Experiment measures how long Claude Code's prompt cache lasts

A developer measured prompt cache lifetime in Claude Code 2.1.295 on Opus 5.5: pauses up to 55 minutes changed nothing, but after 65 minutes the cache was gone and the same message cost 507% more. With a subscription within included usage the cache lasts an hour; with usage credits or an API key it lasts 5 minutes.

• Pauses up to 55 minutes kept cache reads intact at about $0.04 per message
• After 65 minutes the cache was gone: 27,460 tokens rewritten, cost rose to $0.253
• Cache TTL is 1 hour on subscription within included usage, otherwise 5 minutes
• A one-hour cache write on Opus 5.5 costs $8 per million tokens, a read $0.20