Build a SaaS with a coding agent.

What agent skills are, how a SKILL.md file works, and how to install skills in Claude Code, Codex, Cursor and Copilot, from your repo or a marketplace.

A copyable AGENTS.md template, a filled-in example for a Next.js and Supabase SaaS, and how Claude Code, Codex, Cursor and Copilot each find the file.

How to build a SaaS with Claude Code: a solid base, CLAUDE.md, plan mode, checks after every step, permissions, secrets and safe database changes.

What a CLAUDE.md file is, where Claude Code looks for it, what to put in it and leave out, plus a template and a Next.js and Supabase example.

Clerk vs Supabase Auth compared: free limits, sign-in methods, teams, prebuilt UI, where users live, RLS, lock-in, and where Better Auth fits.

Claude Code vs Codex in October 2026: where each runs, plans and prices, instruction files, approvals, sandbox, MCP, and how to pick one for a SaaS.

Expo Supabase setup for sign-in: an encrypted session on the phone, token refresh tied to AppState, auth deep links, and one API shared with Next.js.

Multi-tenant SaaS architecture on Supabase: accounts as tenants, memberships with roles, permissions checked in the database, invites tied to one email.

Six Next.js SaaS boilerplates side by side: price, licence, refunds, stack, sign-in, billing, teams and phone app, each fact sourced and dated.

Add Sign in with Google to a Next.js app with Supabase Auth: the Google OAuth client, both redirect URLs, signInWithOAuth and a PKCE callback route.