Fallback Models

Fallback Model Lists Overview

FastRouter supports fallback model lists, allowing you to provide multiple fallback models in a single request. If the primary model is unavailable or returns an error (e.g., due to rate limits, downtime, or moderation), FastRouter will automatically attempt to route the request to the next available model in your list.

How It Works

• Use the model field to define your primary model and the models array to define one or more fallback models.

• FastRouter will try the primary model first. If it fails, it will iterate through the list in order until a successful response is received or all models fail.

• The final model used is returned in the model field of the response body.

• Billing is based on the model that actually processes the request.

Example

curl --location 'https://go.fastrouter.ai/api/v1/chat/completions' \
--header 'Content-Type: application/json' \
--header 'Authorization: Bearer API-KEY' \
--data '{
    "model": "openai/gpt-4o",
    "models":["openai/o1", "google/gemini-1.5-pro"],
    "messages": [
        {
            "role": "user",
            "content": "Why would I need an LLM judge?"
        }
    ],
    "stream": true
} 
'

What Happens:

1. FastRouter first tries openai/gpt-4o.

2. If it fails (due to downtime, rate limit, moderation, etc.), it tries openai/o1.

3. If that fails, it then tries google/gemini-1.5-pro.

4. If all fail, the final error is returned to the user.