macOS runner: "Too many open files" (ulimit) in CI

By Kaveh Alemi·Latchkey

A process hit the per-process open file descriptor limit. macOS ships a relatively low default ulimit -n, so bundlers, test runners, and watchers that open many files at once fail with EMFILE.

What this error means

A build fails with "Error: EMFILE: too many open files" or "Too many open files" or "Too many open files - getcwd", often from a JS bundler, watcher, or a large parallel build.

Error: EMFILE: too many open files, open '/Users/runner/work/app/node_modules/...'
    at Object.openSync (node:fs:...)

Common causes

The default descriptor limit is too low

macOS sets a modest soft ulimit -n. Tools that open thousands of files concurrently exceed it and get EMFILE.

A file watcher tracks too many paths

Watch-mode tools open a descriptor per watched file; large trees blow past the limit.

How to fix it

Raise the open file limit for the job

1. Check the current limit with ulimit -n.
2. Raise the soft limit at the start of the step (up to the hard limit).
3. Run the build in the same shell so the new limit applies.

ulimit -n 65536
ulimit -n
npm run build

Reduce concurrent open files

Lower parallelism or disable file watching in CI so fewer descriptors are open at once.

CI=true npm test -- --maxWorkers=2

How to prevent it

• Raise ulimit -n early for file-heavy macOS jobs.
• Disable watch mode in CI.
• Cap parallelism so descriptor use stays under the limit.

Frequently asked questions

Related guides

References

• GitHub Actions documentation
• Workflow syntax for GitHub Actions