OpenAI's Codex Cloud just added a new feature for code review, and their new GPT-5-Codex model has been specifically trained for code review: https://openai.com/index/introducing-upgrades-to-codex/
Gemini and Claude both have code review features that work via GitHub Actions: https://developers.google.com/gemini-code-assist/docs/review... and https://docs.claude.com/en/docs/claude-code/github-actions
GitHub have their own version of this pattern too: https://github.blog/changelog/2025-04-04-copilot-code-review...
There are also a whole lot of dedicated code review startups like https://coderabbit.ai/ and https://www.greptile.com/ and https://www.qodo.ai/products/qodo-merge/
Fundamentally, unit tests are using the same system to write your invariants twice, it just so happens that they're different enough that failure in one tends to reveal a bug in another.
You can't reasonably state this won't be the case with tools built for code review until the failure cases are examined.
Furthermore a simple way to help get around this is by writing code with one product while reviewing the code with another.
For unit tests, the parts of the system that are the same are not under test, while the parts that are different are under test.
The problem with using AI to review AI is that what you're checking is the same as what you're checking it with. Checking the output of one LLM with another brand probably helps, but they may also have a lot of similarities, so it's not clear how much.