npm "Tracker idealTree already exists" - Fix Concurrent Install Error

npm builds an in-memory "idealTree" while installing and guards it so only one install runs at a time. This error means a second npm started while another was still going, or a previous one died without cleaning up.

What this error means

An install fails almost immediately with "Tracker "idealTree" already exists". It often shows up when a build script kicks off two installs in parallel, or after a cancelled job left npm in a bad state.

npm output

npm error Tracker "idealTree" already exists
npm error A complete log of this run can be found in: .../_logs/...-debug-0.log

Common causes

Two npm installs running concurrently

Parallel CI steps, a Makefile target, or a postinstall hook that itself runs npm install can start a second install while the first holds the tree, triggering the guard.

A leftover lock from a killed install

If a prior install was cancelled or OOM-killed, a stale _cacache/lock state in ~/.npm can make the next npm think a tree is already in progress.

How to fix it

Serialize installs and clear stale state

Ensure only one install runs, then clear the npm cache locks before retrying.

Terminal

# make sure nothing else runs npm at the same time
rm -rf node_modules
npm cache clean --force
npm ci

Remove the concurrency

Audit postinstall/prepare scripts for nested npm install calls.
In CI, do not run two install steps for the same project in parallel.
If a matrix shares a cache dir, give each leg its own npm cache (--cache).

How to prevent it

Run exactly one install per project, sequentially.
Avoid lifecycle scripts that re-invoke npm install.
Give parallel jobs isolated npm cache directories.