How to build with Cofounder — Cofounder

How To Build A Product

Setting up an engineering department is more than just writing code. You need architecture, a database, infrastructure, testing, design, deployment, and a plan to keep it all running as you grow. Most first-time founders dramatically underestimate this; they think “building” means opening a code editor and hacking until something works. That approach gets you a prototype (which are still important when you start!). It does not get you a company.

This chapter walks through the full lifecycle of building a software product, from initial spec to production deployment to ongoing infrastructure management. At each step, we'll show you what matters, what most people get wrong, and how Cofounder can handle the heavy lifting so you can focus on your product.

Come up with a spec

Before you write a single line of code, you need a plan: a clear, minimal specification of what your product does and who it's for.

The goal at this stage is to define your minimum viable product (MVP). This is the smallest version of your product that delivers real value to a real user. It is not a landing page. It is not a demo. It is a working piece of software that solves a specific problem well enough that someone would actually use it.

To define your MVP, answer three questions:

What is the core problem you're solving?

Be ruthlessly specific. “Helping people manage their finances” is not a problem statement. “Freelancers lose track of quarterly tax estimates because existing tools assume W-2 employment” is. The tighter your problem definition, the easier every subsequent decision becomes.

What is the minimum set of features needed to solve that problem?

List every feature you think you need, then cut half of them. Seriously. If your MVP has more than three to five core features, it's too big. You're not building the final product; you're building the first thing real users will test.

What platform are you building for?

Web, mobile, or desktop? For most startups, start with web. It's the fastest to build, the easiest to deploy, and the simplest to iterate on. You can always go native later once you've validated the product.

Once you have answers, write them down. This is your spec. It doesn't need to be formal — a one-page document or even a detailed bullet list works. What matters is that you've thought through the scope and committed to a boundary.

Create a repository

Your code needs a home. That home is a Git repository, hosted on GitHub.

If you're not familiar with Git, here's the short version: Git is a version control system that tracks every change you make to your code. GitHub is a platform that hosts your Git repositories in the cloud, lets multiple people collaborate on the same codebase, and provides tools for code review, issue tracking, and automation.

Why does this matter? Because without version control, you're one bad edit away from losing hours (or days) of work. With it, you can experiment freely, roll back mistakes, and work with teammates without stepping on each other's code.

Setting up a repository involves a few things:

VERSION CONTROL

A repository is the source of truth for your product.

Every meaningful change moves through a branch, a review, and a protected main branch before production sees it.

A quick note on terminology you'll encounter throughout this chapter: a branch is a parallel copy of your code where you can make changes without affecting the main version. Think of it like a Google Doc suggestion — you're proposing changes that can be reviewed before they're accepted. A pull request(or PR) is how you propose merging a branch's changes into the main codebase. It's a formal “here's what I changed, please review it” request. These two concepts are fundamental to how modern software development works, and you'll see them referenced throughout this guide.

If you want to learn more about Git and GitHub fundamentals, GitHub's own Getting Started guide is excellent. But honestly, for most founders, you don't need to become a Git expert. You need a repo that's set up correctly, and then you need to write code.

Set Up Deployments

Before you start coding, set up deployment. Yes, before. This is counterintuitive but important.

Here's why: if you wait until you've written a bunch of code to figure out deployment, you'll inevitably run into environment-specific bugs, configuration headaches, and “it works on my machine” problems. By setting up deployment first, every change you make is immediately visible in a real environment. This tightens your feedback loop dramatically.

We recommend Vercel for deployment. It's built for modern web frameworks (especially Next.js), handles scaling automatically, and has an excellent developer experience. Vercel is free to start with a generous hobby tier, and paid plans start around $20/month per team member when you need more.

You need three environments:

  1. Deployment — traditionally, this is your local machine where you write and test code. In a traditional setup, this means installing Node.js, a code editor, and running your app locally with commands like npm install and npm run dev. This can be a pain to configure, and “it works on my machine” is one of the most common problems in software development.

  2. Staging — a cloud environment that mirrors production. This is where you test changes before they go live. Staging should be connected to your main development branch.

  3. Production — the live environment your users see. This should only receive code that has been tested in staging.

You should also set up preview environments. Vercel does this natively — every time you push a new branch or open a pull request, Vercel creates a unique URL where you can see exactly what your changes look like. This is incredibly useful for reviewing features before merging them.

