diff --git a/app.js b/app.js
index 8d0b744..5ed9a3a 100644
--- a/app.js
+++ b/app.js
@@ -796,6 +796,25 @@ const I18N = {
 const $ = (selector, root = document) => root.querySelector(selector);
 const $$ = (selector, root = document) => [...root.querySelectorAll(selector)];
 
+function createUuid() {
+  if (typeof window.crypto?.randomUUID === "function") {
+    return window.crypto.randomUUID();
+  }
+
+  const bytes = new Uint8Array(16);
+  if (typeof window.crypto?.getRandomValues === "function") {
+    window.crypto.getRandomValues(bytes);
+  } else {
+    for (let index = 0; index < bytes.length; index += 1) {
+      bytes[index] = Math.floor(Math.random() * 256);
+    }
+  }
+  bytes[6] = (bytes[6] & 0x0f) | 0x40;
+  bytes[8] = (bytes[8] & 0x3f) | 0x80;
+  const hex = [...bytes].map(byte => byte.toString(16).padStart(2, "0")).join("");
+  return `${hex.slice(0, 8)}-${hex.slice(8, 12)}-${hex.slice(12, 16)}-${hex.slice(16, 20)}-${hex.slice(20)}`;
+}
+
 function mapApiOwner(record) {
   return {
     id: record.id,
@@ -1654,7 +1673,7 @@ function calculateForm() {
 
 function resetStayForm(ownerId = owners[0]?.id || "") {
   $("#stayForm").reset();
-  stayIdempotencyKey = IS_API_MODE ? window.crypto.randomUUID() : null;
+  stayIdempotencyKey = IS_API_MODE ? createUuid() : null;
   $("#ownerSelect").value = String(ownerId);
   $("#confirmationInput").value = "";
   $("#checkinInput").value = "2026-08-01";
diff --git a/index.html b/index.html
index b5af7cb..f826502 100644
--- a/index.html
+++ b/index.html
@@ -337,6 +337,6 @@
     <script src="runtime-config.js?v=20260802-login"></script>
     <script src="owners-data.js"></script>
     <script src="api-client.js?v=20260802-login"></script>
-    <script src="app.js?v=20260803-accounts-fill-1"></script>
+    <script src="app.js?v=20260803-uuid-fallback"></script>
   </body>
 </html>
diff --git a/tests/browser-uuid.test.mjs b/tests/browser-uuid.test.mjs
new file mode 100644
index 0000000..3ef2056
--- /dev/null
+++ b/tests/browser-uuid.test.mjs
@@ -0,0 +1,48 @@
+import assert from "node:assert/strict";
+import { readFile } from "node:fs/promises";
+import test from "node:test";
+import vm from "node:vm";
+
+const appSource = await readFile(new URL("../app.js", import.meta.url), "utf8");
+const helperStart = appSource.indexOf("function createUuid");
+const helperEnd = appSource.indexOf("\n\nfunction mapApiOwner", helperStart);
+assert.notEqual(helperStart, -1, "createUuid helper should exist");
+assert.notEqual(helperEnd, -1, "createUuid helper should end before API mappers");
+
+function loadCreateUuid(windowObject) {
+  const context = { window: windowObject };
+  vm.runInNewContext(`${appSource.slice(helperStart, helperEnd)}\nglobalThis.createUuid = createUuid;`, context);
+  return context.createUuid;
+}
+
+test("UUID generation falls back when crypto.randomUUID is unavailable", () => {
+  let next = 0;
+  const createUuid = loadCreateUuid({
+    crypto: {
+      getRandomValues(bytes) {
+        for (let index = 0; index < bytes.length; index += 1) {
+          bytes[index] = next;
+          next = (next + 17) % 256;
+        }
+        return bytes;
+      }
+    }
+  });
+
+  assert.match(
+    createUuid(),
+    /^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/
+  );
+});
+
+test("UUID generation uses native randomUUID when it is available", () => {
+  const createUuid = loadCreateUuid({
+    crypto: {
+      randomUUID() {
+        return "00000000-0000-4000-8000-000000000123";
+      }
+    }
+  });
+
+  assert.equal(createUuid(), "00000000-0000-4000-8000-000000000123");
+});