diff --git a/packages/app/src/hooks/use-settings/storage.test.ts b/packages/app/src/hooks/use-settings/storage.test.ts
index fb7e423120..62903c7433 100644
--- a/packages/app/src/hooks/use-settings/storage.test.ts
+++ b/packages/app/src/hooks/use-settings/storage.test.ts
@@ -705,6 +705,20 @@ describe("appearance settings", () => {
     expect((await loadAppSettingsFromStorage(deps)).uiBaseFontSize).toBe(16);
   });
 
+  it("falls back to a valid legacy interface scale when the explicit base size is invalid", async () => {
+    const deps = makeDeps({
+      storage: createInMemoryKeyValueStorage({
+        [APP_SETTINGS_KEY]: JSON.stringify({ uiBaseFontSize: "abc", uiFontSize: 17 }),
+      }),
+    });
+
+    const result = await loadAppSettingsFromStorage(deps);
+    const persisted = JSON.parse(deps.storage.entries.get(APP_SETTINGS_KEY) ?? "null");
+
+    expect(result.uiBaseFontSize).toBe(15);
+    expect(persisted).toMatchObject({ uiBaseFontSize: 15, uiFontSize: 17 });
+  });
+
   it("clamps the UI base font size into range and rejects non-numeric values", async () => {
     const deps = makeDeps({
       storage: createInMemoryKeyValueStorage({
diff --git a/packages/app/src/hooks/use-settings/storage.ts b/packages/app/src/hooks/use-settings/storage.ts
index db7d015482..d4efe753ac 100644
--- a/packages/app/src/hooks/use-settings/storage.ts
+++ b/packages/app/src/hooks/use-settings/storage.ts
@@ -182,7 +182,7 @@ const StoredAppSettingsSchema = z
     monoFontFamily: sanitizedFontFamily().catch(""),
     uiBaseFontSize: clampedNumber(MIN_UI_BASE_FONT_SIZE, MAX_UI_BASE_FONT_SIZE)
       .optional()
-      .catch(DEFAULT_UI_BASE_FONT_SIZE),
+      .catch(undefined),
     contentFontSize: clampedNumber(MIN_CONTENT_FONT_SIZE, MAX_CONTENT_FONT_SIZE)
       .optional()
       .catch(DEFAULT_CONTENT_FONT_SIZE),