Under the hoodTier: how much it should matter to you
1Useful: my rating, 1 to 5
0Signal: worth watching, 1 to 5
System PromptArea: what it touches
Bug FixesKind: in v2.1.319,
Bug FixesSection of the release
What
Claude Code sometimes rewrites a system message (a message of instructions it sends to the model alongside your conversation). It now makes a copy and edits the copy. Before, it edited the original message itself, which also changed that message wherever else it appeared in the conversation.
Why
Changing an earlier message in place can make the conversation sent to the model differ from one request to the next. That can stop the prompt cache from working well. The prompt cache is a store that lets the model reuse an unchanged start of the conversation instead of reading it again. Keeping the original untouched avoids that.
