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.