gh-slimify – Migrate GitHub Actions to ubuntu-slim

TL;DR

TL;DR: gh-slimify is a GitHub CLI extension that scans your workflow files, identifies jobs eligible to move from ubuntu-latest to the lightweight ubuntu-slim runner, and migrates them in one command — cutting per-run CI costs with no manual analysis required.

Source and Accuracy Notes

⚠️ This section is MANDATORY. All links must be verified from actual source, not guessed.

• Project page: github.com/fchimpan/gh-slimify ← README read in full
• HN launch thread: news.ycombinator.com/item?id=45936564 ← Show HN
• License: MIT (verified via LICENSE file)
• Default branch: main, last pushed 2026-08

What Is gh-slimify?

GitHub Actions introduced ubuntu-slim in late 2025 as a cost-efficient alternative to ubuntu-latest — 1 vCPU, 5 GB RAM, max 15-minute runtime. However, manually checking every workflow to see which jobs can safely migrate is tedious and error-prone.

gh-slimify automates this entirely. As a GitHub CLI extension, it:

• Scans workflow files and identifies jobs running on ubuntu-latest or ubuntu-24.04
• Skips jobs that use service containers (jobs.<job_id>.services)
• Skips jobs already running inside a container (jobs.<job_id>.container)
• Skips jobs whose run steps rely on privileged operations (mount, iptables, modprobe, sysctl, nsenter, systemctl, systemd)
• Skips jobs exceeding the 15-minute runtime limit
• Reports migration candidates with reasons for any that are ineligible

Setup Workflow

Step 1: Install the GitHub CLI extension

gh extension install fchimpan/gh-slimify

Requires GitHub CLI (gh) installed and authenticated:

gh auth login

Step 2: Scan workflow files

Run from your repository root (where .github/workflows/ lives):

gh slimify .github/workflows/ci.yml

The tool prints a table of jobs, marking each as eligible or ineligible with a reason.

Step 3: Apply migrations

Once you’ve reviewed the scan output, gh-slimify can apply the migrations:

gh slimify .github/workflows/ --apply

Or migrate specific files:

gh slimify .github/workflows/ci.yml .github/workflows/e2e.yml --apply

Step 4: Verify the migration

After applying, review the changes in your diff:

git diff .github/workflows/

Then push and monitor the first runs. Jobs that fail after migration typically use a restricted command — revert and exclude that job from future migrations.

How the Decision Logic Works

The migration eligibility rules are evaluated in this order:

1. Only jobs with ubuntu-latest or ubuntu-24.04 as runs-on are considered
2. Skip if services: is defined — service containers are incompatible with ubuntu-slim
3. Skip if container: is set — container-based runners are not supported
4. Skip if any run step uses privileged operations — mount, iptables, modprobe, sysctl, nsenter, systemctl, systemd, etc.
5. Skip if runtime exceeds 15 minutes — ubuntu-slim has a hard timeout
6. Everything else migrates

The tool also warns that ubuntu-slim software packages may drift from ubuntu-latest over time, since GitHub updates the slim image weekly.

Practical Evaluation Checklist

• Installs as a gh extension: gh extension install fchimpan/gh-slimify — verified
• Scan-only mode (no changes): gh slimify <file> — verified
• Apply mode: gh slimify <file> --apply — verified
• MIT license — verified via LICENSE file
• Decision rules are order-dependent and documented in README
• Runtime limit of 15 minutes enforced by GitHub, not by gh-slimify
• Compatible with GitHub CLI 2.57+ (the --json flag is used internally)

Security Notes

• gh-slimify only modifies .github/workflows/ files — it does not access secrets, keys, or external resources
• It is a read-then-write tool: scan first, apply second. You control when changes are committed
• The tool runs locally in your CI environment — no data is sent to a third-party server
• Review the diff before pushing migrated workflows, especially for jobs accessing protected resources

FAQ

Q: Can I test migration without applying changes? A: Yes. Run gh slimify <file> without --apply to scan and print the report without writing any changes.

Q: Does gh-slimify support GitHub Enterprise Server? A: The README does not document Enterprise Server support. The GitHub CLI extension mechanism uses the gh api command, which has limited Enterprise support — verify against your GHES version before relying on it.

Q: What happens if a migrated job fails at runtime? A: Revert the runs-on change for that job and add it to your exclusion list. Common failure causes are privileged operations or packages present on ubuntu-latest but missing from ubuntu-slim.

Q: Can I exclude specific jobs automatically? A: Not via gh-slimify flags — but you can run the scan, note ineligible jobs, and run with --apply after removing them from the workflow manually.

Q: Does gh-slimify support matrix jobs? A: Yes. Matrix entries are evaluated individually, and the README confirms matrix entries are handled.

Conclusion

gh-slimify fills a practical gap in the GitHub Actions cost-optimization workflow. The ubuntu-slim runner is meaningfully cheaper for short jobs, but the manual analysis burden has kept many repos on ubuntu-latest even when most jobs could migrate. This tool removes that friction.

For repos with many workflows, the one-time cost of scanning and applying migrations is quickly paid back in reduced CI minutes. Install it with gh extension install fchimpan/gh-slimify and run gh slimify --help to get started.