Proofbook Gate Action

The pull-request instrumentation gate (mode 3). Not a runtime check: production traces do not exist at PR time. The gate checks that the code still emits the events that back the controls you have been sealing evidence for. If an engineer deletes the last approval checkpoint, the PR fails with the control's name and the removed file:line, before the evidence quietly reads unevaluable a month later.

name: gate
on: pull_request
jobs:
  gate:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: proofbook-dev/gate-action@v1
        with:
          baseline: origin/main
          fail-on: control-regression

What it will and will not flag

Fails only when an event type backing an evidenced control drops to zero emitting call sites relative to the baseline lock. It will not flag removing three of four model-call sites, refactors that move instrumentation, or controls that were never evidenced in the first place. A blocking check earns exactly one false positive before the team adds --skip forever, so the gate under-reports by design.

Where the baseline comes from

.proofbook/instrumentation.lock, read from the baseline ref. The lock is written by proof seal after each evidence run (recording which controls actually reached an evidenced verdict), or bootstrapped with proof gate --write. Commit it. No lock on the baseline means nothing to enforce: the gate passes and says why, so adopting it never breaks a repository's first CI run.

A failure message reads:

✗ Control eu-ai-act-a14-checkpoints (Human oversight is exercised and recorded in operation)
  lost its only evidence source: no code site still emits HumanCheckpoint.
  the last emitting site, now removed:
    src/agents/claims.ts:214 (proofbook.human_checkpoint.type)