diff --git a/packages/parsers/src/hfIds.parseCount.test.ts b/packages/parsers/src/hfIds.parseCount.test.ts
new file mode 100644
index 0000000000..d5bf48b802
--- /dev/null
+++ b/packages/parsers/src/hfIds.parseCount.test.ts
@@ -0,0 +1,66 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const parses = vi.hoisted(() => ({ count: 0 }));
+vi.mock("linkedom", async (actual) => {
+  const real = await actual<typeof import("linkedom")>();
+  return {
+    ...real,
+    parseHTML: (...args: Parameters<typeof real.parseHTML>) => {
+      parses.count += 1;
+      return real.parseHTML(...args);
+    },
+  };
+});
+
+const { ensureHfIds } = await import("./hfIds.js");
+
+const page = (body: string) =>
+  `<!DOCTYPE html><html><head><meta charset="utf-8"><title>t</title></head><body>${body}</body></html>`;
+
+describe("ensureHfIds on a document whose elements all carry their ids", () => {
+  beforeEach(() => {
+    parses.count = 0;
+  });
+
+  it("returns it as it is, without a parse: a stamped bundle has nothing to mint", () => {
+    const html = page(
+      `<div data-hf-id="hf-a1b2"><svg data-hf-id="hf-c3d4"><path data-hf-id="hf-e5f6" d="M0 0"/></svg>` +
+        `<script>var a = "<p>";</script></div>`,
+    );
+    expect(ensureHfIds(html)).toBe(html);
+    expect(parses.count).toBe(0);
+  });
+
+  it("parses once when one element lacks its id, or has an empty one", () => {
+    const missing = page(`<div data-hf-id="hf-a1b2"><p>hi</p></div>`);
+    expect(ensureHfIds(missing)).toMatch(/<p data-hf-id="hf-[0-9a-z]+">hi<\/p>/);
+    expect(parses.count).toBe(1);
+    expect(ensureHfIds(page(`<div data-hf-id="">x</div>`))).toMatch(/data-hf-id="hf-[0-9a-z]+"/);
+    expect(parses.count).toBe(2);
+  });
+
+  it("parses a stamped page not opening <head> then <body>: callers re-parsing the output expect both", () => {
+    const headless = `<!DOCTYPE html><html><body><div data-hf-id="hf-a1b2">x</div></body></html>`;
+    expect(ensureHfIds(headless)).toContain("<head></head><body>");
+    const shapes = [
+      `<html><head><title>t</title><body><p data-hf-id="hf-a1b2">x</p></body></html>`,
+      `<html><head><div data-hf-id="hf-c3d4">x</div></head><body><p data-hf-id="hf-a1b2">x</p></body></html>`,
+      `<html><head></head></html>`,
+      `<html><head><title>t</title><!-- </head> --><body><p data-hf-id="hf-a1b2">x</p></body></html>`,
+    ];
+    for (const html of shapes) ensureHfIds(html);
+    expect(parses.count).toBe(1 + shapes.length);
+  });
+
+  it("checks a fragment from its first tag, so elements before a stray <body> still get their ids", () => {
+    const minted = ensureHfIds(`<p>y</p><body><div data-hf-id="hf-a1b2">x</div>`);
+    expect(minted).toMatch(/<p data-hf-id="hf-[0-9a-z]+">y<\/p>/);
+    expect(parses.count).toBe(1);
+  });
+
+  it("reads an id only from the tag's own attributes, never from a quoted value", () => {
+    const html = page(`<div title='data-hf-id="hf-zzzz"'>x</div>`);
+    expect(ensureHfIds(html)).toMatch(/<div data-hf-id="hf-[0-9a-z]+" title=/);
+    expect(parses.count).toBe(1);
+  });
+});
diff --git a/packages/parsers/src/hfIds.ts b/packages/parsers/src/hfIds.ts
index 52e23db83d..9180f1ea07 100644
--- a/packages/parsers/src/hfIds.ts
+++ b/packages/parsers/src/hfIds.ts
@@ -9,7 +9,7 @@
  * so inserting a non-identical sibling never shifts another element's id.
