I can tell you what helps me and maybe you can incorporate some of this in your own code as well.

1- Ensure you have Changelogs.md, Agents.md properly written to your likings.

2- Depending on what you are building there is always a way to add actual UI tests and takes screenshots of before and after. If its a related to web UI, you can use playwright and or similar but there is always an option to test UI. Worst case scnerio is when none of libs help you and in those cases, you can add automated tests for UI using computer control from agents and take screenshots.

3- When reviewing PRs, always use a totally different agent. For example if you use claude code for creating PR, then perhaps ask Codex or Cursor to review the PR in depth.

4- Prefer smaller multiple PRs over larger ones.

5- Last resort: If you don't have to review and check PRs yourself, perhaps consider hiring a QA engineer if you can afford one.

Last but not least, learn to freeze the scope and remember not all bugs and edge cases are worth covering.