<!-- source: https://modelux.ai/docs/concepts/routing -->

> How routing configs work in Modelux.

# Routing

A **routing config** is a named, versioned resource that tells Modelux how to
handle a request. Your application calls Modelux with a routing config slug
(like `@production`) and Modelux decides which model(s) and provider(s) to
actually invoke.

Routing configs live in Modelux, not in your code. Change the routing
behavior without redeploying your app.

## Calling a routing config

Use the slug prefixed with `@` as the model name:

```python
client.chat.completions.create(
    model="@production",
    messages=[...]
)
```

You can also call raw model names (`gpt-4o`, `claude-sonnet-4-5`) directly —
Modelux auto-routes them to the matching provider using your credentials.

## Strategies

| Strategy | Description |
|---|---|
| `single` | Lock traffic to one model + provider. |
| `fallback` | Ordered list of attempts with per-attempt timeouts. Retries on 429, 5xx, timeout. |
| `cost_optimized` | Pick the cheapest model meeting a quality tier, from an allowlist. |
| `latency_optimized` | Route to the lowest-p50-latency healthy provider. |
| `ensemble` | Parallel fan-out + aggregation (voting, first-valid, weighted). |
| `ab_test` | Percentage-based split across sub-configs. |
| `cascade` | Sequential attempts with early stop on success. |
| `custom_rules` | Programmable DSL over cost, latency, budget, tags. |

## Versioning

Every save creates a new version. You can:

- **Diff** two versions side-by-side
- **Rollback** to any previous version with one click
- **Promote** a candidate from a simulation result

## Model aliases

Instead of hardcoding `gpt-4o-mini` in every request, create a routing config
at `@fast` or `@cheap` and reference those slugs. Change the underlying model
later without touching your app code.

## Tags

Tag requests with arbitrary key-value pairs to scope routing, analytics, and
budgets:

```python
client.chat.completions.create(
    model="@production",
    messages=[...],
    extra_body={
        "mlx:tags": {
            "tenant": "acme",
            "feature": "summarize",
        },
    },
)
```

Custom rules can branch on tags: `if tenant == "enterprise" then use @premium else use @production`.