integration-cost-modelinglisted

# Integration cost modeling ## The constraint Two meters are running. Yours: per SMS, per verification, per API call, per thousand tokens, per gigabyte of egress. Theirs: a prepaid data bundle bought in small increments, from which every byte you send is deducted. Both meters are invisible in code review, and neither appears in a design mock. The result is a well-built feature that costs more per user than it can possibly be worth, discovered a month after launch by someone reading an invoice. In markets where per-message and per-call prices vary several-fold between countries, cost is not a finance concern downstream of the build — it is a design input that determines the architecture. ## Hard rules | ID | Rule | Severity | |---|---|---| | COST-001 | Every user-facing action has a stated marginal cost before it is built. | critical | | COST-002 | Choose the delivery channel by cost per delivered outcome, not by convenience. | critical | | COST-003 | Rate-limit and de-duplicate every paid outbound message. | warning | | COST-004 | Cap AI and inference spend per session with a hard ceiling and a fallback. | warning | | COST-005 | Never spend the user's data allowance on telemetry or prefetch you chose. | warning | | COST-006 | Cache and batch third-party calls against a known per-call price. | warning | | COST-007 | Decide fail-open or fail-closed for every paid dependency, in writing. | warning | | COST-008 | Serve from a region near the user and count egress. | warning