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

# Fallback chains

> Route automatically to a backup model when the primary fails or rate-limits.

Fallback chains let you define **what happens when a model call fails**. Instead of the error bubbling up to your app, Infery transparently retries on a backup model of your choice.

## Use cases

* Primary **provider outage** — OpenAI 429s during peak hours → fall back to Google Gemini Flash
* **Cost-tier progression** — try `gpt-4o`, on rate-limit step down to `gpt-4o-mini`, then `gemini-flash`
* **Capability routing** — use a PDF-native model first; if unavailable, use a vision model with our PDF-to-image preprocessor
* **Regional** — EU customers fall back from an OpenAI model to a Google one hosted in EU

## Setting up a chain

Settings → **Fallbacks** → **New chain** (or edit an existing one).

A chain is attached to a **source model slug** and lists fallback models in priority order:

```
Source: gpt-4o
├─ Priority 1: gpt-4o-mini
├─ Priority 2: gemini-2-5-flash
└─ Priority 3: claude-haiku-4-5
```

## When fallbacks fire

The gateway steps through fallbacks when the primary (or prior fallback) fails with one of:

* `429 rate_limit_exceeded`
* `503 service_unavailable`
* `502 bad_gateway`
* Provider-specific errors tagged as retryable
* Network timeout / connection reset

**Not retried**: `4xx` errors from your code (bad prompt, invalid params, auth failure, quota exceeded).

## Transparent to the caller

Client code doesn't change. The response comes back in OpenAI format as normal, plus an extra header:

```
x-fallback-from: gpt-4o
x-model-used: gpt-4o-mini
x-fallback-depth: 1
```

Use these headers in logs/analytics to see which fallbacks are active in production.

## Cost accounting

The **final model that served the request** is billed. If fallback to a cheaper model succeeds, you pay the cheaper price. Rate-limit attempts don't incur cost.

## Disabling per call

Add header `x-disable-fallback: true` on an individual request to force primary-only behaviour (useful for testing which primary is actually up).