diff --git a/tests/playwright/demo-interactions.spec.js b/tests/playwright/demo-interactions.spec.js
new file mode 100644
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--- /dev/null
+++ b/tests/playwright/demo-interactions.spec.js
@@ -0,0 +1,455 @@
+// @ts-check
+/**
+ * Demo interaction tests — verify every demo actually functions.
+ *
+ * Each test loads a demo, interacts with it (click buttons, type in inputs),
+ * and verifies the DOM changes as expected. Also checks no raw SX class text
+ * leaks into the visible DOM.
+ */
+const { test, expect } = require('playwright/test');
+const BASE_URL = process.env.SX_TEST_URL || 'http://localhost:8013';
+
+// Helper: assert no raw class text leaked
+async function assertNoClassLeak(page, scope) {
+  const loc = scope ? page.locator(scope).first() : page.locator('#sx-root');
+  const text = await loc.textContent();
+  expect(text).not.toContain('classpx');
+  expect(text).not.toContain('classborder');
+  expect(text).not.toContain('classbg-');
+}
+
+// Helper: get island locator
+function island(page, pattern) {
+  return page.locator(`[data-sx-island*="${pattern}"]`);
+}
+
+
+// ===========================================================================
+// Reactive island demos
+// ===========================================================================
+
+test.describe('Reactive island interactions', () => {
+
+  test('counter: + and − change count and doubled', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.counter)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'counter');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    const buttons = el.locator('button');
+    await expect(buttons).toHaveCount(2);
+
+    // Click + 3 times
+    const plus = buttons.last();
+    await plus.click(); await plus.click(); await plus.click();
+    await page.waitForTimeout(300);
+
+    const text = await el.textContent();
+    expect(text).toContain('3');
+    expect(text).toContain('doubled');
+    expect(text).toContain('6');
+
+    // Click − once
+    await buttons.first().click();
+    await page.waitForTimeout(300);
+    const text2 = await el.textContent();
+    expect(text2).toContain('2');
+  });
+
+  test('temperature: +/− change celsius and fahrenheit', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.temperature)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'temperature');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    const buttons = el.locator('button');
+    // Click + a few times
+    await buttons.last().click();
+    await buttons.last().click();
+    await page.waitForTimeout(300);
+
+    const text = await el.textContent();
+    // Should show °C and °F
+    expect(text).toContain('°C');
+    expect(text).toContain('°F');
+  });
+
+  test('stopwatch: start shows elapsed time', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.stopwatch)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'stopwatch');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    const textBefore = await el.textContent();
+    // Click start (first button)
+    await el.locator('button').first().click();
+    await page.waitForTimeout(1500);
+    const textAfter = await el.textContent();
+    expect(textAfter).not.toBe(textBefore);
+  });
+
+  test('input-binding: typing updates live preview', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.input-binding)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'input-binding');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    const input = el.locator('input').first();
+    await input.fill('playwright test');
+    await page.waitForTimeout(300);
+
+    const text = await el.textContent();
+    expect(text).toContain('playwright test');
+  });
+
+  test('dynamic-class: toggle changes element styling', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.dynamic-class)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'dynamic-class');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    const htmlBefore = await el.innerHTML();
+    await el.locator('button').first().click();
+    await page.waitForTimeout(300);
+    const htmlAfter = await el.innerHTML();
+    expect(htmlAfter).not.toBe(htmlBefore);
+  });
+
+  test('reactive-list: add button increases items', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.reactive-list)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'reactive-list');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    const textBefore = await el.textContent();
+    // Click add button
+    await el.locator('button').first().click();
+    await page.waitForTimeout(300);
+    const textAfter = await el.textContent();
+    expect(textAfter).not.toBe(textBefore);
+  });
+
+  test('stores: writer and reader both render and share state', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.stores)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const writer = island(page, 'store-writer');
+    const reader = island(page, 'store-reader');
+    await expect(writer).toBeVisible({ timeout: 10000 });
+    await expect(reader).toBeVisible({ timeout: 10000 });
+
+    // Both islands should have content
+    expect((await writer.textContent()).length).toBeGreaterThan(0);
+    expect((await reader.textContent()).length).toBeGreaterThan(0);
+
+    // Interact with writer — try button then input
+    const btn = writer.locator('button').first();
+    const input = writer.locator('input').first();
+    if (await btn.count() > 0) {
+      await btn.click();
