Standalone TypeScript script (zero deps, native fetch) to verify
Anthropic prompt caching is active. Documents 4 production gotchas
not in official docs: beta header required for Claude 4.x, effective
threshold ~2048+ tokens, cached tokens excluded from input_tokens,
new nested cache_creation response format.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
