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Kevin Codex 87e7e69afd feat(openai): forward prompt_cache_key for server-side prefix cache routing (#515)
* feat(openai): forward prompt_cache_key for server-side prefix cache routing

Send the session ID as OpenAI's prompt_cache_key on every completion
request so consecutive turns route to a backend replica that already
holds the conversation prefix in its prompt cache, instead of re-billing
the full ~11k-token system+tools prefix each turn on OpenAI-compatible
endpoints (which get no cache_control breakpoints, unlike Anthropic).

Keyless requests serialize byte-identically to before, and
ZERO_DISABLE_PROMPT_CACHE_KEY=1 suppresses the field for strict
endpoints that reject unknown parameters. The Codex (Responses API)
flavor is deliberately untouched: the ChatGPT backend is a separate,
stricter surface.

* fix(openai): parse prompt-cache-key kill switch as a boolean flag

ZERO_DISABLE_PROMPT_CACHE_KEY=0 or =false previously disabled forwarding
because any non-empty value tripped the check. Parse it the same way
ZERO_FORMAT_ON_WRITE parses boolean flags, so explicitly-falsy values
are a no-op and only truthy values flip the toggle.
2026-07-05 21:41:03 +08:00
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