Why my single Next.js app runs 4 different domains (and how the proxy.ts decides who sees what)

Youssefroop Posted on May 30           Why my single Next.js app runs 4 different domains (and how the proxy.ts decides who sees what) # nextjs # architecture # saas # buildinpublic > TL;DR — I run four different domains off one Next.js codebase: a marketing site at pagestrike.com , an authenticated app at app.pagestrike.com, a public publishing domain at pagestrike.app, and customer-owned domains. The trick isn't deploying four apps — it's a single proxy.ts that reads the host and rewrites/redirects/passes-through per-request. This post walks through why I chose this shape, the parts I got wrong, and the cookie-domain trick that makes it all stick. Stack: Next.js 16 App Router , Supabase , Vercel , one proxy.ts file (~370 lines). This is the second post in my build-in-public series on PageStrike . Last week I wrote about the 6-CTA architecture — modeling conversion intent as a discriminated union so one launch could be a checkout, a COD form, or a calendar booking. This post is about a different primitive: modeling host as routing context so one codebase can serve four very different audiences. Why four domains, not one Most SaaS apps live at one domain — say myapp.com with /dashboard under it. That works until you grow into edge cases that don't fit: Marketing pages get spammed by your own dashboard headers. Your marketing nav says "Sign in / Pricing / Blog". Your dashboard nav says "Launches / Contacts / Settings". You either A/B them with conditional logic everywhere or you live with the noise. Public user-generated pages share your domain reputation. When a customer publishes a landing page at myapp.com/p/[slug] , every spammy LP from a free-tier user drags down myapp.com 's sender reputation, search trust, and ad-account standing. Google and Meta penalize the host, not the path. Custom domains don't route cleanly. A customer who buys acmewidgets.com and points it at your app expects their LP at acmewidgets.com/ — not myapp.com/p/acm