The cheapest way to try a CI cost gate: run it in warn mode for a week

The objection I hear most about gating agent cost in CI isn't "I don't need it." It's "I'm not wiring up a new check that can fail my builds on day one." Fair. A gate that red-X's a PR the first afternoon it's installed — over a threshold nobody has calibrated yet — gets ripped back out by Friday.

So the way I actually roll one out now is: install it in report-only mode first, and let it be wrong for a week before it's allowed to block anything.

Concretely, that's three properties I'd look for in any cost gate, not just the one I work on:

1. A report-only mode that never fails the build. You want the cost delta to show up on every PR — the number, the top files, the trend — while the check itself stays green no matter what. That's the whole evaluation period: you're watching whether the gate's numbers track reality on your repo before you give it teeth. If a tool only ships in block-or-nothing form, you're calibrating in production against angry teammates.

2. A noise floor, so tiny diffs don't cry wolf. The commonest false positive for a percentage gate is a small base: a helper that goes from 10 tokens to 30 is +200%, and a naive gate fails the build over a fraction of a cent. You want an absolute floor — "only flag when the real increase is at least N tokens" — so the gate speaks up about the fixture someone pasted in, not about a two-line refactor.

3. A report that shows up even when there's no PR. Pushes to main, scheduled runs, manual dispatches — those have no PR to comment on, but they're exactly where a slow cost creep hides. A gate that also writes its table to the run summary (GitHub's $GITHUB_STEP_SUMMARY) keeps a cost record on every event, not just on pull requests.

The rollout that sticks: add it with enforcement off, watch the numbers for a sprint, set the threshold where it would have caught the two regressions you actually remember, then flip it to blocking. By then nobody argues with it, because they've watched it be right.

If you want that off the shelf: wartzar-bee/ci-guardrail is an Apache-2.0 GitHub Action (powered by @wartzar-bee/tokenscope) with a mode: warn that does exactly the report-only-never-fail thing, and it writes the same cost table to the Actions run summary on any event (push, schedule, manual) — not just on PRs — so the first PR you point it at can't break, and you decide when it earns the right to block.

For those of you who gate cost (or coverage, or bundle size) in CI: how long do you run a new gate in warn/report-only before you let it block — a week, a sprint, longer? Or do you just turn it to blocking on day one and eat the noise?