cost-burn

Burn-rate trend observability. The fourth leg of the cost-tracker forward-cost stack:

Algorithm

Implementation: 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

# 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

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.