Zero-Config CI with Describe Affected Auto-Detection

The atmos describe affected command now auto-detects the base commit in CI environments, eliminating the need for verbose flag wiring in your workflows.

What Changed

We introduced the --base flag as a unified replacement for --ref and --sha, and added automatic base resolution when running in CI with ci.enabled: true.

Before

- name: Describe affected
  run: |
    atmos describe affected \
      --ref ${{ github.event.pull_request.head.sha }} \
      --sha ${{ github.event.action == 'closed' && steps.get_parent.outputs.parent_commit || github.event.pull_request.base.sha }}

After

- name: Describe affected
  run: atmos describe affected

That's it. No flags needed. Atmos reads GitHub Actions environment variables and event payloads to determine the correct base commit for each event type.

How It Works

When ci.enabled is true in your atmos.yaml and no explicit --base flag is provided, each CI provider resolves the base commit automatically:

GitHub Actions Event	Base Resolution
Pull request (open/sync)	git merge-base(HEAD, origin/<target>) — auto-fetches the target branch in shallow CI checkouts so it works without fetch-depth: 0
Pull request (closed/merged)	git merge-base, falling back to HEAD~1 for merge-commit checkouts
Push	Previous HEAD from event payload (event.before)
Force push	Parent commit (HEAD~1)
Merge group	Target branch from GITHUB_BASE_REF

git merge-base is the gold standard for PR base resolution: it returns the fork point of the PR branch regardless of how out of date the PR is with <target>. If the PR was forked at commit B and main has since advanced to E, the diff is still computed against B — never against E — so commits on main that the PR hasn't pulled in do not show up as "affected".

The --base Flag

The new --base flag replaces both --ref and --sha with a single, intuitive flag that accepts either format:

atmos describe affected --base main
atmos describe affected --base refs/tags/v1.16.0
atmos describe affected --base 3a5eafeab90426bd82bf5899896b28cc0bab3073

The old --ref and --sha flags still work but are now deprecated.

Provider-Agnostic Architecture

Base resolution is part of the CI provider interface — each provider implements its own ResolveBase() method. GitHub Actions is the first implementation, with the architecture ready for GitLab CI, Jenkins, and other providers.

Get Involved

Try it out in your GitHub Actions workflows and let us know how it works. Open an issue at github.com/cloudposse/atmos with any feedback.