diff --git a/apps/web/src/appearanceFonts.browser.test.ts b/apps/web/src/appearanceFonts.browser.test.ts
index 9bdc5c2ef9864..289e48ecad050 100644
--- a/apps/web/src/appearanceFonts.browser.test.ts
+++ b/apps/web/src/appearanceFonts.browser.test.ts
@@ -52,4 +52,40 @@ describe("applyAppearanceFontVariables", () => {
       else root.setAttribute("style", style);
     }
   });
+
+  it("widens the numbered-list gutter only when the interface font's marker would clip", () => {
+    const root = document.documentElement;
+    const style = root.getAttribute("style");
+    const gutter = (sans: string) => {
+      applyAppearanceFontVariables(root, {
+        sans,
+        code: "",
+        composer: "",
+        sizeInterface: 16,
+        sizePrompt: 14,
+        sizeCode: 13,
+        smoothing: true,
+        separate: false,
+      });
+      const markdown = document.createElement("div");
+      markdown.className = "chat-markdown";
+      markdown.style.fontSize = "14px";
+      const list = document.createElement("ol");
+      markdown.append(list);
+      document.body.append(markdown);
+      try {
+        return Number.parseFloat(getComputedStyle(list).paddingInlineStart);
+      } finally {
+        markdown.remove();
+      }
+    };
+    try {
+      // `0. ` in a 14px monospace font is about 25px, past the 20px default.
+      expect(gutter("monospace")).toBeGreaterThan(24);
+      expect(gutter("sans-serif")).toBe(20);
+    } finally {
+      if (style === null) root.removeAttribute("style");
+      else root.setAttribute("style", style);
+    }
+  });
 });
diff --git a/apps/web/src/appearanceFonts.ts b/apps/web/src/appearanceFonts.ts
index e7c2867cb279f..19f2d88d3c29d 100644
--- a/apps/web/src/appearanceFonts.ts
+++ b/apps/web/src/appearanceFonts.ts
@@ -88,6 +88,16 @@ export function applyAppearanceFontVariables(
     }
   }
 
+  const markerWidth = probeWidth(
+    appearanceFontStack(preferences.sans, DEFAULT_SANS_FONT_STACK),
+    LIST_MARKER_PROBE_TEXT,
+  );
+  if (markerWidth === null) {
+    root.style.removeProperty("--list-marker-sans");
+  } else {
+    root.style.setProperty("--list-marker-sans", `${(markerWidth / PROBE_FONT_SIZE).toFixed(3)}em`);
+  }
+
   root.style.fontSize = `${clampInterfaceFontSize(preferences.sizeInterface)}px`;
   root.style.setProperty(
     "--font-size-prompt",
@@ -127,15 +137,18 @@ function clampCodeFontSize(value: number): number {
 }
 
 const FONT_PROBE_TEXT = "mmmmmmmmMMWli1O0@# fjord";
+// The widest single-digit `ol` marker; CSS can't measure it to size the list gutter.
+const LIST_MARKER_PROBE_TEXT = "0. ";
+const PROBE_FONT_SIZE = 16;
 let fontProbeContext: CanvasRenderingContext2D | null | undefined;
 
-function probeWidth(fontList: string): number | null {
+function probeWidth(fontList: string, text = FONT_PROBE_TEXT): number | null {
   if (fontProbeContext === undefined) {
     fontProbeContext = document.createElement("canvas").getContext("2d");
   }
   if (fontProbeContext === null) return null;
-  fontProbeContext.font = `16px ${fontList}`;
-  return fontProbeContext.measureText(FONT_PROBE_TEXT).width;
+  fontProbeContext.font = `${PROBE_FONT_SIZE}px ${fontList}`;
+  return fontProbeContext.measureText(text).width;
 }
 
 /** Probes canvas metrics because document.fonts.check() accepts uninstalled families. */
diff --git a/apps/web/src/index.css b/apps/web/src/index.css
index 21c3816f8d34b..654ef108e8e75 100644
--- a/apps/web/src/index.css
+++ b/apps/web/src/index.css
@@ -1933,13 +1933,11 @@ code {
   list-style-type: disc;
 }
 
-/* --list-gutter defaults to the same 1.25rem as .chat-markdown ul, but
-   ChatMarkdown's `ol` renderer widens it (via inline style) for lists whose
-   widest marker has multiple characters so it isn't clipped by list-style-position:
-   outside painting the marker past the padding box. Reset it here too so a
-   nested ol without its own widened marker doesn't inherit the outer one. */
+/* Outside markers paint past the padding box, so the gutter must fit `0. `,
+   measured per font in appearanceFonts. ChatMarkdown widens it inline for
+   multi-digit markers; setting it here stops a nested ol inheriting that. */
 :is(.chat-markdown, .composer-tiptap) ol {
-  --list-gutter: 1.25rem;
+  --list-gutter: max(1.25rem, var(--list-marker-sans, 0px));
   padding-inline-start: var(--list-gutter, 1.25rem);
   list-style-type: decimal;
 }