I just did something similar and it went swimmingly by doing this: Keep the plan and status in an md file. Tell it to finish one file at a time and run tests and fix issues and then to ask whether to proceed with the next file. You can then easily start a new chat with the same instructions and plan and status if the context gets poisoned.

I might give that a go in the future, but in this case it would've been faster for me to just do the work than to coach it for each file.

Also as this was an architectural change there are no tests to run until it's done. Everything would just fail. It's only done when the whole thing is done. I think that might be one of the reasons it got stuck: it was trying to solve issues that it did not prove existed yet. If it had just finished the job and run the tests it would've probably gotten further or even completed it.

It's a bit like stopping half way through renaming a function and then trying to run the tests and finding out the build does not compile because it can't find 'old_function'. You have to actually finish and know you've finished before you can verify your changes worked.

I still haven't actually addressed this tech debt item (it's not that important :)). But I might try again and either see if it succeeds this time (with plan in an md) or just do the work myself and get Opus to fix the unit tests (the most tedious part).