Opus 4.8 Mid-Convo System Messages in Python (Cache-Safe)

Why this matters right now

Anthropic shipped Claude Opus 4.8 on May 28, 2026 and quietly turned on a feature that long-running agents have been asking for since Opus 4.0: a role: "system" message you can drop into the middle of a conversation. The new instruction takes system-level priority from that point onward, and the conversation history before it stays byte-identical, so the prefix you already paid to cache still hits on the next request.