Opus 5 is remarkably bad at instruction following over long chats. I have to repeat “Be succint”, “talk like a friend or colleague would”, “no rambling” or some variant of it every few messages
Have you tried a custom output style? CLAUDE.md instructions are appended to the system prompt. A custom output style can replace the system prompt. At least the part of it pertaining to voice and persona. The reason it forgets over long chats is the context size starts getting too large. Instructions weigh more strongly the later they appear in the context. This is necessarily true otherwise you couldn't change your mind in a conversation. The model would stick with what you originally said. For the output styles, there is a per-turn "reminder" that gets added to the context asking it to "remember" the content in the system prompt. That's why it has more staying power than the CLAUDE.md instructions in long conversations.
