The billing header fingerprint was computed from the first user message text
via computeFingerprint(), which changes every conversation turn. This mutated
the system[] prefix on each request, invalidating Anthropic's prompt-cache
prefix and forcing ~100% cache_create (vs 96% cache_read with stable prefix).
Now uses a per-day SHA-256 hash of the date + ccVersion, keeping the billing
header format while preserving prompt-cache prefix stability across turns.
Includes 6 unit tests.
19edb8efa4