Astro 4.2

Emanuele Stoppa
Matthew Phillips
Nate Moore
Bjorn Lu

Astro 4.2 is now available! This release includes new experimental features to try out, improvements to accessibility rules, the ability for remark plugins to customize image optimization in Markdown, and many more improvements and bug fixes.

Highlights include:

This marks the first Astro release of almost all community-built features, with only one highlight brought to you by an Astro core maintainer. This is a huge milestone for us, as it means that Astro is now big enough to have a thriving community of contributors capable of spearheading releases. Thank you to everyone who contributed to this release!

How to upgrade

To take advantage of the latest features, make sure you’re running the latest version of Astro. You can upgrade to Astro 4.2 by running the @astrojs/upgrade command:

npx @astrojs/upgrade

or by running the upgrade command for your package manager:

npm install astro@latest
pnpm upgrade astro --latest
yarn upgrade astro --latest

(Experimental) Prerendering pages using the Speculation Rules API

Thanks to Ross Robino, Astro’s prefetch feature now has experimental support for prerendering pages using the currently Chromium-exclusive Speculation Rules API. This API allows you to prerender pages on the client, and even run client-side JavaScript on pages that the user is likely to visit next, allowing for a faster browsing experience.

To enable this feature, add the following to your astro.config.mjs file:

import { defineConfig } from 'astro/config';
export default defineConfig({
prefetch: true,
experimental: {
clientPrerender: true,

This will respect your existing prefetch configuration options, but will now prerender all links with the data-astro-prefetch attribute on the client, instead. A built-in fallback is provided for browsers that do not yet support the Speculation Rules API.

Additionally, you can use the document.prerendering property in <script> tags to check whether the page is being prerendered:

if (document.prerendering) {
// prerendering is enabled

For more about this experimental feature, see the experimental.clientPrerender documentation.

(Experimental) Reworked routing priority for injected routes

Previously, routes injected using the injectRoute() API would be given the highest priority, meaning that they would be matched before any other routes. While this seemed like a good idea at the time, it unfortunately caused a lot of hard-to-debug issues where routes wouldn’t be matched as expected.

Historically, we’ve been hesitant to change this behavior due to how many unintended issues changes to the routing system can cause. However, thanks to Luiz Ferraz carefully implementing a fix, Astro 4.2 brings an experimental flag that lays the foundation for a new and improved default routing system in Astro!

With this flag enabled, routes injected using the injectRoute() API, as well as redirects, will now follow the same priority order that routes from the filesystem do. This should provide more stable, consisting priority ordering rules for all your project routes.

Try the future of Astro today by adding the new experimental.globalRoutePriority option to astro.config.mjs file to opt-in to the new behavior:

import { defineConfig } from 'astro/config';
export default defineConfig({
experimental: {
globalRoutePriority: true,

Additional logging has also been added to inform you of any route collisions where two routes are attempting to build the same URL.

Please see the experimental.globalRoutePriority documentation for more details.

New redirectToDefaultLocale config option

A common request since the introduction of internationalization support in Astro 4.0 has been the ability to disable automatic redirects of the root URL (/) to the default locale when prefixDefaultLocale: true is set.

This is now possible with the new redirectToDefaultLocale option:

import { defineConfig } from 'astro/config';
export default defineConfig({
defaultLocale: "en",
locales: ["en", "fr"],
routing: {
prefixDefaultLocale: true,
redirectToDefaultLocale: false

Redirect away, or don’t! It’s up to you now. This option is enabled by default.

Improved accessibility rules

The accessibility rules added to the Astro Dev Toolbar in 4.0 have been improved to be more accurate. Thanks to Oliver Speir for meticulously reading through the WCAG guidelines and improving these rules! Accessibility issues wrongfully reported as false positives by the Dev Toolbar in previous releases are fixed in this update, and more improvements are in active development.

Customizable image optimization in Markdown

Also by Oliver, remark plugins can now customize how images are optimized in Markdown files. Previously, every image included in Markdown files using the native syntax (![alt](src)) used Astro’s default settings for image optimization. With this update, remark plugins can now add properties to image nodes to customize how they are optimized.

For example, the following remark plugin will set the width and height properties of every image to 100:

import { visit } from "unist-util-visit";
export default function remarkPlugin() {
return (tree) => {
visit(tree, 'image', (node) => { = || {}; = "100"; = "100";

This is currently exclusive to Markdown files, but we’re hoping to add support for MDX as well in a future release.

Bug Fixes

As always, additional bug fixes are included in this release. Check out the release notes to learn more.