Technically, format=auto makes the ImagePipe engine resolve the final format (AVIF, WebP, JPEG…) from the Accept header sent by the browser at request time, then caches the generated variant with a very long Cache-Control (immutable, one year). Fine for a single real visitor, but dangerous once that same URL gets reused for several different visitors.
The concrete risk: a cached HTML page (WordPress page cache, CDN, proxy) containing a format=auto URL freezes the format resolved at the moment THAT request filled the cache, potentially a search engine crawler or a cache warmer with no AVIF support, then serves that same frozen format to every subsequent visitor of that page, including those whose browser doesn't support it.
ImagePipe protects against this by design on its official integrations: the Gutenberg block and the WordPress shortcode render a real <picture> with one <source> per format, negotiation then happens when the browser loads the image, not when the page is generated. The API for custom themes deliberately defaults to webp rather than auto, precisely to avoid freezing a format into a URL meant to be reused.
If you call ImagePipe's low-level API directly, a custom theme, a bespoke integration, one simple rule to remember: never pass format=auto on a URL meant to be injected into cached HTML or served to multiple visitors. Prefer a <picture>/<source> for real browser-side negotiation, or an explicit fixed format if you only need a single <img>.