I'm implementing Apple Sign-In in my Next.js application with a NestJS backend. After the user authenticates with Apple, instead of redirecting to my configured callback URL, the browser makes a POST request to a mysterious endpoint /appleauth/auth/federate that doesn't exist in my codebase, resulting in a 404 error. Tech Stack Frontend: Next.js 16.0.10, React 19.2.0 Backend: NestJS with Passport (using @arendajaelu/nestjs-passport-apple) Frontend URL: https://myapp.example.com Backend URL: https://api.example.com Apple Developer Configuration Service ID: (configured correctly in Apple Developer Console) Return URL (only one configured): https://api.example.com/api/v1/auth/apple/callback Domains verified in Apple Developer Console: myapp.example.com api.example.com example.com Backend Configuration NestJS Controller (auth.controller.ts): typescript @Public() @Get('apple') @UseGuards(AuthGuard('apple')) async appleAuth() { // Initiates Apple OAuth flow } @Public() @Post('apple/callback') // Changed from @Get to @Post for form_post @UseGuards(AuthGuard('apple')) async appleAuthCallback(@Req() req: any, @Res() res: any) { const result = await this.authService.socialLogin(req.user, ipAddress, userAgent); // Returns HTML with tokens that uses postMessage to send to opener window } Environment Variables: typescript APPLE_CLIENT_ID=<service_id> APPLE_TEAM_ID=<team_id> APPLE_KEY_ID=<key_id> APPLE_PRIVATE_KEY_PATH=./certs/AuthKey_XXX.p8 APPLE_CALLBACK_URL=https://api.example.com/api/v1/auth/apple/callback FRONTEND_URL=https://myapp.example.com The passport-apple strategy uses response_mode: 'form_post', so Apple POSTs the authorization response to the callback URL. Frontend Implementation Next.js API Route (/src/app/api/auth/apple/route.js): javascript export async function GET(request) { const backendUrl = new URL(`${API_URL}/auth/apple`); const response = await fetch(backendUrl.toString(), { method: "GET", headers: { "Content-Type": "application/json", }, }); const responseText = await response.text(); return new NextResponse(responseText, { status: response.status, headers: { "Content-Type": contentType || "text/html" }, }); } Frontend Auth Handler: javascript export const handleAppleLogin = (router, setApiError) => { const frontendUrl = window?.location?.origin; // Opens popup to /api/auth/apple window.open( `${frontendUrl}/api/auth/apple`, "appleLogin", "width=500,height=600" ); }; The Problem Expected Flow: User clicks "Login with Apple" Frontend opens popup → https://myapp.example.com/api/auth/apple Frontend proxies to → https://api.example.com/api/v1/auth/apple Backend redirects to Apple's authentication page User authenticates with Apple ID Apple POSTs back to → https://api.example.com/api/v1/auth/apple/callback Backend processes and returns success HTML Actual Behavior: After step 5 (user authentication with Apple), instead of Apple redirecting to my callback URL, the browser makes this unexpected request: POST https://myapp.example.com/appleauth/auth/federate?isRememberMeEnabled=false Status: 404 Not Found Request Payload: json { "accountName": "user@example.com", "rememberMe": false } Network Tab Analysis From Chrome DevTools, the call stack shows: send @ app.js:234 ajax @ app.js:234 (anonymous) @ app.js:10 Ee.isFederated @ app.js:666 _callAuthFederate @ app.js:666 The Ee.isFederated and _callAuthFederate functions appear to be minified library code, but I cannot identify which library. What I've Verified ✅ The /appleauth/auth/federate endpoint does not exist anywhere in my codebase: bash grep -r "appleauth" src/ # No results grep -r "federate" src/ # No results ✅ Apple Developer Console shows only ONE Return URL configured (verified multiple times) ✅ Changed callback route from @Get to @Post to handle form_post response mode ✅ Rebuilt frontend completely multiple times: bash rm -rf .next npm run build ✅ Tested in: Incognito/Private browsing mode Different browsers (Chrome, Firefox, Safari) Different devices After clearing all cache and cookies ✅ No service workers registered in the application ✅ No external <script> tags or CDN libraries loaded ✅ package.json contains no AWS Amplify, Auth0, Cognito, or similar federated auth libraries ✅ Checked layout.js and all root-level files - no external scripts Additional Context Google Sign-In works perfectly fine using the same approach The mysterious endpoint uses a different path structure (/appleauth/ vs /api/auth/) The call appears to originate from client-side JavaScript (based on the call stack) The app.js file with the mysterious functions is the built Next.js bundle Questions Where could this /appleauth/auth/federate endpoint be coming from? Why is the browser making this POST request instead of following Apple's redirect to my configured callback URL? Could this be related to the response_mode: 'form_post' in the Apple Passport strategy? Is there something in the Apple Developer Primary App ID configuration that could trigger this behavior? Could this be a Next.js build artifact or some hidden dependency? The mysterious call stack references (Ee.isFederated, _callAuthFederate) suggest some library is intercepting the Apple authentication flow, but I cannot identify what library or where it's being loaded from. The minified function names suggest federated authentication, but I have no such libraries in my dependencies. Has anyone encountered similar issues with Apple Sign-In where an unexpected endpoint is being called?