npm "409 Conflict" on Publish - Fix Duplicate/Concurrent Publish in CI

A 409 Conflict on publish means the registry rejected the PUT because it conflicts with current state - almost always the version already exists, or two release jobs tried to publish it at the same moment.

What this error means

npm publish fails with E409 409 Conflict - PUT. The release pipeline may have re-run, or two parallel jobs raced; either way the registry refuses to create a version that conflicts with what is already there.

npm error code E409
npm error 409 Conflict - PUT https://registry.npmjs.org/@acme%2fpkg
npm error Conflict: version 2.3.0 already exists

Common causes

The version already exists

A re-run of the release job tries to publish a version that a previous run already published. Registries are immutable, so the duplicate PUT conflicts.

Two release jobs published concurrently

Parallel pipelines (e.g. a matrix or a duplicate trigger) race to publish the same version, and one loses with a 409.

How to fix it

Guard publish to a new, single version

Only publish when the version is not already on the registry, and serialize the release job.

VER=$(node -p "require('./package.json').version")
if npm view "@acme/pkg@$VER" version >/dev/null 2>&1; then
  echo "already published; skipping"
else
  npm publish
fi

Serialize and de-duplicate releases

1. Use a concurrency guard so only one release job runs at a time.
2. Bump the version before publishing so each run targets a new one.
3. Make the publish step idempotent (skip-if-exists).

How to prevent it

• Run publish from one serialized release job.
• Skip publish when the version already exists.
• Bump the version on every release.

Related guides