Terrantula + Atlantis
Keep Atlantis. It listens for PRs, plans in comments, and applies on approval — self-hosted, free, battle-tested. Terrantula doesn't replace any of that; it produces the PRs.
How they compose. A Terrantula Action validates the request, places the new instance into the right cell, and opens a pull request against your repository. Atlantis sees the PR on its existing webhook subscription and takes over — plan comment, review, atlantis apply — exactly as it does for a hand-written PR. No configuration changes on the Atlantis side.
What Terrantula adds that Atlantis doesn't have: everything before and after the run. Atlantis has no concept of a fleet, capacity, or lifecycle — it runs whatever PR appears. Terrantula decides which Terraform to change and where the instance lands, enforces the ceilings before the PR opens, and tracks the population afterward.
Zero SaaS required. Terrantula OSS + Atlantis + your own state backend is a complete setup with no subscriptions — self-hosting is a first-class path, not a downgrade.
Not for you if you don't stamp anything — a pets-only Atlantis setup needs no fleet layer.