This is so perplexing to me. I've definitely hit these kinds of issues (which usually result in me cursing at the agent in all caps while telling it to get its shit together!), but it's almost always a long ways into a session where I know context rot is an issue, and the assumption it's making is a dumb one but it's also in the middle of a complex task...I just haven't had anything remotely like the situation you're describing, where Opus 4.6 can't make a simple change and verify that it compiles, can't look up docs, can't follow your instructions, etc. Bizarre.