Cache lifetime restricts the age of eligible cache entries. The driver can require implementations no older than one hour:
$ llmc++ \
-fllm-cache-lifetime=3600 \
-fllm-offline main.cpp -o main
Age is measured from the cache file's modification time. Reading a hit does not refresh it. In this offline build, an expired entry causes failure; a normal online build can generate a replacement.
Function-level cache_lifetime(3600) overrides the driver value. Zero disables expiry, and explicit keys are still subject to the age check. Changing the command-line lifetime changes eligibility rather than cache identity.
Freshness is a policy, not a correctness test: a new implementation can be wrong, while an older reviewed implementation may remain suitable.