Claude Code keeps two cache buckets: the main conversation and everything else (subagents). Each control takes 5m or 1h; any other value is ignored. Requires v2.1.242 or later.
Use
If you sign in with an API key or a cloud provider, give the main conversation a one-hour cache:
{
"promptCacheTtl": "1h",
"subagentPromptCacheTtl": "5m"
}Environment variable equivalents: CLAUDE_CODE_PROMPT_CACHE_TTL and CLAUDE_CODE_SUBAGENT_PROMPT_CACHE_TTL. A bucket you do not set keeps the five-minute default. FORCE_PROMPT_CACHING_5M=1 overrides both.
Check the effect in /cost, which shows a per-session cache line. Mechanics: Don’t break the prompt cache.