned a silent no-op into a crash on a bad file path.

Verification

The issue's repro, against the built ESM dist:

$ node probe.mjs      # before
2026-07-17T11:27:18.312Z INFO esm file probe
file exists: false

$ node probe.mjs      # after
2026-07-17T11:41:36.072Z INFO esm file probe
file exists: true
contents: "2026-07-17T11:41:36.072Z INFO esm file probe\n"

Unwritable path — warns, keeps console logging, exits 0:

2026-07-17T11:41:36.099Z | INFO | console still alive
logger: file logging disabled — cannot write to /private/tmp/…: EISDIR: illegal operation on a directory
process survived, exit 0

Also checked: browser safety holds (no static fs/path import in the ESM dist; createLogger({file}) with no process global doesn't throw), @objectstack/client builds, @objectstack/core 24/24 test files pass with --force, eslint clean.

Note on the test

logger-esm-file.test.ts bundles the entry with esbuild and spawns a real node. That indirection is load-bearing: under vitest the source runs through vite-node where require is defined, so an in-process test of this code passes against the broken version. It fails on all 4 assertions at origin/main. It bundles rather than reading dist/ because turbo's test only dependsOn: ["^build"] (not its own build) and excludes dist/** from its inputs — a dist-reading test would be both unbuilt and mis-cached. This adds esbuild to core's devDeps (already in the lockfile via tsup; pnpm's strict linking wouldn't resolve it otherwise).

Rebased onto #3111, which landed on the same file and already makes the file destination plain-text — so no ANSI codes reach the now-working log file.

🤖 Generated with Claude Code