diff --git a/tests/test_webui_katex_assets.py b/tests/test_webui_katex_assets.py
new file mode 100644
index 000000000..e69808a53
--- /dev/null
+++ b/tests/test_webui_katex_assets.py
@@ -0,0 +1,14 @@
+from pathlib import Path
+import re
+
+
+PROJECT_ROOT = Path(__file__).resolve().parents[1]
+
+
+def test_katex_css_font_assets_are_complete():
+    katex = PROJECT_ROOT / "webui" / "vendor" / "katex"
+    css = (katex / "katex.min.css").read_text(encoding="utf-8")
+    referenced = set(re.findall(r"fonts/(KaTeX_[^)\"']+\.(?:woff2|woff|ttf))", css))
+    available = {path.name for path in (katex / "fonts").iterdir() if path.is_file()}
+
+    assert referenced == available
diff --git a/webui/vendor/katex/AGENTS.md b/webui/vendor/katex/AGENTS.md
index 983d15276..de950f661 100644
--- a/webui/vendor/katex/AGENTS.md
+++ b/webui/vendor/katex/AGENTS.md
@@ -9,11 +9,13 @@
 - `katex.min.js` owns core rendering behavior.
 - `katex.auto-render.min.js` owns auto-render support.
 - `katex.min.css` owns KaTeX styling.
+- `fonts/` owns every font asset referenced by `katex.min.css`.
 
 ## Local Contracts
 
 - Do not hand-edit minified vendor files.
 - Keep markdown/message rendering imports synchronized with asset paths.
+- Keep the CSS font references fully available under `fonts/`.
 
 ## Work Guidance
 
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