How to Generate an Image URL Using Auto=Format

Based on Sanity's image transformation documentation, there isn't a built-in fallback format parameter when using auto=format.

Here's how auto=format works:

When you use ?auto=format, Sanity automatically serves the most optimized format based on the browser's Accept header:

• WebP for browsers that support it
• AVIF for browsers that support it (though this is generated asynchronously, so the first few requests might get WebP instead)
• Falls back to PNG/JPG depending on the original source image format if the browser doesn't support modern formats

The key point: The fallback is determined by your original uploaded image format, not by a parameter you can specify. If you upload a PNG, browsers that don't support WebP/AVIF will get PNG. If you upload a JPG, they'll get JPG.

Your options:

1. Control the fallback by choosing your upload format: Upload your images as JPG if you want JPG as the fallback format for browsers that don't support WebP/AVIF.

2. Use the fm parameter explicitly: Instead of auto=format, you can specify the format directly with fm=jpg or fm=webp, but this means you lose the automatic format selection benefit.

3. Handle it client-side: Use picture elements with multiple source tags to specify your own fallback chain:

   <picture>
     <source srcset="image.jpg?fm=webp" type="image/webp">
     <source srcset="image.jpg?fm=jpg" type="image/jpeg">
     <img src="image.jpg?fm=jpg" alt="...">
   </picture>

Unfortunately, there's no single parameter like ?auto=format&fallback=jpg in Sanity's image pipeline. The fallback format is inherently tied to your source image format when using auto=format.