This seam sits between finished work and the review of it. It exists so that an independent reviewer spends the session judging the work rather than verifying that it was ready to be looked at.
What crosses — one open pull request, and everything that makes it reviewable. The brief in its body, pasted whole rather than summarised, because it is the statement of intent the diff has to be judged against. The impact tier, which sets how deep the review goes. Green checks: the type checker, the linter, the tests and the documentation gate all passing already, so a failure found in review is the work's, not the environment's. A diff that stayed inside the file surface the brief named. A completed checklist. Every test-plan item an agent can run, actually run, with the real command output posted rather than a claim about it. And every document the brief named, updated in the same change.
The hand-off is malformed when — the brief is missing, paraphrased or edited; when the checks are red; when the diff reaches outside the named surface; when a checklist item is ticked without evidence; when a test-plan item is claimed rather than shown; or when a promised document did not move. Each of those sends the change back rather than becoming a review finding, because none of them is a judgement call.
What it does not carry — the authority to fix anything. The reviewer reports and the author repairs; the reviewer never edits the code, never merges, and never writes status. It also does not carry a second opinion on taste: a change is judged against its brief, the product's specification, and safety, not against how the reviewer would have written it.
How it physically runs — the carrier is the open pull request: the diff, plus the body holding the brief. Coverage of the documents a change must touch is already enforced mechanically before review begins, which is why the reviewer's remaining job is the question no check can answer — whether the update is true, or a no-op edit that silenced the gate without describing the change. The verdict lands as comments on the pull request, with a severity on every finding, and a change cannot merge without a clean one.