> how do you review all the code?

Code review is a skill, as is reading code. You're going to quickly learn to master it.

> It's like 20k of line changes over 30-40 commits.

You run it, in a debugger and step through every single line along your "happy paths". You're building a mental model of execution while you watch it work.

> One solution is to start from scratch again, using this branch as a reference, to reimplement in smaller PRs. I'm not sure this would actually save time overall though.

Not going to be a time saver, but next time you want to take nibbles and bites, and then merge the branches in (with the history). The hard lesson here is around task decomposition, in line documentation (cross referenced) and digestible chunks.

But if you get step debugging running and do the hard thing of getting through reading the code you will come out the other end of the (painful) process stronger and better resourced for the future.

Oh I didn't mean literally how do I review code. I meant, if an agent can write a lot of code to achieve a large task that seemingly works (from manual testing), what's the point if we haven't really solved code review? There's still that bottleneck no matter how fast you can get working code down.