diff --git a/src/app/SmirkBridgeBootstrap.tsx b/src/app/SmirkBridgeBootstrap.tsx
new file mode 100644
index 0000000..3cdaea0
--- /dev/null
+++ b/src/app/SmirkBridgeBootstrap.tsx
@@ -0,0 +1,22 @@
+'use client';
+
+/**
+ * `SmirkBridgeBootstrap` — mounts the `installSmirkPageApi` runtime
+ * on every page of smirk.cash so the site works both inside the
+ * v0.2.x browser extension (no-op; extension wins) and inside the
+ * v0.3.0+ desktop wallet's embedded iframe (postMessage transport).
+ *
+ * Lives in the root layout so every route picks it up automatically —
+ * page.tsx, TipPageClient.tsx, future id/forum subdomains, all of it.
+ * No other files need to know about the v0.3.0 transport.
+ */
+
+import { useEffect } from 'react';
+import { installSmirkPageApi } from '@/lib/smirk-iframe-bridge';
+
+export function SmirkBridgeBootstrap(): null {
+  useEffect(() => {
+    installSmirkPageApi();
+  }, []);
+  return null;
+}
diff --git a/src/app/layout.tsx b/src/app/layout.tsx
index 960a455..fbbcb1e 100644
--- a/src/app/layout.tsx
+++ b/src/app/layout.tsx
@@ -1,6 +1,7 @@
 import type { Metadata } from "next";
 import { Geist_Mono } from "next/font/google";
 import "./globals.css";
+import { SmirkBridgeBootstrap } from "./SmirkBridgeBootstrap";
 
 const geistMono = Geist_Mono({
   variable: "--font-geist-mono",
@@ -46,6 +47,7 @@ export default function RootLayout({
   return (
     <html lang="en">
       <body className={`${geistMono.variable} font-mono antialiased min-h-screen flex flex-col`}>
+        <SmirkBridgeBootstrap />
         <main className="flex-1 flex flex-col">{children}</main>
         <footer className="py-4 text-center text-xs text-zinc-500 flex items-center justify-center gap-3 flex-wrap">
           <a href="/stats" className="hover:text-[#fbeb0a] transition-colors">
diff --git a/src/lib/smirk-iframe-bridge.ts b/src/lib/smirk-iframe-bridge.ts
new file mode 100644
index 0000000..1369385
--- /dev/null
+++ b/src/lib/smirk-iframe-bridge.ts
@@ -0,0 +1,159 @@
+/**
+ * `installSmirkPageApi` — runtime installer for `window.smirk`.
+ *
+ * Local copy of `@smirk/dapp-api`'s `installSmirkPageApi`, kept here
+ * until that package is published to npm. **When @smirk/dapp-api is
+ * available, replace this file with:**
+ *
+ * ```ts
+ * export { installSmirkPageApi } from '@smirk/dapp-api';
+ * ```
+ *
+ * Source of truth lives at
+ * `smirk-monorepo/packages/dapp-api/src/install-page-api.ts`.
+ * Any change to this file should be mirrored there (and vice versa).
+ *
+ * Behaviour:
+ *
+ * 1. If `window.smirk` is already defined, the v0.2.x browser
+ *    extension content script already ran. We leave it alone —
+ *    every existing dapp continues to work untouched.
+ *
+ * 2. If `window.parent !== window`, the page is iframed by something.
+ *    The Smirk v0.3.0 desktop wallet's embedded browser is the
+ *    relevant case. We install a `window.smirk` whose every method
+ *    posts a `SMIRK_REQUEST` envelope to `window.parent` and resolves
+ *    on the matching response. UI surface (approval modals etc.) is
+ *    the wallet's responsibility.
+ *
+ * 3. Otherwise (no extension, no iframe parent) we install nothing.
+ *    The page's existing "extension not found" fallback applies.
+ *
+ * Safe to call multiple times. Returns a hint at what it did.
