The problem with auto compaction is that you aren’t given the opportunity to review its compacted understanding to confirm that it’s correct or doesn’t contain large omissions. I try to avoid letting it compact whenever possible and stick to plans that I review because it seems to get extremely dumb after an auto compaction.

Yeah, I still find Opus to be pretty unreliable once you get past around 150K tokens, so I usually run a custom hand-off command at that point that extracts specific elements to specialized documents. The command contains a "Documentation Map" with single line summaries of each of those documents to help the agent sort everything out. Like most memory systems, it works pretty well around 80% of the time. I messed around with RAG and other complex solutions, and I never got much better results than my KISS system.