Intercepting Fetch Requests

Many providers support setting a custom fetch function using the fetch argument in their factory function.

A custom fetch function can be used to intercept and modify requests before they are sent to the provider's API, and to intercept and modify responses before they are returned to the caller.

Use cases for intercepting requests include:

Logging requests and responses
Adding authentication headers
Modifying request bodies
Caching responses
Using a custom HTTP client

Example

import { generateText } from 'ai';
import { createOpenAI } from '@ai-sdk/openai';

const openai = createOpenAI({
  // example fetch wrapper that logs the input to the API call:
  fetch: async (url, options) => {
    console.log('URL', url);
    console.log('Headers', JSON.stringify(options!.headers, null, 2));
    console.log(
      `Body ${JSON.stringify(JSON.parse(options!.body! as string), null, 2)}`,
    );
    return await fetch(url, options);
  },
});

const { text } = await generateText({
  model: openai('gpt-3.5-turbo'),
  prompt: 'Why is the sky blue?',
});