diff --git a/scripts/ci_font_gate.py b/scripts/ci_font_gate.py
new file mode 100644
index 0000000..c4c7777
--- /dev/null
+++ b/scripts/ci_font_gate.py
@@ -0,0 +1,170 @@
+#!/usr/bin/env python3
+"""CI font gate - assert the patched binary exposes exactly the Windows font
+persona on EVERY host OS (Windows / Linux / macOS), with zero host-font leak.
+
+The patched binary is bundle-only: at font-list construction it drops every host
+system font and exposes only the bundled Windows-11 family set (the exposed set
+IS the bundle). This gate launches the binary on its NATIVE runner - so
+macOS/CoreText, Linux/fontconfig and Windows/DWrite are each tested for real -
+enumerates the visible families with the same width-probe web detectors use, and
+asserts three things:
+
+  1. the detected family set == the canonical Windows set (EXPECTED): the SAME
+     set on all three platforms. A leaked host font or a missing Windows one
+     fails here. This is the "identical on every OS" contract.
+  2. no known host family is visible (macOS: Helvetica Neue / Geneva / Menlo ...;
+     Linux: DejaVu / Ubuntu ...) - a POSITIVE proof that block-at-birth ran for
+     this platform's backend, not just "no obvious tell".
+  3. the CSS generics resolve to Windows fonts (serif=Times New Roman,
+     sans-serif=Arial, monospace=Consolas) and system-ui=Segoe UI.
+
+This is the macOS validator the local Win/Linux gate cannot be - there is no
+local Mac, so CoreText is only ever exercised here. Headless, no proxy, no
+secrets, loopback-free (about:blank + arrow-function evaluate, which is not
+eval and carries no CSP problem) -> safe in public CI.
+
+Usage:  python ci_font_gate.py <firefox-binary>
+Exit 0 + "FONT GATE OK ..." on success; non-zero + the diff on failure.
+"""
+from __future__ import annotations
+
+import sys
+
+# The canonical Windows-11 family set the bundle exposes. Verified byte-for-byte
+# identical on Windows/DWrite and Linux/fontconfig; macOS/CoreText must match it
+# too. Keep this sorted and in sync with the bundle (browser/fonts/moz.build).
+EXPECTED = [
+    "Arial", "Arial Black", "Bahnschrift", "Calibri", "Cambria", "Cambria Math",
+    "Candara", "Comic Sans MS", "Consolas", "Constantia", "Corbel",
+    "Courier New", "Ebrima", "Franklin Gothic", "Franklin Gothic Medium",
+    "Gabriola", "Gadugi", "Georgia", "Impact", "Ink Free", "Javanese Text",
+    "Leelawadee", "Leelawadee UI", "Lucida Console", "Lucida Sans Unicode",
+    "MS Gothic", "MS PGothic", "MS UI Gothic", "MV Boli", "Malgun Gothic",
+    "Marlett", "Microsoft Himalaya", "Microsoft JhengHei",
+    "Microsoft JhengHei UI", "Microsoft New Tai Lue", "Microsoft PhagsPa",
+    "Microsoft Sans Serif", "Microsoft Tai Le", "Microsoft Uighur",
+    "Microsoft YaHei", "Microsoft YaHei UI", "Microsoft Yi Baiti",
+    "MingLiU-ExtB", "Mongolian Baiti", "Myanmar Text", "NSimSun", "Nirmala UI",
+    "PMingLiU-ExtB", "Palatino Linotype", "Segoe Fluent Icons",
+    "Segoe MDL2 Assets", "Segoe Print", "Segoe Script", "Segoe UI",
+    "Segoe UI Emoji", "Segoe UI Historic", "Segoe UI Symbol", "SimSun",
+    "SimSun-ExtB", "Sitka Small", "Sylfaen", "Symbol", "Tahoma",
+    "Times New Roman", "Trebuchet MS", "Verdana", "Webdings", "Wingdings",
+    "Wingdings 2", "Wingdings 3", "Yu Gothic", "Yu Gothic UI",
+]
+
+# Host families that must NEVER be visible - one per backend. Their presence is a
+# hard fail (block-at-birth did not run for this OS). These are decoys added to
+# the probe list; they must all come back absent.
+HOST_MUST_BE_ABSENT = [
+    # macOS / CoreText
+    "Helvetica Neue", "Geneva", "Menlo", "Monaco", "Avenir", "Lucida Grande",
+    "Apple SD Gothic Neo", "PingFang SC",
+    # Linux / fontconfig
+    "DejaVu Sans", "Liberation Sans", "Ubuntu", "Nimbus Sans", "Noto Sans",
+    # Office / non-standard families intentionally dropped from the bundle
+    "Century Gothic", "Agency FB", "Monotype Corsiva", "Pristina",
+]
+
+# CSS generic -> the Windows family it must resolve to under bundle-only.
