# Osysharp.LlmRouter — the cheapest model that is good enough for each kind of work

> `Osysharp.LlmRouter` is a routing policy for an app with more than one model: filters by what a model can do, a quality floor for each kind of work, a cost ceiling, then the cheapest model that clears them — in an order that is also the order they are tried in when one fails.

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## Summary        {#summary}
`Osysharp.LlmRouter` is a routing policy for an app with more than one model: filters by what a model can do, a quality floor for each kind of work, a cost ceiling, then the cheapest model that clears them — in an order that is also the order they are tried in when one fails.

Without a router an app uses its default model for everything. With this one, no call site names a model: classifying an inbox goes to a small model and drafting a contract clause to a careful one, because the policy says so in one place.

|  |  |
|---|---|
| Package | `Osysharp.LlmRouter`, version 0.3.0 |
| You add it with | `use Osysharp.LlmRouter;` in the app's manifest. |
| It sits on | [[agent-kit-llm\|Llm]] |
| Its own test app | A print shop with three models and a policy over them — `kits/llm-router/tests` |

## Description    {#description}

### What do you get without writing anything?   {#free}
- Every call is routed, including the calls agents make.
- The reason a model was chosen is written on the meter beside the call.
- When money for a piece of work runs short, the router steps down to a cheaper model that still clears the floor.

### When do you need it?   {#when}
When your app lists more than one model and you do not want each call to choose. Any shape.

### What does it expose?   {#exposes}
The tables below are generated from the kit itself, not written by hand: `osy kits Osysharp.LlmRouter --surface` lists each of
them with the code of the kit's own test app that uses it, and the tests that show it working.

**Types you use**

| Type | What it is |
|---|---|
| `RoutedModel` | One model this policy may serve with, and what it can do — declared by the app that names the router, because only the app knows which models it has credentials for. |
| `QualityFloor` | The lowest `Quality` that may serve one kind of work. |
| `StepFloor` | The lowest `Quality` that may serve one STEP of a larger job — "plan with rank 3, fill with anything". |
| `CostTieredRouter` | A cost-tiered router — the policy most apps actually want, in the order the decisions have to happen. |

## What to read next   {#read-next}
- [Osysharp.Llm — every call to a model: which one, what it may cost, what it did cost](https://osysharp.com/reference/agent/kit-llm/) — every call to a model: which one, what it may cost, what it did cost
- [Osysharp.Models.Anthropic — Anthropic's models as a provider](https://osysharp.com/reference/agent/kit-anthropic/) — Anthropic's models as a provider
- [The map of the kits — which sits on which, and what each one is for](https://osysharp.com/reference/agent/guide-kits/) — the map of the kits — which sits on which, and what each one is for

## Go deeper   {#see-also}
- [[agent-llm-calls#routing]] — how the model for a call is chosen
- [Model spend — budgets, the meter and the price book (Llm kit)](https://osysharp.com/reference/agent/llm-spend/) — what each call cost