Under the hood
Custom system prompts can now be split into multiple cache segments
What
When Claude Code builds the system prompt used for analyzing context usage, a custom system prompt string is now passed through a new splitSystemPromptAtBoundary() helper instead of being included as one single block.
Why
Splitting the prompt at a boundary allows it to be broken into multiple segments for prompt caching (where Anthropic's API can reuse unchanged parts of a prompt across requests instead of reprocessing them), which can make repeated requests with a custom system prompt more efficient.