fix: set revalidate cache header on 404'd static assets

Some adapters (like Vercel) apply a immutable cache header by default to static assets. They respect explicitly set headers though. Therefore apply a "must immediately revalidate" cache header to the 404 response of static assets in the SvelteKit runtime related to #9089
sveltejs · Aug 1, 2024 · 0a6d436 · 0a6d436
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diff --git a/.changeset/light-pears-help.md b/.changeset/light-pears-help.md
@@ -0,0 +1,5 @@
+---
+"@sveltejs/kit": patch
+---
+
+fix: set revalidate cache header on 404'd static assets
diff --git a/packages/kit/src/runtime/server/respond.js b/packages/kit/src/runtime/server/respond.js
@@ -116,6 +116,9 @@ export async function respond(request, options, manifest, state) {
 	}
 
 	if (decoded.startsWith(`/${options.app_dir}`)) {
+		// Ensure that 404'd static assets are not cached - some adapters might apply caching by default
+		const headers = new Headers();
+		headers.set('cache-control', 'public, max-age=0, must-revalidate');
 		return text('Not found', { status: 404 });
 	}