diff --git a/tests/font-settings-rtl.test.mts b/tests/font-settings-rtl.test.mts
new file mode 100644
index 000000000..4e71665e5
--- /dev/null
+++ b/tests/font-settings-rtl.test.mts
@@ -0,0 +1,85 @@
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+import { readFileSync } from 'node:fs';
+import { dirname, resolve } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+import { applyFont } from '../src/services/font-settings.ts';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const mainCss = readFileSync(resolve(__dirname, '../src/styles/main.css'), 'utf-8');
+
+function withMockDocument(
+  run: (calls: { set: string[]; remove: string[]; dataset: Record<string, string | undefined> }) => void,
+): void {
+  const dataset: Record<string, string | undefined> = {};
+  const calls = { set: [] as string[], remove: [] as string[], dataset };
+  const originalDocument = globalThis.document;
+
+  Object.defineProperty(globalThis, 'document', {
+    configurable: true,
+    value: {
+      documentElement: {
+        dataset,
+        style: {
+          setProperty: (name: string) => {
+            calls.set.push(name);
+          },
+          removeProperty: (name: string) => {
+            calls.remove.push(name);
+          },
+        },
+      },
+    },
+  });
+
+  try {
+    run(calls);
+  } finally {
+    if (originalDocument === undefined) {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      delete (globalThis as any).document;
+    } else {
+      Object.defineProperty(globalThis, 'document', {
+        configurable: true,
+        value: originalDocument,
+      });
+    }
+  }
+}
+
+describe('font settings RTL-safe contract', () => {
+  it('applies the system preference through the data-font attribute only', () => {
+    withMockDocument((calls) => {
+      applyFont('system');
+      assert.equal(calls.dataset.font, 'system');
+      assert.deepEqual(calls.set, []);
+      assert.deepEqual(calls.remove, []);
+    });
+  });
+
+  it('removes the data-font attribute when switching back to mono', () => {
+    withMockDocument((calls) => {
+      calls.dataset.font = 'system';
+      applyFont('mono');
+      assert.equal(calls.dataset.font, undefined);
+      assert.deepEqual(calls.set, []);
+      assert.deepEqual(calls.remove, []);
+    });
+  });
+
+  it('lets system preference, RTL, and CJK rules compose through --font-body-base', () => {
+    assert.match(
+      mainCss,
+      /\[data-font="system"\]\s*\{\s*--font-body-base:\s*system-ui, -apple-system, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;/s,
+    );
+    assert.match(
+      mainCss,
+      /\[dir="rtl"\]\s*\{\s*--font-body:\s*'Tajawal', 'Geeza Pro', 'SF Arabic', 'Tahoma', var\(--font-body-base\);/s,
+    );
+    assert.match(
+      mainCss,
+      /:lang\(zh-CN\),\s*:lang\(zh\)\s*\{\s*--font-body:\s*'PingFang SC', 'Microsoft YaHei', 'Noto Sans SC', var\(--font-body-base\);/s,
+    );
+  });
+});