LLM Best Practices

Astro: Image Optimization

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Overview

astro:assets optimizes images with Sharp (the default service). <Image /> transforms a local or authorized remote image, infers dimensions from the file to prevent layout shift, and outputs WebP by default; <Picture /> adds a <picture> element with multiple formats. Both default to loading="lazy" and decoding="async" and require alt (use alt="" for decorative images).

Import local images and let Astro infer the size

Keep source images in src/assets/ and import them. Astro reads width and height from the file.

---
import { Image } from "astro:assets";
import hero from "../assets/hero.jpg";
---
<Image src={hero} alt="A description of the image" />

Pass format="avif" to change the output format. Images in public/ are served as is and never transformed; put favicons, pre-sized OG images, and files referenced by external URLs there. A large hero JPEG in public/ is the usual cause of a heavy Lighthouse image warning. Remote and public/ images need explicit width and height, or inferSize, which fetches remote dimensions only for authorized domains.

Use <Picture /> for several formats and widths

<Picture /> takes formats (default ["webp"]) and a fallbackFormat (PNG for static images). Pass widths and sizes together, because without sizes the browser assumes 100vw.

<Picture src={hero} widths={[400, 800, 1200]} sizes="(max-width: 720px) 100vw, 800px"
  formats={["avif", "webp"]} alt="Hero image" />

Mark the LCP image with priority

Add priority to the above-the-fold image (Astro 5.10+). It sets loading="eager", decoding="sync", and fetchpriority="high". You can still override each attribute. Use it on one image per page.

<Image src={hero} alt="Hero" priority />

Let responsive layout generate srcset and sizes

Set layout to constrained, full-width, or fixed on a component, or image.layout in the config for a default (Astro 5.10+). Astro then generates srcset and sizes for you. Enable image.responsiveStyles: true to add the matching resizing CSS.

<Image src={hero} alt="Hero" layout="constrained" width={800} height={600} />

Allowlist remote image sources

Only authorized remote images are optimized. Add hosts to image.domains or patterns to image.remotePatterns in astro.config.mjs.

export default defineConfig({
  image: {
    domains: ["cdn.sanity.io"],
    remotePatterns: [{ protocol: "https", hostname: "images.ctfassets.net" }],
  },
});

Prefer a specific hostname over a bare protocol pattern, which authorizes every HTTPS host. On Cloudflare, the adapter’s default image service is cloudflare-binding (adapter v13), not Sharp.

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