diff --git a/test/content-engine.js b/test/content-engine.js
new file mode 100644
index 0000000..4cbf3fb
--- /dev/null
+++ b/test/content-engine.js
@@ -0,0 +1,192 @@
+"use strict";
+
+const assert = require("node:assert/strict");
+const fs = require("node:fs");
+const path = require("node:path");
+const { JSDOM } = require("jsdom");
+
+const source = fs.readFileSync(path.join(__dirname, "../src/content.js"), "utf8");
+
+function createExtensionDom(body, overrides = {}) {
+  const settings = {
+    enabled: true,
+    fixDirection: true,
+    useFont: true,
+    fixInputs: true,
+    disabledSites: [],
+    ...overrides
+  };
+  let onChanged;
+  const dom = new JSDOM(`<!doctype html><html><head></head><body>${body}</body></html>`, {
+    url: "https://chat.example/",
+    runScripts: "dangerously",
+    pretendToBeVisual: true
+  });
+  dom.window.requestIdleCallback = (callback) => callback({ didTimeout: false, timeRemaining: () => 50 });
+  dom.window.chrome = {
+    runtime: {
+      getURL: (file) => `chrome-extension://test/${file}`,
+      lastError: null
+    },
+    storage: {
+      local: { get: (_defaults, callback) => callback(settings) },
+      onChanged: { addListener: (callback) => { onChanged = callback; } }
+    }
+  };
+  const script = dom.window.document.createElement("script");
+  script.textContent = source;
+  dom.window.document.head.appendChild(script);
+  return {
+    dom,
+    changeSettings(patch) {
+      for (const [key, value] of Object.entries(patch)) settings[key] = value;
+      const changes = Object.fromEntries(Object.entries(patch).map(([key, value]) => [key, { newValue: value }]));
+      onChanged(changes, "local");
+    }
+  };
+}
+
+function element(body, id = "target", overrides) {
+  const fixture = createExtensionDom(body, overrides);
+  return { fixture, target: fixture.dom.window.document.getElementById(id) };
+}
+
+const tests = [];
+function test(name, fn) { tests.push({ name, fn }); }
+function nextTurn() { return new Promise((resolve) => setTimeout(resolve, 0)); }
+
+test("real Persian text is corrected to RTL", () => {
+  const { target } = element('<p id="target">این یک متن فارسی است</p>');
+  assert.equal(target.getAttribute("dir"), "rtl");
+});
+
+test("Arabic-only text is not treated as Persian", () => {
+  const { target } = element('<p id="target">هذا نص عربي للاختبار</p>');
+  assert.equal(target.hasAttribute("dir"), false);
+  assert.equal(target.hasAttribute("data-rtlx-font"), false);
+});
+
+test("English-leading Persian text uses auto direction", () => {
+  const { target } = element('<p id="target">The answer is متن فارسی برای آزمایش.</p>');
+  assert.equal(target.getAttribute("dir"), "auto");
+  assert.equal(target.hasAttribute("data-rtlx-rtl"), false);
+});
+
+test("Latin digits inside Persian text remain byte-identical", () => {
+  const text = "نسخه 1234567890 برای آزمایش";
+  const { target } = element(`<p id="target">${text}</p>`);
+  assert.equal(target.textContent, text);
+});
+
+test("a sensible explicit RTL direction is preserved without ownership markers", () => {
+  const { target } = element('<p id="target" dir="rtl">این متن از قبل راست‌چین است</p>');
+  assert.equal(target.getAttribute("dir"), "rtl");
+  assert.equal(target.hasAttribute("data-rtlx-setdir"), false);
+});
+
+test("a sensible explicit auto direction is preserved without ownership markers", () => {
+  const { target } = element('<p id="target" dir="auto">این متن جهت خودکار دارد</p>');
+  assert.equal(target.getAttribute("dir"), "auto");
+  assert.equal(target.hasAttribute("data-rtlx-setdir"), false);
+});
+
+test("a clearly wrong explicit LTR direction is corrected reversibly", () => {
+  const { target } = element('<p id="target" dir="ltr">این متن فارسی راست‌به‌چپ است</p>');
+  assert.equal(target.getAttribute("dir"), "rtl");
+  assert.equal(target.getAttribute("data-rtlx-dir"), "ltr");
+});
+
+test("an inline author font is never replaced", () => {
