What regressions will a CI gate never catch?

Any regression whose cause never shows up in a diff. A CI gate runs the modified agent against a set of checks before a change merges, so its whole method depends on there being a change in the branch to run against. A provider updating the model behind a stable API name, a dependency’s behavior shifting on its own release schedule, or a downstream service changing its response format all regress an agent with nothing in any pull request to point the gate at. A CI gate was built to check a change your team is about to ship, so a regression with no diff behind it simply falls outside what that check can see. That’s why production monitoring exists as its own practice, separate from pre-merge testing: it watches what the agent actually does on a rolling basis, and catches a regression the gate never had a diff to run.