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AssistantsModel configurations

Model fallback configuration

Configure a backup language model that activates automatically if your primary model fails

Overview

Model fallback lets a backup language model take over automatically if your primary model fails, so the call keeps going instead of ending in an error.

Model fallback is currently supported for the OpenAI and Anthropic Bedrock providers only. It is not yet a cross-provider fallback like voice fallback or transcriber fallback, which can fall back to a different provider entirely. Without fallbackModels configured, your call ends with an error if your chosen model provider fails.

How it works

When a model failure occurs, Vapi will:

  1. Detect the failure of the primary model.
  2. If fallbackModels is configured, switch to the first fallback model in the list.
  3. For OpenAI, continue through the rest of your specified list if subsequent failures occur. For Anthropic Bedrock, only one fallback model can be configured, so Vapi tries that single model.
  4. Terminate the call only if every configured fallback has also failed.

Fallback models stay on the same provider as your primary model. To protect against a provider-wide outage, pair model fallback with voice fallback and transcriber fallback on the other two pipeline components, which do support cross-provider fallback.

Configure via API

Add fallbackModels directly to your assistant’s model object. There is no separate fallbackPlan wrapper for models — unlike voice.fallbackPlan.voices and transcriber.fallbackPlan.transcribers, fallbackModels is a sibling property of model.model itself.

OpenAI

Set model.fallbackModels to an ordered list of OpenAI model IDs. Vapi tries them in order if the primary model fails.

{
  "model": {
    "provider": "openai",
    "model": "gpt-5.5",
    "fallbackModels": ["gpt-5.4", "gpt-4.1"]
  }
}

You shouldn’t need to set this unless you have a specific reason to — Vapi automatically finds the fastest fallback models that make sense for your primary model.

Anthropic Bedrock

Set model.fallbackModels to at most one same-provider Claude model ID available through Bedrock.

{
  "model": {
    "provider": "anthropic-bedrock",
    "model": "claude-sonnet-4-6",
    "fallbackModels": "claude-haiku-4-5-20251001"
  }
}

The fallback model is resolved using the call’s Bedrock credential region (or ANTHROPIC_BEDROCK_AWS_REGION). A fallback name with no inference profile in that region is skipped and a warning is logged — Vapi never silently remaps it to a US or global profile. On Vapi EU, a fallback name without an EU inference profile is rejected at write time. fallbackModels cannot be combined with thinking in this release.

Other providers

Vapi’s other language model providers — including Anthropic (non-Bedrock), Google, Groq, xAI, Mistral, MiniMax, and Custom LLM — do not currently expose a fallbackModels field. If your primary model is on one of these providers and it fails, the call ends with an error unless you handle retries at the application layer.

FAQ

Not yet. fallbackModels only lets you specify backup models from the same provider as your primary model. Cross-provider model fallback isn’t currently supported — use voice fallback and transcriber fallback to protect those two pipeline components against a provider-wide outage.

You’re billed normally for whichever model actually generates the response, whether that’s your primary model or a fallback.

fallbackModels is currently only available on OpenAIModel and AnthropicBedrockModel. It isn’t exposed for other providers yet.

Voice fallback configuration

Configure fallback voices that activate automatically when your primary voice provider fails.

Transcriber fallback configuration

Configure fallback transcribers so calls keep working if your primary speech-to-text provider fails.

Model providers

Compare every language model provider Vapi supports.