+GENERICS = {
+    "serif": "Times New Roman",
+    "sans-serif": "Arial",
+    "monospace": "Consolas",
+    "system-ui": "Segoe UI",
+}
+
+# Width+height probe (the offsetWidth method real detectors use): a family is
+# "present" if styling text in it renders at a different size than the three CSS
+# base generics. For the generics, return the measured size of each generic and
+# of its target Windows family so the caller can assert they coincide.
+DETECT_JS = r"""(arg) => {
+  const bases = ['monospace', 'sans-serif', 'serif'];
+  const sample = 'mmmmmmmmmmlli WwQ 0123456789 gjpqy';
+  const sp = document.createElement('span');
+  sp.style.cssText =
+    'position:absolute;left:-9999px;font-size:72px;white-space:nowrap;';
+  sp.textContent = sample;
+  document.body.appendChild(sp);
+  const size = (ff) => { sp.style.fontFamily = ff; return sp.offsetWidth + 'x' + sp.offsetHeight; };
+  const bw = {};
+  for (const b of bases) bw[b] = size(b);
+  const present = {};
+  for (const f of arg.cands) {
+    present[f] = bases.some((b) => size("'" + f + "'," + b) !== bw[b]);
+  }
+  const gen = {};
+  for (const g of arg.generics) gen[g] = size(g);
+  const genref = {};
+  for (const w of arg.targets) genref[w] = size("'" + w + "'");
+  document.body.removeChild(sp);
+  return { present, gen, genref };
+}"""
+
+# Suppress the new-tab machinery so the launch is quiet (mirrors ci_drive_gate).
+_PREFS = {
+    "browser.startup.page": 0,
+    "browser.newtabpage.enabled": False,
+    "browser.newtab.preload": False,
+    "browser.newtabpage.activity-stream.enabled": False,
+}
+
+
+def main(exe: str) -> int:
+    from playwright.sync_api import sync_playwright
+
+    cands = EXPECTED + HOST_MUST_BE_ABSENT
+    arg = {
+        "cands": cands,
+        "generics": list(GENERICS.keys()),
+        "targets": list(GENERICS.values()),
+    }
+    with sync_playwright() as p:
+        browser = p.firefox.launch(executable_path=exe, headless=True,
+                                   firefox_user_prefs=_PREFS)
+        try:
+            page = browser.new_page()
+            page.goto("about:blank")
+            r = page.evaluate(DETECT_JS, arg)
+        finally:
+            browser.close()
+
+    detected = {f for f, v in r["present"].items() if v}
+    expected = set(EXPECTED)
+    missing = sorted(expected - detected)
+    # Anything detected that isn't in EXPECTED (host leaks land here too).
+    extra = sorted(detected - expected)
+    leaked_host = [h for h in HOST_MUST_BE_ABSENT if r["present"].get(h)]
+    gen_bad = []
+    for g, want in GENERICS.items():
+        got, ref = r["gen"].get(g), r["genref"].get(want)
+        if got != ref:
+            gen_bad.append(f"{g} -> {got} (expected {want} = {ref})")
+
+    n = len(detected)
+    print(f"[font-gate] {exe}")
+    print(f"[font-gate] detected {n} families (expected {len(EXPECTED)})")
+    if missing:
+        print(f"[font-gate] MISSING (in bundle, not exposed): {missing}")
+    if extra:
+        print(f"[font-gate] UNEXPECTED (exposed, not in canonical set): {extra}")
+    if leaked_host:
+        print(f"[font-gate] HOST LEAK (block-at-birth did not run!): {leaked_host}")
+    if gen_bad:
+        print(f"[font-gate] GENERIC MISMATCH: {gen_bad}")
+
+    ok = not missing and not extra and not leaked_host and not gen_bad
+    if ok:
+        print(f"FONT GATE OK - exactly the {n} Windows families, host-leak 0, "
+              f"generics map to Windows (serif/sans/mono/system-ui).")
+        return 0
+    print("FONT GATE FAILED - the exposed set does not match the Windows "
+          "persona on this OS (see the diff above).")
+    return 1
+
+
+if __name__ == "__main__":
+    if len(sys.argv) < 2:
+        print("usage: python ci_font_gate.py <firefox-binary>", file=sys.stderr)
+        raise SystemExit(2)
+    raise SystemExit(main(sys.argv[1]))