getPromptCachingEnabled() returned true for all providers including
Azure Foundry, OpenAI, Gemini, and GitHub. This caused cache_control
blocks to be injected into every request sent to 3P endpoints.
Azure Foundry's strict Responsible AI content filter treats unexpected
Anthropic-specific fields (cache_control: { type: "ephemeral" }) as
a jailbreak signal and rejects the request with a 400 error — even
for a simple prompt like "hi".
Fix: return false early when provider is not firstParty, bedrock, or
vertex — the only providers that understand and support prompt caching.
Fixes#273
Related: #267 (Finding 1)
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
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