标签：#costengineering

Every AI video pipeline eventually has to answer an unglamorous question: what did we actually pay for that clip? On the main video-generation service, the answer for months had been "a hardcoded constant." That's fine until the vendor changes its own pricing, or a code path pays for the same synthesis twice, or a voice engine mints a clone, bills for it, and never sends it downstream. Over a ten-PR run I audited and rebuilt the voice and lip-sync pipeline from the billing layer up, then used the vendor's own SKU tiers to cut cost 7x without touching output quality. A cost model built from hardcoded constants isn't a cost model. It's a guess that happens to compile. Billing what the vendor actually charges PR #224 was workstream one of three from a sibling-tool audit: port the cost-accounting fixes that Presenter Generation and Variant Multiplier had already found, verifying each one against this repo's own code rather than assuming the same defect existed in the same place. Anthropic returns exact token counts on every response. Nothing in the pipeline read them — every charge was a hardcoded per-call constant, so the ledger and the vendor invoice diverged the moment usage drifted from whatever number had been typed in at launch. The same PR closed a second gap: two editor-facing routes could spend money — kicking off a generation, retrying a step — outside any run . A run is the unit everything else (budgets, audit trail, the cost ledger) is keyed to. A spend with no run attached is a spend the ledger can't even see, which is worse than a wrong number. Paying twice for a take the model returns unchanged PR #225 found the sibling bug's twin: some vendor calls return the exact same asset on a retry — no new synthesis happened — and the pipeline billed a second time anyway because "call succeeded" and "call did new work" were treated as the same fact. The fix is the boring, correct kind: hash the output, and only charge when the hash changes from the take you already