A regenerate-and-commit job must produce output that is a pure function of its input

Fact

A scheduled job that regenerates a file and commits it when the file changed only stays quiet if the generator is deterministic with respect to its upstream input. A build timestamp — Generated at 2026-09-10 14:22 — makes every byte of the output a function of the clock as well, so the diff is never empty and the job commits on every tick.

The same applies to any incidental non-determinism: an unsorted dict or set iteration, a random id, a locale-dependent format.

Why it matters

The failure is not loud. The job works, the site is correct, and the only symptom is a commit history filling with identical-looking commits — which then makes the real change, the one that added a new entry, impossible to find. It also defeats the git diff --quiet guard people reach for to keep the job cheap, so every run pushes and every push re-triggers downstream builds.

How to apply

• Derive any displayed "freshness" from the data, not from the clock: the newest updated_at across the fetched records says something true and changes only when the records do.
• Sort every collection before rendering it.

• Guard the commit on content, and let an unchanged run be a no-op:

  git diff --quiet -- out.html && { echo "No change."; exit 0; }

• Test it by running the generator twice with no upstream change and diffing; identical output is the contract.