---
alwaysApply: false
globs: ["**/*"]
description: "Monthly cost per billed resource, its assumed traffic and its ceiling. Use when adding paid infrastructure or before going live."
---

# Cost And Capacity Ceilings

**Produce a monthly cost estimate per billed resource.** One line per thing
that can appear on an invoice: compute, storage, egress, queue operations,
per-request paid calls, e-mail sends, managed databases, log ingestion. Each
line states the price basis, the assumed monthly volume, and the resulting
amount.

**State the traffic the estimate assumes.** Write the number of requests,
users or records the estimate is built on. An estimate without its assumed
volume cannot be checked and cannot be exceeded on paper.

**State the load at which the estimate stops holding.** Name the first
resource that runs out and what happens then: a slower response, a rejected
request, a larger bill, or a stopped service. Name which of those it is.

**Give every resource that grows with usage a ceiling.** Storage, queues, log
volume and per-request paid calls each get a stated limit and the behaviour at
the limit. A limit with no behaviour is not a ceiling: write whether the
product refuses new work, sheds the cheapest work, or keeps spending.

**Alert the owner before the bill surprises them.** Configure the alert to
fire before spend passes 150% of the estimate `[ASSUMPTION A-8]`. The alert
names the resource that grew, states the amount, and states the one action the
owner can take. Send it to a channel the owner reads daily `[ASSUMPTION]`.

**State the cost before going live, in the owner's currency and per month.**
Include one sentence on what happens to the cost when the product succeeds and
traffic multiplies.

**A reviewer checks:**

- every billed resource has an amount, a price basis and an assumed volume;
- every resource that grows with usage has a stated ceiling and a stated
  behaviour at that ceiling;
- an alert exists that fires before 150% of the estimate, and it has been
  tested by lowering the limit once;
- the alert reaches the owner and names one action;
- the breaking load is stated as a number, not as "high traffic".