diff --git a/apps/app/src/components/generative-ui/open-generative-ui/__tests__/frame-content.test.ts b/apps/app/src/components/generative-ui/open-generative-ui/__tests__/frame-content.test.ts
new file mode 100644
index 0000000..db441fa
--- /dev/null
+++ b/apps/app/src/components/generative-ui/open-generative-ui/__tests__/frame-content.test.ts
@@ -0,0 +1,29 @@
+import { describe, it, expect } from "vitest";
+import { buildFinalFrameContent } from "../frame-content";
+
+describe("buildFinalFrameContent", () => {
+  // Websandbox rejects frame content without a literal "<head>" and injects
+  // its bootstrap at that tag.
+  it.each([
+    ['<html lang="en"><head lang="en"><title>T</title></head><body>x</body></html>'],
+    ["<HTML><HEAD><TITLE>T</TITLE></HEAD><BODY>x</BODY></HTML>"],
+  ])("normalizes a head tag websandbox would not find: %s", (html) => {
+    const frame = buildFinalFrameContent(html);
+    expect(frame).toContain("<head>");
+    expect(frame.indexOf("Content-Security-Policy")).toBeGreaterThan(
+      frame.indexOf("<head>")
+    );
+    expect(frame.indexOf("Content-Security-Policy")).toBeLessThan(
+      frame.search(/<title>/i)
+    );
+  });
+
+  it("does not mistake <header> for the head", () => {
+    const frame = buildFinalFrameContent("<body><header>Top</header></body>");
+    expect(frame.indexOf("<head>")).toBe(0);
+    expect(frame).toContain("<header>Top</header>");
+    expect(frame.indexOf("Content-Security-Policy")).toBeLessThan(
+      frame.indexOf("<header>")
+    );
+  });
+});
diff --git a/apps/app/src/components/generative-ui/open-generative-ui/frame-content.ts b/apps/app/src/components/generative-ui/open-generative-ui/frame-content.ts
index 246e696..d2e7b02 100644
--- a/apps/app/src/components/generative-ui/open-generative-ui/frame-content.ts
+++ b/apps/app/src/components/generative-ui/open-generative-ui/frame-content.ts
@@ -38,8 +38,16 @@ export const CSP_META_TAG = `<meta http-equiv="Content-Security-Policy" content=
       https://unpkg.com;
   ">`;
 
+const HEAD_OPEN_TAG = /<head(?:\s[^>]*)?>/i;
+
+/**
+ * Websandbox requires a literal lowercase "<head>" in its frame content and
+ * injects its bootstrap there, so "<HEAD>" or "<head lang=...>" is normalized
+ * (attributes on <head> have no effect) and a missing head is added.
+ */
 export function ensureHead(html: string): string {
-  if (/<head[\s>]/i.test(html)) return html;
+  if (html.includes("<head>")) return html;
+  if (HEAD_OPEN_TAG.test(html)) return html.replace(HEAD_OPEN_TAG, "<head>");
   return `<head></head>${html}`;
 }
 
@@ -58,11 +66,7 @@ export function buildFinalFrameContent(html: string, css?: string): string {
     DESIGN_SYSTEM_STYLE_TAG +
     (css ? `<style>${css}</style>` : "");
   const withHead = ensureHead(html);
-  const openTag = withHead.match(/<head[^>]*>/i);
-  if (!openTag || openTag.index === undefined) {
-    return `<head>${headContent}</head>${withHead}`;
-  }
-  const insertAt = openTag.index + openTag[0].length;
+  const insertAt = withHead.indexOf("<head>") + "<head>".length;
   return withHead.slice(0, insertAt) + headContent + withHead.slice(insertAt);
 }