New CLAUDE_CODE_MESSAGES_PROMPT_CACHE_TTL variable can force 5-minute caching

What

The prompt cache lets the model reuse earlier parts of a conversation instead of processing them again, for a set time called the TTL (time to live). CLAUDE_CODE_MESSAGES_PROMPT_CACHE_TTL is a new environment variable, a setting handed to Claude Code when it starts:

• When it equals 5m, the main conversation's cache TTL is forced to 5 minutes. Otherwise the usual TTL, 1h or 5m, is used unchanged.
• When the variable would change the TTL, the main conversation no longer gets its usual cache setting.
• It sits on the same list as CLAUDE_CODE_PROMPT_CACHE_TTL, CLAUDE_CODE_SUBAGENT_PROMPT_CACHE_TTL, CLAUDE_CODE_EFFORT_LEVEL and DISABLE_INTERLEAVED_THINKING, and in the configuration map.

Why

It is a separate control from CLAUDE_CODE_PROMPT_CACHE_TTL and FORCE_PROMPT_CACHING_5M, so you can shorten caching for the main conversation's messages on their own.