Thanks. You're right that it's the weak spot, because the gate runs whatever tests are in the commit.

It isn't in RULES.md. It's in the review prompt Codex gets: every test change has to fail on the old behaviour, nothing can be weakened without a stated reason, and the reviewer has to check the diff for any weakened or deleted test. So a loosened assertion with no reason comes back as CHANGES REQUIRED, but it's the reviewer's call rather than a hard check in the script. A script that flags removed asserts in the diff would be a good addition, and I'll look at it.