Composer "proc_open(): fork failed" in CI

Composer shells out to Git and other tools via proc_open. "fork failed - Cannot allocate memory" means the OS could not fork a child process because free memory (or the process limit) is exhausted. It is a memory-pressure symptom, not a Composer bug.

What this error means

composer install fails mid-run with "proc_open(): fork failed - Cannot allocate memory", often while cloning a Git dependency.

Composer

  [ErrorException]
  proc_open(): fork failed - Cannot allocate memory

Common causes

The runner is out of usable memory

A large solve plus a forked Git clone pushes memory past what the runner can grant a new process.

No swap and a tight memory ceiling

Small runners without swap fail to fork once resident memory is high.

How to fix it

Lower peak memory during install

Prefer dist archives so Composer downloads zips instead of forking Git clones.
Install from the lock to avoid the heavy solve.
Re-run once peak memory is reduced.

Terminal

composer install --no-interaction --prefer-dist

Use a runner with more memory or swap

Run the job on a larger runner so forking a child process has headroom.

How to prevent it

Prefer dist installs to avoid forking Git for every package.
Install from a committed lock to cut peak memory.
Size runners so composer plus its child processes fit in RAM.