cost-tracking

# Cost Tracking Use this skill to analyze Claude Code cost and usage history from the metrics log that ECC's `stop:cost-tracker` hook writes. ## Where the data lives The tracker appends one JSON object per session-stop to `~/.claude/metrics/costs.jsonl`. Each row is a **cumulative snapshot for that session**, so to total spend you take the **latest row per `session_id`** and sum across sessions — summing every row multiply-counts. ECC also maintains internal per-session files under `~/.claude/metrics/cost-snapshots/` so runtime hooks can read the current session total without rescanning all history. Treat those files as a rebuildable cache; each snapshot stores a byte cursor so only newly appended rows are scanned. Stable reads are O(1), while updates are O(new bytes). Stale entries are pruned after 30 days or when the directory exceeds 512 sessions. Cold catch-up work is limited to 16 MiB per hook invocation, and malformed unterminated rows larger than 1 MiB are discarded with a resumable cursor. Reports and exports should continue to use `costs.jsonl`. Row schema: | Field | Meaning | | --- | --- | | `timestamp` | ISO timestamp of the snapshot | | `session_id` | Claude Code session identifier | | `transcript_path` | Path to the session transcript | | `model` | Model used | | `input_tokens` / `output_tokens` | Token counts | | `cache_write_tokens` / `cache_read_tokens` | Prompt-cache token counts | | `estimated_cost_usd` | Precomputed cumulative cost in USD for the session | Prefer `estimated_cost_usd` over hand-calculating pricing — model and cache prices change, and the tracker is the source of truth. ## When to Use - The user asks "how much have I spent?", "what did this session cost?", or "what is my token usage?" - The user mentions budgets, spending limits, overruns, or cost controls. - The user wants a cost breakdown by model, session, or date, or a CSV export. ## How It Works First verify the log exists (use `node`, not `sqlite3` — the tracker writes JSONL, and `node` is cross-platform): ```bash node -e 'const fs=require("fs"),os=require("os"),p=require("path");const f=p.join(os.homedir(),".claude","metrics","costs.jsonl");console.log(fs.existsSync(f)?"cost log found":"cost log not found: "+f)' ``` If the log is missing, do not fabricate usage data. Tell the user that cost tracking populates after the first session ends with the `stop:cost-tracker` hook enabled. ## Example — summary, by model, last 7 days ```bash node -e ' const fs=require("fs"),os=require("os"),path=require("path"); const f=path.join(os.homedir(),".claude","metrics","costs.jsonl"); if(!fs.existsSync(f)){console.log("cost log not found: "+f);process.exit(0);} const rows=fs.readFileSync(f,"utf8").split(/\r?\n/).filter(Boolean).map(l=>{try{return JSON.parse(l)}catch{return null}}).filter(Boolean); const bySession=new Map(); for(const r of rows){const k=r.session_id||r.transcript_path||r.timestamp;const p=bySession.get(k);if(!p||String(r.timestamp)>String(p.timestamp))bySession.set(k,r);} const latest=[...bySession.values()]; const cost=r=>Number(r.estimated_cost_usd)||0, day=r=>String(r.timestamp||"").slice(0,10), sum=a=>a.reduce((s,r)=>s+cost(r),0), f4=n=>"$"+n.toFixed(4); const today=new Date().toISOString().slice(0,10), yest=new Date(Date.now()-864e5).toISOString().slice(0,10); console.log("today: "+f4(sum(latest.filter(r=>day(r)===today)))+" | yesterday: "+f4(sum(latest.filter(r=>day(r)===yest)))+" | total: "+f4(sum(latest))+" ("+latest.length+" sessions)"); const m=new Map();for(const r of latest){const k=r.model||"(unknown)";m.set(k,(m.get(k)||0)+cost(r));} console.log("by model:");[...m.entries()].sort((a,b)=>b[1]-a[1]).forEach(([k,v])=>console.log(" "+f4(v)+" "+k)); ' ``` For a session drilldown or CSV export, iterate the same `latest` set (or the raw rows for CSV) and print the fields you need. ## Reporting Guidance When presenting cost data, include today's spend vs yesterday, total across all sessions, a by-model breakdown, and session count. Format sub-dollar amounts with four decimals, larger amounts with two. ## Anti-Patterns - Do not sum every row — they are cumulative per session; reduce to the latest row per `session_id` first. - Do not estimate costs from raw token counts when `estimated_cost_usd` is present. - Do not assume the log exists without checking. - Do not hard-code current model pricing in user-facing answers. - Do not recommend installing unreviewed hooks or plugins that execute arbitrary code. ## Related - `/cost-report` - Command-form report over the same metrics log. - `cost-aware-llm-pipeline` - Model-routing and budget-design patterns. - `token-budget-advisor` - Context and token-budget planning. - `strategic-compact` - Context compaction to reduce repeated token spend.