Claude Code carries the session in metadata.user_id, which the Responses
API translation drops before the executor resolves a cache session. The
request then fell through to the assistant-text hash and the per-connection
fallback, so one conversation was split across several prompt_cache_key
values and the upstream prefix cache kept missing.
Fall back to the x-claude-code-session-id header, which survives every
translation. The body stays authoritative when both are present.
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