The fallback models to try, in order.
A middleware instance that handles model failures with fallbacks
import { createAgent, modelFallbackMiddleware } from "langchain";
// Create middleware with fallback models (not including primary)
const fallback = modelFallbackMiddleware(
"openai:gpt-4o-mini", // First fallback
"anthropic:claude-sonnet-4-5-20250929", // Second fallback
);
const agent = createAgent({
model: "openai:gpt-4o", // Primary model
middleware: [fallback],
tools: [],
});
// If gpt-4o fails, automatically tries gpt-4o-mini, then claude
const result = await agent.invoke({
messages: [{ role: "user", content: "Hello" }]
});
Middleware that provides automatic model fallback on errors.
This middleware attempts to retry failed model calls with alternative models in sequence. When a model call fails, it tries the next model in the fallback list until either a call succeeds or all models have been exhausted.