I built kerf-cli because Claude Code told me not to worry about cost

A few weeks ago I logged into Claude Code, typed /cost, and got back this: With your Claude Max subscription, no need to monitor cost. Two days later I had used 91% of my weekly limit in a single morning of work. I had no idea which session, which project, or which model was responsible. I tried ccusage (which is great) and it gave me totals — but I wanted to ask questions like "which of my projects is eating Opus tokens unnecessarily?" and "what's my actual cache hit rate over the last 30 days?" Those answers weren't there. So I built kerf-cli — a local-first cost intelligence tool for Claude Code. This post is about why it exists, what it does, and what I learned about Claude Code billing along the way. The actual problem Anthropic gives you a lot of data and almost no analytics on top of it. Every Claude Code session is logged to ~/.claude/projects/<encoded-cwd>/<session-id>.jsonl with full token breakdowns: input, output, cache_read, cache_creation, model, timestamp, gi