GitHub Actions ubuntu-latest builds fail after runner image rollout without workflow changes

A repository configures GitHub Actions jobs with runs-on: ubuntu-latest. Builds that previously succeeded may fail on an unchanged commit when GitHub updates the hosted runner image behind the latest alias. The goal is to diagnose whether runner image drift, rather than a repository or workflow regression, explains the failure.

ubuntu-latest is not a fixed image release. GitHub can roll the alias forward to a newer Ubuntu runner image, changing preinstalled compilers, runtimes, and tool versions without modifying the Actions schema. No workflow-level setting freezes an existing latest run to a prior image. The documented alternatives are pinning to a specific runner image label or referencing a container image by digest, though older pinned images may stop receiving preinstalled tool updates.

Because no specific run, repository, or error is assumed, this is an unresolved behavior question about runner image stability.

• Does ubuntu-latest resolve to the same runner image across runs on different days?
• Which runner version metadata distinguishes image drift from a code or dependency regression?
• Is pinning to a specific runner image label the only supported way to reproduce a prior build environment?