> ## Documentation Index
> Fetch the complete documentation index at: https://oximy.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Routing and fallbacks

> Configure pinned or automatic model selection, candidate sets, tradeoffs, and fallback behavior.

Routing policy controls requests that ask Relay to choose a model. Direct requests for a provider-prefixed model do not invoke automatic selection.

## Pinned routing

Pinned routing sends eligible requests to one configured model. Use it when consistency matters more than per-request optimization, or while establishing a baseline.

## Automatic routing

Automatic routing evaluates each request against the project's routable candidates. Configure:

* **Candidates**: the models Relay may select
* **Tradeoff**: cost, quality, or latency
* **Fallback model**: the deterministic model to use when the preferred selection cannot be completed

Start with a small candidate set whose modalities and tool support match the application. Expand only after comparison data shows a reason.

## Fallbacks

A fallback protects availability when the preferred route is unavailable or the routing engine cannot complete a selection. It is not a substitute for:

* provider retry and timeout policy
* a provider credential
* application-level handling in route-only mode
* compatible model capabilities

Exercise the fallback before production by using a controlled unavailable candidate or equivalent test environment.

## Understand a decision

Traffic records distinguish:

* `engine`: selected by automatic routing
* `policy`: selected by configured policy
* `fallback`: selected by fallback behavior
* no routing reason: the request named a model directly

When a custom router participates, Relay also records its identifier and name.

<CardGroup cols={2}>
  <Card title="Train a custom router" icon="brain-circuit" href="/docs/relay/custom-routers">
    Learn a workload-specific selector from real project traffic.
  </Card>

  <Card title="Replay decisions" icon="rotate-ccw" href="/docs/relay/reports-evals">
    Estimate how another policy would have routed historical traffic.
  </Card>
</CardGroup>