diff --git a/eleventy.config.js b/eleventy.config.js
index 9d376a37..e9df8e17 100644
--- a/eleventy.config.js
+++ b/eleventy.config.js
@@ -23,6 +23,7 @@ export default async function (eleventyConfig) {
   eleventyConfig.addPassthroughCopy("src/img");
   eleventyConfig.addPassthroughCopy("src/manifest.json");
   eleventyConfig.addPassthroughCopy("src/_headers");
+  eleventyConfig.addPassthroughCopy("src/_routes.json");
   eleventyConfig.addPassthroughCopy("src/plugin-sandbox.html");
 
   // Prevent sandbox from being treated as a template
diff --git a/functions/_middleware.js b/functions/_middleware.js
new file mode 100644
index 00000000..53f5ef3e
--- /dev/null
+++ b/functions/_middleware.js
@@ -0,0 +1,19 @@
+// Send 404s for missing assets in Cloudflare pages
+
+const ASSET_EXTENSIONS =
+  /\.(m?js|css|json|md|svg|png|jpe?g|gif|webp|ico|woff2?)$/;
+
+export async function onRequest(context) {
+  const response = await context.next();
+  const { pathname } = new URL(context.request.url);
+  const isHtml = (response.headers.get("Content-Type") || "").includes(
+    "text/html",
+  );
+  if (ASSET_EXTENSIONS.test(pathname) && isHtml) {
+    return new Response(`Not found: ${pathname}`, {
+      status: 404,
+      headers: { "Content-Type": "text/plain; charset=utf-8" },
+    });
+  }
+  return response;
+}
diff --git a/src/_routes.json b/src/_routes.json
new file mode 100644
index 00000000..dd5d2220
--- /dev/null
+++ b/src/_routes.json
@@ -0,0 +1,5 @@
+{
+  "version": 1,
+  "include": ["/js/*", "/css/*", "/img/*"],
+  "exclude": []
+}