Preview environments become even more powerful when you're working with AI agents. In a traditional development workflow, you'd review code in a pull request, try to visualize the changes in your head, and maybe run the branch locally to actually see them. With preview environments, every PR an agent opens is immediately deployed to its own live URL. You can see exactly what the agent built, click through it, and decide whether to merge — all without pulling a single branch to your local machine.

This matters because agents work fast. A single agent can open multiple PRs in the time it takes you to review one. Preview environments let you keep up: you can review several changes in parallel, each in its own live environment, and merge the ones that look right. It turns code review from a bottleneck into a lightweight quality gate. Instead of being the person who writes every line, you become the person who steers — directing agents, reviewing their output, and shipping the good stuff. This is what agent-native development looks like, and preview environments are the infrastructure that makes it practical.

DEPLOYMENT PIPELINE

Code moves through gates before users see it.

Secret Management

Your app will need API keys, database connection strings, auth secrets, and other sensitive configuration values. These should never be hardcoded in your codebase — not even in a private repo. If a secret ends up in a Git commit, it's in your history forever (or at least until you go through the painful process of rewriting history to remove it).

The right approach is to use environment variables. These are values stored outside your code that get injected into your app at runtime. Vercel has built-in support for environment variables, and you can set different values for each environment (development, staging, production). This means your staging app can talk to a staging database while your production app talks to the real one, all without changing a line of code.

A few rules of thumb for secrets management:

Start Building: Scaffold Your App

Now it's time to actually write code. The first step is scaffolding — setting up the foundational structure of your application.

Any modern web app has two sides:

For your tech stack, we recommend:

This stack — Next.js + Supabase — is battle-tested, well-documented, and one of the fastest ways to go from zero to a working product. It's also what Cofounder is optimized for.

On cost: GitHub is free for private repositories. Supabase has a free tier that's more than enough for development and early users, with paid plans starting at $25/month when you need more storage or compute. Between Vercel and Supabase, you can realistically run a production app for under $50/month until you have meaningful traffic. Don't let infrastructure costs scare you — the expensive part of building a startup is your time, not your server bill.

The Backend: What You Need at Minimum

Your backend is the foundation everything else sits on. Get it wrong and you'll be dealing with security vulnerabilities, data loss, and architectural rewrites down the line. Get it right and it quietly does its job while you focus on the product.

At minimum, your backend needs four things:

Authentication

Supabase Auth handles all of this out of the box. It supports email/password login, magic links (passwordless login via email), and social login with providers like Google, GitHub, and Apple. It also integrates with Supabase's database security so you can control what data each user is allowed to see — for example, ensuring users can only access their own records, not anyone else's.

A Database

You need somewhere to store your data. Supabase gives you a full database that can handle everything from simple user profiles to complex data with many relationships between different types of records. It's the same type of database (PostgreSQL) that powers some of the largest apps in the world, so it won't be something you outgrow.

Think carefully about your data structure early. What are the main things your app tracks? Users, orders, messages, projects? How do they connect to each other? For example, a user has many orders, and each order has many items. Getting this structure right early saves you from painful restructuring later.

That said, don't overthink it. Your data structure will evolve as your product evolves. Start with what you need for your MVP and iterate.

A note on database migrations: when you need to change your database schema after you've already launched (adding a new column, renaming a field, changing a relationship), you can't just edit the schema like you would a document. You need to write a migration — a set of instructions that transforms the database from the old structure to the new one without losing existing data. Supabase has built-in migration support, and this is one area where being careful pays off. A botched migration on a production database with real user data is one of the worst situations you can be in as a founder.

An API Layer

Your API layer is how the pages your users see communicate with your server and database. When a user clicks “submit” on a form, or a page needs to load a list of items, those requests go through your API. In Next.js, you define these directly in your project — no separate server needed.

Your API handles things like: loading data for a page, saving form submissions, running business logic (“when a user does X, also do Y and Z”), and talking to third-party services like Stripe or Postmark.

Keep these routes clean and well-organized. Make sure they check that incoming data is valid before doing anything with it, and that they return helpful error messages when something goes wrong. These are the boring fundamentals that separate a production-quality app from a hackathon project.