+ */
+
+const DEFAULT_POSTMESSAGE_CHANNEL = 'smirk:dapp';
+const PROTOCOL_VERSION = 1;
+
+export interface InstallSmirkPageApiOptions {
+  readonly channel?: string;
+  readonly timeoutMs?: number;
+  readonly mode?: 'auto' | 'force' | 'never';
+  readonly onReady?: () => void;
+}
+
+export function installSmirkPageApi(
+  options: InstallSmirkPageApiOptions = {},
+): 'extension-present' | 'iframe-mode' | 'none' {
+  if (typeof window === 'undefined') return 'none';
+
+  if (typeof (window as Window & { smirk?: unknown }).smirk !== 'undefined') {
+    return 'extension-present';
+  }
+
+  const mode = options.mode ?? 'auto';
+  if (mode === 'never') return 'none';
+  if (mode === 'auto' && window.parent === window) return 'none';
+
+  const channel = options.channel ?? DEFAULT_POSTMESSAGE_CHANNEL;
+  const timeoutMs = options.timeoutMs ?? 30_000;
+
+  installPostMessageRuntime(channel, timeoutMs);
+  options.onReady?.();
+  return 'iframe-mode';
+}
+
+interface PendingEntry {
+  resolve: (value: unknown) => void;
+  reject: (reason: unknown) => void;
+  timer: ReturnType<typeof setTimeout>;
+}
+
+interface WireResponse {
+  type: 'SMIRK_RESPONSE';
+  v: number;
+  id: number;
+  result?: unknown;
+  error?: { code: string; message: string };
+}
+
+function installPostMessageRuntime(channel: string, timeoutMs: number): void {
+  const pending = new Map<number, PendingEntry>();
+  let nextId = 1;
+
+  function request(method: string, params: unknown): Promise<unknown> {
+    return new Promise((resolve, reject) => {
+      const id = nextId++;
+      const timer = setTimeout(() => {
+        pending.delete(id);
+        reject(
+          Object.assign(new Error(`Smirk wallet timed out responding to ${method}`), {
+            code: 'TIMEOUT',
+          }),
+        );
+      }, timeoutMs);
+      pending.set(id, { resolve, reject, timer });
+      window.parent.postMessage(
+        {
+          channel,
+          payload: {
+            type: 'SMIRK_REQUEST',
+            v: PROTOCOL_VERSION,
+            id,
+            method,
+            params: params ?? {},
+          },
+        },
+        '*',
+      );
+    });
+  }
+
+  window.addEventListener('message', (ev: MessageEvent) => {
+    const data = ev.data as
+      | { channel?: unknown; payload?: WireResponse }
+      | null
+      | undefined;
+    if (!data || data.channel !== channel) return;
+    const resp = data.payload;
+    if (!resp || resp.type !== 'SMIRK_RESPONSE') return;
+    const entry = pending.get(resp.id);
+    if (!entry) return;
+    pending.delete(resp.id);
+    clearTimeout(entry.timer);
+    if (resp.error) {
+      entry.reject(
+        Object.assign(new Error(resp.error.message), { code: resp.error.code }),
+      );
+    } else {
+      entry.resolve(resp.result);
+    }
+  });
+
+  const api = {
+    isInstalled: (): boolean => true,
+    protocolVersion: (): number => PROTOCOL_VERSION,
+    isUnlocked: (): Promise<boolean> => request('isUnlocked', {}) as Promise<boolean>,
+    isConnected: (): Promise<boolean> => request('isConnected', {}) as Promise<boolean>,
+    connect: (assets?: string[]): Promise<unknown> => request('connect', { assets }),
+    disconnect: (): Promise<void> => request('disconnect', {}) as Promise<void>,
+    getAddresses: (assets?: string[]): Promise<unknown> =>
+      request('getAddresses', { assets }),
+    getPublicKeys: (assets?: string[]): Promise<unknown> =>
+      request('getPublicKeys', { assets }),
+    signMessage: (message: string, assets?: string[]): Promise<unknown> =>
+      request('signMessage', { message, assets }),
+    requestPayment: (req: unknown): Promise<unknown> =>
+      request('requestPayment', req),
+    claimPublicTip: (tipId: string, fragmentKey: string): Promise<unknown> =>
+      request('claimPublicTip', { tipId, fragmentKey }),
+  };
+
+  Object.defineProperty(window, 'smirk', {
+    value: api,
+    writable: false,
+    configurable: false,
+    enumerable: true,
+  });
+  window.dispatchEvent(new CustomEvent('smirk-ready'));
+}