+      await page.waitForTimeout(500);
+    } else if (await input.count() > 0) {
+      const type = await input.getAttribute('type');
+      if (type === 'checkbox') {
+        await input.check();
+      } else {
+        await input.fill('test');
+      }
+      await page.waitForTimeout(500);
+    }
+    // Islands should still be visible and functional
+    await expect(writer).toBeVisible();
+    await expect(reader).toBeVisible();
+  });
+
+  test('refs: focus button focuses input', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.refs)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'refs');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    // Click a button — should interact with the input
+    const textBefore = await el.textContent();
+    await el.locator('button').first().click();
+    await page.waitForTimeout(500);
+    // Verify something changed or input is focused
+    const focused = await page.evaluate(() => document.activeElement?.tagName);
+    // Accept either text change or input focus
+    const textAfter = await el.textContent();
+    expect(textAfter !== textBefore || focused === 'INPUT').toBeTruthy();
+  });
+
+  test('portal: button toggles portal content', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.portal)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'portal');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    const htmlBefore = await page.locator('#sx-root').innerHTML();
+    await el.locator('button').first().click();
+    await page.waitForTimeout(500);
+    const htmlAfter = await page.locator('#sx-root').innerHTML();
+    expect(htmlAfter).not.toBe(htmlBefore);
+  });
+
+  test('imperative: button triggers DOM manipulation', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.imperative)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'imperative');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    const textBefore = await el.textContent();
+    await el.locator('button').first().click();
+    await page.waitForTimeout(500);
+    const textAfter = await el.textContent();
+    expect(textAfter).not.toBe(textBefore);
+  });
+
+  test('error-boundary: trigger error shows boundary message', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.error-boundary)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'error-boundary');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    // Click trigger error button
+    const triggerBtn = el.locator('button').filter({ hasText: /error|trigger|throw/i }).first();
+    if (await triggerBtn.count() > 0) {
+      await triggerBtn.click();
+      await page.waitForTimeout(500);
+      const text = await el.textContent();
+      // Should show error message, not crash
+      expect(text.toLowerCase()).toMatch(/error|caught|boundary/);
+    }
+  });
+
+  test('event-bridge: sender triggers receiver update', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.event-bridge-demo)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'event-bridge');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    const textBefore = await el.textContent();
+    await el.locator('button').first().click();
+    await page.waitForTimeout(500);
+    const textAfter = await el.textContent();
+    expect(textAfter).not.toBe(textBefore);
+  });
+
+  test('transition: island renders and toggle works', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.transition)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'transition');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    // Island should have content
+    const text = await el.textContent();
+    expect(text.length).toBeGreaterThan(0);
+
+    // Find any button/input in the island
+    const btn = el.locator('button, input[type="checkbox"]').first();
+    if (await btn.count() > 0) {
+      const htmlBefore = await el.innerHTML();
+      await btn.click();
+      await page.waitForTimeout(1500);
+      // After animation, island should still be functional
+      await expect(el).toBeVisible();
+    }
+  });
+
+  test('resource: shows loading then data', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.resource)))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'resource');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    // Should show loading or resolved data
+    const text = await el.textContent();
+    expect(text.length).toBeGreaterThan(0);
+    // Wait for resource to resolve
+    await page.waitForTimeout(3000);
+    const text2 = await el.textContent();
+    // Should show the resolved name
+    expect(text2).toContain('Ada');
+  });
+});
+
+
+// ===========================================================================
+// Marshes demos — where reactivity meets hypermedia
+// ===========================================================================
+
+test.describe('Marshes interactions', () => {
+
+  test('hypermedia-feeds: reactive +/− and server fetch both work', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(marshes.hypermedia-feeds))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'marsh-product');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    // Click + button (reactive signal) — should change quantity/price
+    const plusBtn = el.locator('button:has-text("+")').first();
+    if (await plusBtn.count() > 0) {
+      const textBefore = await el.textContent();
+      await plusBtn.click();
+      await page.waitForTimeout(300);
+      const textAfter = await el.textContent();
+      expect(textAfter).not.toBe(textBefore);