+  const { target } = element('<p id="target" style="font-family: Arial">این متن فونت دارد</p>');
+  assert.equal(target.style.fontFamily, "Arial");
+  assert.equal(target.hasAttribute("data-rtlx-font"), false);
+});
+
+test("an inherited author font is never replaced", () => {
+  const { target } = element('<div style="font-family: sans-serif"><p id="target">این متن فونت ارث‌بری دارد</p></div>');
+  assert.equal(target.style.fontFamily, "");
+  assert.equal(target.hasAttribute("data-rtlx-font"), false);
+});
+
+test("a stylesheet font on the Persian element is never replaced", () => {
+  const { target } = element('<style>.authored { font-family: system-ui; }</style><p id="target" class="authored">این متن فونت دارد</p>');
+  assert.equal(target.style.fontFamily, "");
+  assert.equal(target.hasAttribute("data-rtlx-font"), false);
+});
+
+test("an inherited stylesheet font is never replaced", () => {
+  const { target } = element('<style>.chat { font-family: Tahoma; }</style><article class="chat"><p id="target">این پاسخ هوش مصنوعی است</p></article>');
+  assert.equal(target.style.fontFamily, "");
+  assert.equal(target.hasAttribute("data-rtlx-font"), false);
+});
+
+test("unstyled Persian text receives Vazirmatn", () => {
+  const { target } = element('<p id="target">این متن بدون فونت است</p>');
+  assert.match(target.style.fontFamily, /Vazirmatn/i);
+});
+
+test("streamed Persian text is corrected and Latin digits are untouched", async () => {
+  const { fixture, target } = element('<p id="target">Waiting</p>');
+  target.textContent = "پاسخ زنده 1234567890";
+  await nextTurn();
+  assert.equal(target.getAttribute("dir"), "rtl");
+  assert.equal(target.textContent, "پاسخ زنده 1234567890");
+  fixture.dom.window.close();
+});
+
+test("a streamed block that becomes English is restored", async () => {
+  const { fixture, target } = element('<p id="target">پاسخ فارسی</p>');
+  assert.equal(target.getAttribute("dir"), "rtl");
+  target.textContent = "The final answer is English.";
+  await nextTurn();
+  assert.equal(target.hasAttribute("dir"), false);
+  assert.equal(target.hasAttribute("data-rtlx-rtl"), false);
+  assert.equal(target.style.fontFamily, "");
+  fixture.dom.window.close();
+});
+
+test("disabling the extension restores owned direction and font", () => {
+  const { fixture, target } = element('<p id="target">پاسخ فارسی</p>');
+  fixture.changeSettings({ enabled: false });
+  assert.equal(target.hasAttribute("dir"), false);
+  assert.equal(target.style.fontFamily, "");
+});
+
+test("unstyled inputs receive Vazirmatn only after Persian is present", () => {
+  const { fixture, target } = element('<input id="target" type="text" />');
+  assert.equal(target.style.fontFamily, "");
+  target.value = "متن فارسی";
+  target.dispatchEvent(new fixture.dom.window.Event("input", { bubbles: true }));
+  assert.match(target.style.fontFamily, /Vazirmatn/i);
+});
+
+test("author-styled Persian inputs preserve their font", () => {
+  const { target } = element('<input id="target" type="text" style="font-family: Arial" value="متن فارسی" />');
+  assert.equal(target.style.fontFamily, "Arial");
+  assert.equal(target.hasAttribute("data-rtlx-font"), false);
+});
+
+test("input listeners do not reapply styling while the extension is disabled", () => {
+  const { fixture, target } = element('<input id="target" type="text" />');
+  fixture.changeSettings({ enabled: false });
+  target.value = "متن فارسی";
+  target.dispatchEvent(new fixture.dom.window.Event("input", { bubbles: true }));
+  assert.equal(target.style.fontFamily, "");
+  assert.equal(target.hasAttribute("data-rtlx-font"), false);
+});
+
+(async () => {
+  let passed = 0;
+  for (const { name, fn } of tests) {
+    try {
+      await fn();
+      passed += 1;
+      console.log(`PASS ${name}`);
+    } catch (error) {
+      console.error(`FAIL ${name}`);
+      console.error(error.message);
+    }
+  }
+  console.log(`\n${passed}/${tests.length} passed`);
+  process.exitCode = passed === tests.length ? 0 : 1;
+})();