cost-burn

Burn-rate trend observability. The fourth leg of the cost-tracker forward-cost stack: | Question | Skill | |---|---| | "Have we crossed a threshold?" (reactive) | `cost-budget-check` | | "When will we cross a threshold?" (predictive) | `cost-projection` | | "Could we have spent less?" (comparative) | `cost-counterfactual` | | "Is daily burn ACCELERATING?" (trend) | **`cost-burn`** ← this | ## Algorithm Implementation: [`scripts/burn.mjs`](../../scripts/burn.mjs). 1. Read all `session-*` records from `cost-tracking` namespace. 2. Bin into `--bucket` duration windows (default `1d`) over `--lookback` (default `14d`). 3. For each bucket: `{n: sessions, spendUsd: sum(total_cost_usd)}`. 4. Compute `delta = latest.spendUsd - mean(prior non-empty buckets)`. 5. If `--alert-on-acceleration-pct N` is set: exit 1 when `deltaPct > N`. ## Smoke transcript (5 days @ $0.10/day, today $0.50 — 400% acceleration) ``` | Latest bucket spend | $0.500000 (1 sessions) | | Prior bucket mean | $0.100000 (4 non-empty buckets) | | **Delta (latest vs prior mean)** | **+$0.400000 (400.00%)** | # | Window | Sessions | Spend 0 | 2026-06-15 14:16 → 2026-06-16 14:16 | 1 | $0.500000 1 | 2026-06-14 14:16 → 2026-06-15 14:16 | 0 | $0.000000 2 | 2026-06-13 14:16 → 2026-06-14 14:16 | 1 | $0.100000 3 | 2026-06-12 14:16 → 2026-06-13 14:16 | 1 | $0.100000 ... ``` ## Drift alert exit code $ cost burn --bucket 1d --lookback 7d --alert-on-acceleration-pct 50 ⚠ ALERT: latest bucket $0.500000 is 400.0% above prior mean $0.100000 (threshold +50%) exit 1 $ cost burn --bucket 1d --lookback 7d --alert-on-acceleration-pct 500 ✓ latest bucket within +500% of prior mean (actual delta: 400.0%) — OK exit 0 ## CI integration ```bash # Fail the build if today's spend accelerated > 100% over the weekly mean cost burn --bucket 1d --lookback 7d --alert-on-acceleration-pct 100 || alert-oncall ``` The alert is independent of budget — it triggers on rate ACCELERATION even when total spend is well under budget. Catches "we shipped a hot loop that burns 10× normal" before the budget alarm goes off. ## Distinct from `cost-trend` | Skill | Data source | Question | |---|---|---| | `cost-trend` | `docs/benchmarks/runs/*.json` | "Is the benchmark drifting (win rate, latency)?" | | `cost-burn` | `cost-tracking` namespace | "Is production spend accelerating?" | Both useful; they answer different questions on different data. ## Edge cases - Sparse history (no prior non-empty buckets): alert is SKIPPED with a reason string, exit 0. Operators don't get spurious alerts on cold-start. - Prior buckets all $0 but latest > $0: delta is `Infinity`/`null` in JSON, marked `new` in the table. No alert fires (no baseline to compare against). - `--bucket` > `--lookback`: hard error, exit 2.