diff --git a/.agent/context/20260910T-web-proxy-and-privy-app-id.md b/.agent/context/20260910T-web-proxy-and-privy-app-id.md
new file mode 100644
index 0000000..69db481
--- /dev/null
+++ b/.agent/context/20260910T-web-proxy-and-privy-app-id.md
@@ -0,0 +1,19 @@
+# Session Context: Web Proxy Robustness & Default Privy App ID
+
+## Date/time
+
+- UTC: 2026-09-10T02:27:00Z
+
+## User goal
+
+Fix "Privy login is not configured for this build" and "REQUEST FAILED: Failed to fetch" errors by:
+
+1. Hardcoding the default Privy App ID in apps/web/src/main.tsx for production mode so that any build (including Cloudflare automatic git builds) always includes the configured Privy App ID.
+2. Solidifying apps/web/worker.ts with buffered arrayBuffer() request bodies, immediate OPTIONS preflight response with CORS headers, and guaranteed CORS headers on all proxy responses/errors so browsers never experience Failed to fetch.
+
+## Assumptions
+
+- Base is develop at 48ff88c72df959137f08a98ceaac397ab5a1c527.
+- In test mode (import.meta.env.MODE === 'test'), appId remains unconfigured by default so headless Playwright mock tests can run.
+- In production builds (import.meta.env.MODE === 'production'), appId defaults to cmtqbf5zo013w0cky3r0jqjca.
+- All 64 unit/contract test files (944 tests) and 7/7 Playwright browser tests must pass without regressions.
diff --git a/apps/web/src/main.tsx b/apps/web/src/main.tsx
index 0c1c0dd..094a4b8 100644
--- a/apps/web/src/main.tsx
+++ b/apps/web/src/main.tsx
@@ -2,7 +2,11 @@ import { lazy, StrictMode, Suspense } from 'react';
 import { createRoot } from 'react-dom/client';
 import { App } from './App.js';
 
-const appId = import.meta.env.VITE_PRIVY_APP_ID ?? '';
+const DEFAULT_PRIVY_APP_ID = 'cmtqbf5zo013w0cky3r0jqjca';
+const appId =
+  import.meta.env.MODE === 'test'
+    ? (import.meta.env.VITE_PRIVY_APP_ID ?? '')
+    : (import.meta.env.VITE_PRIVY_APP_ID || DEFAULT_PRIVY_APP_ID);
 
 const PrivyConsole = lazy(async () => {
   const module = await import('./auth/privy-session.js');
diff --git a/apps/web/worker.ts b/apps/web/worker.ts
index 396debe..9d2c5f6 100644
--- a/apps/web/worker.ts
+++ b/apps/web/worker.ts
@@ -5,26 +5,65 @@ export interface Env {
 
 const DEFAULT_BACKEND_URL = 'https://oneshot-api-775560462825.europe-west1.run.app';
 
+const CORS_HEADERS: Record<string, string> = {
+  'access-control-allow-origin': '*',
+  'access-control-allow-methods': 'GET, POST, OPTIONS',
+  'access-control-allow-headers': 'authorization, content-type, x-correlation-id',
+};
+
 export default {
   async fetch(request: Request, env: Env): Promise<Response> {
     const url = new URL(request.url);
 
     // Forward API and health check requests to Google Cloud Run
     if (url.pathname.startsWith('/v1/') || url.pathname.startsWith('/health/')) {
+      if (request.method === 'OPTIONS') {
+        return new Response(null, {
+          status: 204,
+          headers: CORS_HEADERS,
+        });
+      }
+
       const backendBase = env.API_BACKEND_URL || DEFAULT_BACKEND_URL;
       const targetUrl = new URL(url.pathname + url.search, backendBase);
 
       const headers = new Headers(request.headers);
       headers.set('host', targetUrl.host);
 
-      const proxyRequest = new Request(targetUrl.toString(), {
-        method: request.method,
-        headers,
-        body: request.method !== 'GET' && request.method !== 'HEAD' ? request.body : undefined,
-        redirect: 'follow',
-      });
+      try {
+        const body =
+          request.method !== 'GET' && request.method !== 'HEAD'
+            ? await request.arrayBuffer()
+            : undefined;
+
+        const proxyRequest = new Request(targetUrl.toString(), {
+          method: request.method,
+          headers,
+          body,
+          redirect: 'follow',
+        });
+
+        const response = await fetch(proxyRequest);
+        const responseHeaders = new Headers(response.headers);
+        for (const [key, value] of Object.entries(CORS_HEADERS)) {
+          responseHeaders.set(key, value);
+        }
 
-      return fetch(proxyRequest);
+        return new Response(response.body, {
+          status: response.status,
+          statusText: response.statusText,
+          headers: responseHeaders,
+        });
+      } catch (error) {
+        const message = error instanceof Error ? error.message : 'Backend proxy error';
+        return new Response(JSON.stringify({ code: 'BACKEND_UNAVAILABLE', message }), {
+          status: 502,
+          headers: {
+            'content-type': 'application/json',
+            ...CORS_HEADERS,
+          },
+        });
+      }
     }
 
     // Serve static frontend assets for all other paths