OpenAI / Codex / Azure-OpenAI use automatic prefix caching: the upstream
caches the longest matching prefix of a request (system prompt + earliest
messages) WITHOUT any explicit cache_control markers. The cache-aware
compression guard only protected that prefix when the body carried explicit
cache_control, so for automatic-cache providers the guard was skipped — and
with compression active + preserveSystemPrompt:false (or a prefix-compressing
mode) it rewrote the prefix, guaranteeing a cache miss and higher token spend
through OmniRoute than going direct.
getCacheAwareStrategy now treats isCachingProvider alone as sufficient to skip
the system prompt and downgrade aggressive/ultra (the explicit cache_control
path is a subset). openai/codex/azure are added to CACHING_PROVIDERS so they
are recognized as automatic-cache providers (this also activates the intended
prompt_cache_key cache-routing hint for OpenAI in chatCore).
Compression remains off by default — this only affects operators who enabled
it with prefix preservation turned off.
TDD: tests/unit/compression-cache-guard-3955.test.ts (RED 5/7 fail → GREEN
7/7). Aligned the existing cachingAware / strategySelector-cache-aware /
cache-control-policy / cache-control-claude-providers tests that encoded the
old (buggy) "openai is non-caching" behavior.
Refs #3955
62e0920e5e