Opus 5 doesn't really even speak coherent English. I'm not sure what's going on, but it can't explain anything. It still does an excellent job with code and writing tests and code review and creating and completing a plan, and it seems to be able to understand English instructions, but it sure as hell can't explain what it did or how to use the code it wrote.

That was true before they announced the watermarking, I'd already started to back off of using Opus as much because I like to understand what the model is doing and have it write documentation I can use to reproduce its results, but maybe watermarking was already in there unannounced.