Which Skill Is Quietly Burning Your Tokens? Find Out From transcript.jsonl

Your monthly Claude Code bill went up 20%. You know that much. What you don't know is which Skill did it — and nothing in the tooling will tell you. Run /usage in Claude Code and you get claude-sonnet-4-6: ¥3,240 — a per-model total and nothing else . "More expensive than last week" is visible. "Which Skill caused it" is not. usage-breakdown.sh closes that gap. It's a 106-line shell script that parses transcript.jsonl with Python and tallies call counts per Skill, Agent, and MCP server using Counter . This article walks through how the script works and how to run it, with the actual code and actual numbers. Why This Approach Works What Claude Code Is Actually Recording Claude Code streams every operation during a session into .jsonl files under ~/.claude/projects/ . It's JSONL — one event per line, one file per session. The files sit under a <project-id>/ directory. The skeleton of a single record looks like this: { "message" : { "role" : "assistant" , "content" : [ { "type" : "tool_use" , "name" : "Skill" , "input" : { "skill" : "pre-completion-self-audit" } } ] } } Inside message.content[] sit "type": "tool_use" blocks. The name field is the name of the tool that was invoked. The Bash tool, the Edit tool, the Skill tool, the Agent tool, MCP calls — all of it is recorded in this same format. Once I noticed that, the thought was: run this through a Counter and everything becomes visible. For the Skill tool, the skill name lives in input.skill ; for the Agent tool it's input.subagent_type ; and for MCP servers, the tool-name convention mcp__<server>__<tool> lets you extract the server name by splitting on __ . The structure is consistent, so the parser comes out surprisingly simple. What /usage Doesn't Tell You What Claude Code's /usage command outputs is a per-model cost total for a period. Model Cost claude-sonnet-4-6 ¥3,240 claude-opus-4-8 ¥ 892 Useful as far as it goes, but the breakdown of that cost is invisible . You can't see which session, which Skill, how ma