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eyre
7b3ed2a961
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.