diff --git a/index.html b/index.html
index 168c85b..f885722 100644
--- a/index.html
+++ b/index.html
@@ -31,6 +31,7 @@
   </head>
   <body>
     <div id="root">
+      <!-- share-html:fallback:start -->
       <main>
         <h1>Share HTML — upload one HTML file, get a sandboxed shareable link</h1>
         <p>Upload a single self-contained HTML file and get a public, sandboxed preview URL. No signup required; anonymous uploads are kept for 365 days.</p>
@@ -39,6 +40,7 @@ <h2>Use it programmatically</h2>
 # returns JSON: { "share": { "share_url", "preview_url", "slug", "id" }, "claimToken" }</pre>
         <p>Full API: <a href="/openapi.json">/openapi.json</a> · AI guide: <a href="/llms.txt">/llms.txt</a> · MCP: <a href="/mcp">/mcp</a></p>
       </main>
+      <!-- share-html:fallback:end -->
     </div>
     <script>
       (() => {
diff --git a/src/worker/index.ts b/src/worker/index.ts
index c45f59d..d24c4e8 100644
--- a/src/worker/index.ts
+++ b/src/worker/index.ts
@@ -143,7 +143,7 @@ export default {
       const assetResponse = await env.ASSETS.fetch(request);
       const contentType = assetResponse.headers.get("content-type") ?? "";
       if (contentType.includes("text/html") && request.method === "GET") {
-        return withDiscoveryHeaders(await injectConfig(assetResponse, env));
+        return withDiscoveryHeaders(await injectConfig(assetResponse, env, request));
       }
       return withDiscoveryHeaders(assetResponse);
     } catch (error) {
@@ -158,13 +158,14 @@ export default {
  * can initialise the Supabase client without a separate /api/config fetch.
  * The injected script sets window.__APP_CONFIG__ before any module scripts run.
  */
-async function injectConfig(response: Response, env: Env): Promise<Response> {
+async function injectConfig(response: Response, env: Env, request: Request): Promise<Response> {
   const configScript = `<script>window.__APP_CONFIG__=${JSON.stringify({
     supabaseUrl: env.SUPABASE_URL,
     supabasePublishableKey: env.SUPABASE_PUBLISHABLE_KEY
   })}</script>`;
   const html = await response.text();
-  const injected = html.replace("</head>", `${configScript}</head>`);
+  const bodyAdjusted = routeFallbackHtml(html, request);
+  const injected = bodyAdjusted.replace("</head>", `${configScript}</head>`);
   return new Response(injected, {
     status: response.status,
     statusText: response.statusText,
@@ -172,6 +173,37 @@ async function injectConfig(response: Response, env: Env): Promise<Response> {
   });
 }
 
+function routeFallbackHtml(html: string, request: Request): string {
+  const pathname = new URL(request.url).pathname;
+  if (!isShareRoute(pathname)) return html;
+  return replaceAppFallback(html, shareRouteFallback());
+}
+
+function isShareRoute(pathname: string): boolean {
+  return /^\/s\/[^/]+\/?$/.test(pathname);
+}
+
+function replaceAppFallback(html: string, replacement: string): string {
+  const startMarker = "<!-- share-html:fallback:start -->";
+  const endMarker = "<!-- share-html:fallback:end -->";
+  const start = html.indexOf(startMarker);
+  const end = html.indexOf(endMarker);
+  if (start === -1 || end === -1 || end < start) return html;
+
+  const contentStart = start + startMarker.length;
+  return `${html.slice(0, contentStart)}\n${replacement}\n      ${html.slice(end)}`;
+}
+
+function shareRouteFallback(): string {
+  return [
+    '      <main class="app-shell" aria-busy="true">',
+    '        <section class="flex flex-col gap-6 pt-8 border-t border-border">',
+    '          <p class="text-sm text-muted" aria-live="polite">Loading share...</p>',
+    "        </section>",
+    "      </main>",
+  ].join("\n");
+}
+
 function discoveryRoute(request: Request, url: URL): Response | null {
   if (request.method !== "GET" && request.method !== "HEAD") {
     return null;
@@ -308,4 +340,3 @@ export type { AuthUser } from "./auth.ts";
 export { cleanTitle, sanitizeShortText, looksLikeHtml, isUploadFile, getClientIp, sha256Hex, hashText, base64Url, numberEnv, formatBytes, escapeHtml, errorMessage, logBackgroundError } from "./utils.ts";
 export { maxUploadBytes, appOrigin, previewOrigin } from "./config.ts";
 export { createShare, createShareRecord, checkUploadRate, listMyShares, getPublicShare, reportShare, claimShare, deleteShare, listReports, moderateShare, previewShare, previewHeaders, previewMessage } from "./shares.ts";
-
diff --git a/tests/worker-index.test.ts b/tests/worker-index.test.ts
new file mode 100644
index 0000000..588f88b
--- /dev/null
+++ b/tests/worker-index.test.ts
@@ -0,0 +1,69 @@
+import assert from "node:assert/strict";
+import { test, vi } from "vitest";
+
+import worker from "../src/worker/index.ts";
+
+const INDEX_HTML = `<!doctype html>
+<html lang="en">
+  <head>
+    <title>Share HTML</title>
+  </head>
+  <body>
+    <div id="root">
+      <!-- share-html:fallback:start -->
+      <main>
+        <h1>Share HTML — upload one HTML file, get a sandboxed shareable link</h1>
+      </main>
+      <!-- share-html:fallback:end -->
+    </div>
+    <script type="module" src="/assets/index.js"></script>
+  </body>
+</html>`;
+
+function makeEnv() {
+  return {
+    SUPABASE_URL: "https://proj.supabase.co",
+    SUPABASE_PUBLISHABLE_KEY: "publishable-key",
+    ASSETS: {
+      fetch: vi.fn(async () => new Response(INDEX_HTML, {
+        headers: { "content-type": "text/html" },
+      })),
+    },
+  } as any;
+}
+
+function makeCtx() {
+  return { waitUntil: vi.fn() } as any;
+}
+
+test("worker fetch: share SPA fallback does not expose homepage copy before React renders", async () => {
+  const env = makeEnv();
+  const response = await worker.fetch(
+    new Request("https://sharehtml.zhenjia.dev/s/example"),
+    env,
+    makeCtx()
+  );
+
+  assert.equal(response.status, 200);
+  const html = await response.text();
+
+  assert.ok(html.includes("Loading share..."));
+  assert.ok(!html.includes("Share HTML — upload one HTML file"));
+  assert.ok(html.includes("window.__APP_CONFIG__"));
+});
+
+test("worker fetch: homepage SPA fallback keeps agent-readable static copy", async () => {
+  const env = makeEnv();
+  const response = await worker.fetch(
+    new Request("https://sharehtml.zhenjia.dev/"),
+    env,
+    makeCtx()
+  );
+
+  assert.equal(response.status, 200);
+  const html = await response.text();
+
+  assert.ok(html.includes("Share HTML — upload one HTML file"));
+  assert.ok(!html.includes("Loading share..."));
+  assert.ok(html.includes("window.__APP_CONFIG__"));
+});