Third-Party Integrations

Almost every real product needs to integrate with external services. Two are nearly universal:

Payments. If you're charging users, you need a payment processor. We recommend Stripe. It handles credit card processing, subscriptions, invoicing, and tax compliance. Stripe's API is well-documented and battle-tested — it powers payments for companies from early-stage startups to Fortune 500s. Stripe charges 2.9% + 30¢ per transaction with no monthly fee, so you only pay when you make money.

Setting up Stripe involves creating an account, adding Stripe's code library to your app, building the checkout experience your users will see, and setting up webhooks. Webhooks are how Stripe tells your app about events — “this payment succeeded,” “this subscription was cancelled,” “this card was declined.” Your app needs to listen for these notifications and respond appropriately (e.g., granting access after a payment, or sending a follow-up email after a cancellation). This is one of those integrations that seems simple but has a lot of edge cases: failed payments, refunds, subscription upgrades, partial charges, and so on.

Transactional email. Your app will need to send emails — welcome emails, password resets, receipts, notifications. We recommend Postmark for this. Postmark is fast, reliable, and focused specifically on transactional email (as opposed to marketing email). It has excellent deliverability, which means your emails actually land in inboxes instead of spam folders. Postmark offers a free tier for development, with paid plans starting at $15/month for 10,000 emails.

Integration Map

Your app needs to react when outside services send updates.

Security

This is the one that trips up most first-time founders. You are responsible for making sure your app doesn't leak user data. This means:

This is not optional. A data breach can kill a startup — both legally and reputationally. Even if you're using Cofounder to help set things up, it's still on you as the founder to understand your security posture and ensure user data is protected.

The Frontend: Building What Users See

The frontend is where your product comes to life. It's what users interact with, and it's often the difference between a product people love and one they abandon after thirty seconds.

Frontend development in 2026 is primarily done in TypeScript (a typed superset of JavaScript) using component-based frameworks like React (which Next.js is built on). The good news: this is an area where AI agents excel. Translating designs and user flows into working UI code is one of the things agents do best.

That said, the agent is only as good as your direction. You need to deeply understand your UX and business logic:

Design Without Designers

Traditionally, building a frontend meant hiring a designer to create mockups in Figma, then handing those designs to a developer to implement. This was slow, expensive, and created constant back-and-forth between design and engineering.

In 2026, agents can design directly in code. Instead of creating a static mockup and then translating it, you describe what you want and the agent builds a working version immediately. You can see it, click through it, and iterate on it in real time. This collapses the design-to-code pipeline into a single step.

Once you have this clarity, building the frontend is largely execution. You're translating your flows and product thinking into components, pages, and interactions.

Deploy to Production

You've got a working app in staging. It's time to go live.

Deployment is the process of taking your tested code and making it available to real users. If you set up Vercel earlier (which you should have), this process is straightforward:

  1. Quickly test in preview environments. Before anything gets merged to staging, review it in its preview environment. Click through the changes, test the flows, make sure it looks and works the way you expect.
  2. Test thoroughly in staging. Once branches are merged, test the integrated result in staging. Use your app like a real user would. Try to break it. Click things you're not supposed to click. Enter data you're not supposed to enter. Find the bugs now, not after launch.
  3. Merge to main. Once you're confident staging is solid, merge your code to the main branch. This should go through a pull request with a code review (even if you're reviewing your own code).
  4. Deploy. Vercel automatically deploys your main branch to production. If you've set things up correctly, this is a one-click operation.
  5. Verify in production. After deployment, check that everything works in the live environment. Sometimes things that work in staging break in production due to environment differences (different API keys, different database, different domain configuration).

Testing and Quality: Avoiding AI Slop

Automated tests verify that your code works as expected. There are several types:

You don't need 100% test coverage on day one. Start with tests for your core flows — the critical paths that, if broken, would make your app unusable. Then expand coverage over time.

A crucial warning: when using agents to write tests, verify that the tests actually test something meaningful. When something breaks, you can describe the problem and Cofounder will help you locate and fix it.

Infrastructure Management

Congratulations, your app is live. Now you have to keep it running.

Post-deployment, infrastructure management is primarily about two things: reliability and cost. You need to watch for slow data lookups, storage growth, and too many simultaneous users. There’s a classic founder dilemma here: do you throw money at infrastructure to move fast, or do you optimize ruthlessly for efficiency? The answer depends on your stage. Early on, optimize for speed — your time is more valuable than your server bill. As you scale, efficiency becomes critical because infrastructure costs can eat into your margins.