  */
 import { parseHTML } from "linkedom";
-import { assignHfIds, getChildElements } from "./hfIdAssignment.js";
+import { EXCLUDED_TAGS, assignHfIds, getChildElements } from "./hfIdAssignment.js";
 export {
   EXCLUDED_TAGS,
   mintHfId,
@@ -18,17 +18,63 @@ export {
 } from "./hfIdAssignment.js";
 
 export function ensureHfIds(html: string): string {
+  if (alreadyStamped(html)) return html;
   const { document, wrapped } = parseFragmentOrDocument(html);
   if (!document.body) return html;
   assignHfIds(document.body);
   return wrapped ? document.body.innerHTML || "" : document.toString();
 }
 
+const isFragment = (html: string) =>
+  !/^(?:<!doctype|<html[\s>/])/i.test(html.replace(/^(?:\s|<!--(?:>|->|[\s\S]*?-->))*/, ""));
+
+function alreadyStamped(html: string): boolean {
+  const tags = startTags(html);
+  if (!tags) return false;
+  const body = isFragment(html) ? -1 : tags.findIndex((tag) => tag.name === "body");
+  if (!isFragment(html) && !opensHeadThenBody(html, tags, body)) return false;
+  return tags
+    .slice(body + 1)
+    .every(
+      (tag) =>
+        !IMPLIED.has(tag.name) &&
+        tag.name !== "table" &&
+        (EXCLUDED_TAGS.has(tag.name) || hasHfId(html, tag)),
+    );
+}
+
+const HEAD_CONTENT = new Set(["meta", "link", "base", "title", "style", "script"]);
+
+// The parse path's document.body read adds any missing <head>/<body>, and callers re-parsing its output expect both.
+function opensHeadThenBody(html: string, tags: StartTag[], body: number): boolean {
+  const [first, head] = tags;
+  if (body < 0 || first?.name !== "html" || head?.name !== "head") return false;
+  const inHead = tags.slice(2, body);
+  return (
+    inHead.every((tag) => HEAD_CONTENT.has(tag.name)) &&
+    closesHead(html, inHead.at(-1) ?? head, tags[body]!)
+  );
+}
+
+function closesHead(html: string, last: StartTag, body: StartTag): boolean {
+  const from = RAW_TEXT.has(last.name) ? rawTextEnd(html, last.name, last.bracket) : last.bracket;
+  const between = html.slice(from, html.lastIndexOf("<", body.nameEnd));
+  const marks = between.matchAll(/<!--[\s\S]*?-->|<\/head\s*>/gi);
+  return from >= 0 && [...marks].some((mark) => !mark[0].startsWith("<!--"));
+}
+
+function hasHfId(html: string, tag: StartTag): boolean {
+  const open = html.slice(tag.nameEnd, tag.bracket);
+  const name = /\sdata-hf-id\s*=\s*/i.exec(outsideQuotes(open));
+  if (!name) return false;
+  const value = open.slice(name.index + name[0].length);
+  const quote = value[0] === '"' || value[0] === "'" ? value[0] : "";
+  return quote ? value.length > 1 && value[1] !== quote : /^[^\s>]/.test(value);
+}
+
 function parseFragmentOrDocument(html: string) {
   // Wrap fragments so every body element participates in stable ID assignment.
-  const wrapped = !/^(?:<!doctype|<html[\s>/])/i.test(
-    html.replace(/^(?:\s|<!--(?:>|->|[\s\S]*?-->))*/, ""),
-  );
+  const wrapped = isFragment(html);
   const { document } = wrapped
     ? parseHTML(`<!DOCTYPE html><html><head></head><body>${html}</body></html>`)
     : parseHTML(html);