I wired an AI code reviewer into CI, felt clever, and then looked at the bill after a busy week of PRs. Every push, every commit, sending full diffs to a big frontier model. It added up faster than I expected, and most of what it was reviewing was formatting changes and README edits that did not need a genius reading them. So I rebuilt it as two tiers, and the cost dropped hard without losing the catches that mattered.
The idea is simple. Do not send everything to the expensive model. Use a cheap model as a bouncer that decides what is even worth escalating, and reserve the big model (with caching turned on) for the files that could actually hurt you.
Tier one: a cheap triage pass
The first tier looks at the diff and answers one question: is anything here risky enough to justify a real review? That is a classification task, and classification does not need a frontier model.
I run this tier on a small local model. On my own machine that is Ollama with qwen2.5-coder, but in CI it can be a self-hosted small model or the cheapest cloud tier your provider offers. The job is triage, not judgment.
