Why your Anthropic prompt caching probably isn't working (and the npm package I built to fix it)

I'm a solo developer with about five years of experience, mostly outside AI. The last few months I've been getting serious about it — reading docs, building small things with Claude, learning how it differs from the web APIs I'm used to. While I was setting up Anthropic prompt caching for a project, I got stuck on a question I couldn't easily answer: how do I know it's actually working? The docs explained the cache_control API and the 90% discount on cached tokens. But the only way to verify a...