When an upstream API gateway (e.g. new-api) relays real Claude Code
requests, the User-Agent becomes Go-http-client while the body retains
the full Claude Code fingerprint (billing attribution block +
metadata.user_id + cache_control breakpoints).
Previously, the OAuth mimicry path relied solely on UA matching to
detect Claude Code clients. Without a matching UA, the gateway would
rewrite the system prompt — replacing the client's carefully structured
system blocks and cache_control breakpoints with its own injection.
This breaks Anthropic's prefix-based prompt cache: since the cache key
evaluates tools → system → messages in order, a changed system
invalidates all downstream message caching.
Symptoms observed:
- cache_read permanently locked at ~25K (only system prompt cached)
- cache_creation growing monotonically every turn (full messages rewrite)
- Single-request costs $17-27 instead of normal $1-2
Fix: when UA does not match but the body contains a valid billing
attribution block (x-anthropic-billing-header with cc_entrypoint=),
treat the request as proxied Claude Code traffic and skip mimicry.
This preserves the client's original system structure and cache_control
breakpoints, allowing Anthropic's prompt cache to function correctly.
eyre
7b3ed2a961