AI Cost CalculatorHow to route between AI models without confusing users
Practical model-routing decisions based on task risk, quality, cost and latency, plus ways to keep product behavior consistent when multiple models work behind the scenes.
Sending every request to the strongest model is simple and expensive. Sending everything to the cheapest one is also simple - right up until the hard requests fail and people start repeating themselves. Routing is the middle ground: a cheap model for the routine work, a stronger one for the tasks that actually need it.
The technical switch is usually the easy part. The hard part is the product design, because the customer sees one feature, not your routing table.
Route by task, not by "personality"
Start with the kinds of task you can name - sorting, pulling out facts, short drafts, fixing code, analysing several documents at once. Test the candidate models on real examples of each. A model that's brilliant at one doesn't get a permanent promotion to all of them.
Don't route on prompt length alone. A short legal question can carry far more risk than a long request to tidy up some notes. Factor in the tools needed, the format, the customer's plan, how fast it has to be, and how bad a wrong answer would be.
Keep the promise the same
Different models have different habits - one loves headings, one is terse, one refuses anything vague. Your app should smooth that over: decide what fields an answer must contain, reject malformed ones, keep the tone instructions in one place, and decide what happens when a model just can't finish.
Consistency doesn't mean every answer sounds identical. It means the feature keeps its promises. A date extractor returns the same fields every time. A writing tool respects the length you chose. A support assistant doesn't invent a policy just because one model is less careful.
Step up to a bigger model on evidence, not a hunch
Use the small model first where you can reliably tell when it failed. Format checks and task-specific tests are far stronger signals than asking the model "are you confident?" - self-reported confidence sounds precise but rarely is.
For genuinely risky tasks, going straight to the stronger model can be cheaper than trying the cheap one and then trying again. Cost the whole route, including the failed first attempts.
Test the routing like a product
Run a fixed set of examples through the whole router and measure good answers, speed, retries and total cost. Then quietly test changes on live traffic without showing customers the alternate answer. That tells you whether real traffic actually looks like the examples you designed the rules around.
Watch where requests land over time. If nearly everything gets bumped up to the big model, the small one is adding delay and cost for nothing. If nothing ever gets bumped up, your checks are probably too soft.
Explain the difference only when it matters
Most people don't need to know which provider answered an automatic background choice. They do need to know when a mode changes speed or quality. Describe that trade-off in plain terms they can act on.
Routing works when it cuts cost without making the experience unpredictable. The customer should notice that the feature works - not that three different models held a meeting behind the button.