diff --git a/app/public/assets/posts/california-wealth-tax-calculator.png b/app/public/assets/posts/california-wealth-tax-calculator.png
new file mode 100644
index 000000000..efb89b551
Binary files /dev/null and b/app/public/assets/posts/california-wealth-tax-calculator.png differ
diff --git a/app/src/components/IframeContent.tsx b/app/src/components/IframeContent.tsx
index b044f146a..7d6abff4d 100644
--- a/app/src/components/IframeContent.tsx
+++ b/app/src/components/IframeContent.tsx
@@ -24,7 +24,7 @@ export default function IframeContent({
   // Derive allowed origin from iframe URL for postMessage validation
   const iframeOrigin = useMemo(() => {
     try {
-      return new URL(url).origin;
+      return new URL(url, window.location.origin).origin;
     } catch {
       return '';
     }
diff --git a/app/src/data/apps/appTransformers.test.ts b/app/src/data/apps/appTransformers.test.ts
index b34f3d400..16cf5963a 100644
--- a/app/src/data/apps/appTransformers.test.ts
+++ b/app/src/data/apps/appTransformers.test.ts
@@ -28,8 +28,8 @@ describe('appTransformers', () => {
 
   test('all apps have valid source URLs', () => {
     apps.forEach((app) => {
-      // Allow external URLs (https://) or local paths (/assets/)
-      expect(app.source).toMatch(/^(https?:\/\/|\/assets\/)/);
+      // Allow external URLs, asset files, or same-origin app proxy paths.
+      expect(app.source).toMatch(/^(https?:\/\/|\/assets\/|\/[a-z]{2}\/)/);
     });
   });
 
diff --git a/app/src/data/apps/apps.json b/app/src/data/apps/apps.json
index 2b982f884..b33997b10 100644
--- a/app/src/data/apps/apps.json
+++ b/app/src/data/apps/apps.json
@@ -75,6 +75,19 @@
     "date": "2026-03-06 12:00:00",
     "authors": ["max-ghenis", "pavel-makarchuk"]
   },
+  {
+    "type": "iframe",
+    "slug": "california-wealth-tax",
+    "title": "California wealth tax fiscal impact calculator",
+    "description": "Estimate how California's proposed billionaire wealth tax changes revenue once avoidance, migration, return flows, and income-tax offsets are taken into account",
+    "source": "/us/california-wealth-tax/embed",
+    "tags": ["us", "us-ca", "policy", "interactives", "tax"],
+    "countryId": "us",
+    "displayWithResearch": true,
+    "image": "california-wealth-tax-calculator.png",
+    "date": "2026-03-25 12:00:00",
+    "authors": ["max-ghenis"]
+  },
   {
     "type": "iframe",
     "slug": "uk-land-value-tax",
diff --git a/app/src/tests/unit/components/IframeContent.test.tsx b/app/src/tests/unit/components/IframeContent.test.tsx
new file mode 100644
index 000000000..7bd6397c2
--- /dev/null
+++ b/app/src/tests/unit/components/IframeContent.test.tsx
@@ -0,0 +1,43 @@
+import { render } from '@testing-library/react';
+import { MemoryRouter, Route, Routes } from 'react-router-dom';
+import { afterEach, describe, expect, test, vi } from 'vitest';
+import IframeContent from '@/components/IframeContent';
+
+describe('IframeContent', () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  test('accepts same-origin relative iframe URLs for hash sync messages', () => {
+    const replaceStateSpy = vi.spyOn(window.history, 'replaceState');
+
+    render(
+      <MemoryRouter initialEntries={['/us/california-wealth-tax']}>
+        <Routes>
+          <Route
+            path="/:countryId/:slug"
+            element={
+              <IframeContent url="/us/california-wealth-tax/embed" title="California wealth tax" />
+            }
+          />
+        </Routes>
+      </MemoryRouter>
+    );
+
+    window.dispatchEvent(
+      new MessageEvent('message', {
+        origin: window.location.origin,
+        data: {
+          type: 'hashchange',
+          hash: '/scenario/rauh',
+        },
+      })
+    );
+
+    expect(replaceStateSpy).toHaveBeenCalledWith(
+      null,
+      '',
+      '/us/california-wealth-tax/scenario/rauh'
+    );
+  });
+});
diff --git a/app/vite.config.mjs b/app/vite.config.mjs
index 643f6f995..b88d01222 100644
--- a/app/vite.config.mjs
+++ b/app/vite.config.mjs
@@ -89,6 +89,13 @@ export default defineConfig({
     // Use discovered ports in dev, defaults otherwise
     port: appMode === 'calculator' ? (calculatorPort ?? 3001) : (websitePort ?? 3000),
     strictPort: true,
+    proxy: {
+      '/us/california-wealth-tax/embed': {
+        target: 'https://california-wealth-tax.vercel.app',
+        changeOrigin: true,
+        secure: true,
+      },
+    },
   },
   define: viteDefines,
   // Use separate cache directories for website and calculator to avoid conflicts
diff --git a/vercel.json b/vercel.json
index 50ccfe947..9296c2134 100644
--- a/vercel.json
+++ b/vercel.json
@@ -65,6 +65,14 @@
       "source": "/us/keep-your-pay-act/:path*",
       "destination": "https://keep-your-pay-act.vercel.app/us/keep-your-pay-act/:path*"
     },
+    {
+      "source": "/us/california-wealth-tax/embed",
+      "destination": "https://california-wealth-tax.vercel.app/us/california-wealth-tax/embed"
+    },
+    {
+      "source": "/us/california-wealth-tax/embed/:path*",
+      "destination": "https://california-wealth-tax.vercel.app/us/california-wealth-tax/embed/:path*"
+    },
     {
       "source": "/us/taxsim",
       "destination": "https://policyengine-taxsim-policy-engine.vercel.app/us/taxsim"