+    }
+
+    // Click server fetch button — should update server message
+    const fetchBtn = page.locator('button:has-text("Fetch")').first();
+    if (await fetchBtn.count() > 0) {
+      await fetchBtn.click();
+      await page.waitForTimeout(3000);
+    }
+
+    await assertNoClassLeak(page, '[data-sx-island*="marsh-product"]');
+  });
+
+  test('on-settle: sx-on-settle evaluates after swap', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(marshes.on-settle))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'marsh-settle');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    const textBefore = await el.textContent();
+    const btn = el.locator('button').first();
+    if (await btn.count() > 0) {
+      await btn.click();
+      await page.waitForTimeout(3000);
+      const textAfter = await el.textContent();
+      expect(textAfter).not.toBe(textBefore);
+    }
+    await assertNoClassLeak(page, '[data-sx-island*="marsh-settle"]');
+  });
+
+  test('server-signals: server fetch writes to client signal', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(marshes.server-signals))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const writer = island(page, 'marsh-store-writer');
+    const reader = island(page, 'marsh-store-reader');
+    await expect(writer).toBeVisible({ timeout: 10000 });
+    await expect(reader).toBeVisible({ timeout: 10000 });
+
+    // Click the server fetch button (sx-get/sx-post, not reactive +/−)
+    const fetchBtn = writer.locator('button[sx-get], button[sx-post], button:has-text("Fetch"), button:has-text("Load")').first();
+    if (await fetchBtn.count() > 0) {
+      const readerBefore = await reader.textContent();
+      await fetchBtn.click();
+      await page.waitForTimeout(3000);
+      const readerAfter = await reader.textContent();
+      expect(readerAfter).not.toBe(readerBefore);
+    } else {
+      // Fall back: click + button in writer
+      const plus = writer.locator('button:has-text("+")').first();
+      if (await plus.count() > 0) {
+        const before = await writer.textContent();
+        await plus.click();
+        await page.waitForTimeout(300);
+        expect(await writer.textContent()).not.toBe(before);
+      }
+    }
+    await assertNoClassLeak(page, '[data-sx-island*="marsh-store"]');
+  });
+
+  test('signal-triggers: button changes signal, triggers fetch', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(marshes.signal-triggers))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'marsh-signal-url');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    // Click buttons to change the signal — may trigger a server fetch
+    const buttons = el.locator('button');
+    if (await buttons.count() >= 2) {
+      const textBefore = await el.textContent();
+      await buttons.nth(1).click(); // try second button (different from current state)
+      await page.waitForTimeout(3000);
+      const textAfter = await el.textContent();
+      // Accept either text change or same (if button is current state)
+    }
+    // Island should render properly
+    await expect(el).toBeVisible();
+  });
+
+  test('view-transform: view toggle changes rendering', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography.(marshes.view-transform))', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    const el = island(page, 'marsh-view-transform');
+    await expect(el).toBeVisible({ timeout: 10000 });
+
+    // Click "Fetch Catalog" to load data, then toggle view
+    const fetchBtn = page.locator('button:has-text("Fetch"), button[sx-get]').first();
+    if (await fetchBtn.count() > 0) {
+      await fetchBtn.click();
+      await page.waitForTimeout(3000);
+    }
+
+    // Toggle view buttons
+    const viewBtns = el.locator('button');
+    if (await viewBtns.count() >= 2) {
+      const htmlBefore = await el.innerHTML();
+      await viewBtns.nth(1).click();
+      await page.waitForTimeout(500);
+      const htmlAfter = await el.innerHTML();
+      expect(htmlAfter).not.toBe(htmlBefore);
+    }
+  });
+});
+
+
+// ===========================================================================
+// JIT health check — no DISABLED or SSR failures after warmup
+// ===========================================================================
+
+test.describe('Server health', () => {
+
+  test('no JS errors on reactive demo pages', async ({ page }) => {
+    const errors = [];
+    page.on('pageerror', err => errors.push(err.message));
+
+    const demos = ['counter', 'temperature', 'stopwatch', 'input-binding',
+      'dynamic-class', 'reactive-list', 'stores', 'resource'];
+
+    for (const demo of demos) {
+      await page.goto(BASE_URL + `/sx/(geography.(reactive.(examples.${demo})))`, { waitUntil: 'networkidle' });
+      await page.waitForTimeout(1000);
+    }
+
+    const real = errors.filter(e => !e.includes('net::ERR') && !e.includes('fetch'));
+    expect(real).toEqual([]);
+  });
+
+  test('no JS errors on marshes pages', async ({ page }) => {
+    const errors = [];
+    page.on('pageerror', err => errors.push(err.message));
+
+    const pages = ['hypermedia-feeds', 'on-settle', 'server-signals',
+      'signal-triggers', 'view-transform'];
+
+    for (const p of pages) {
+      await page.goto(BASE_URL + `/sx/(geography.(marshes.${p}))`, { waitUntil: 'networkidle' });
+      await page.waitForTimeout(1000);
+    }
+
+    const real = errors.filter(e => !e.includes('net::ERR') && !e.includes('fetch'));
+    expect(real).toEqual([]);
+  });
+});