How to Build an API as a Solo Developer

What You're Building

An API (Application Programming Interface) is a service that other software talks to. It receives requests, processes data, and returns responses. APIs power everything from mobile apps to integrations to entire platforms. Building and selling API access is one of the most scalable solo developer businesses because your customers are other developers, and developers pay for tools that save them time.

I've built several APIs, both internal ones that power my own products and a public one that third-party developers pay to use. The process is more straightforward than you'd think.

Difficulty & Timeline

Recommended Tech Stack

FastAPI (Python) for the framework, PostgreSQL for the database, and Railway or Render for hosting. FastAPI auto-generates your API documentation from your code, which is a massive time-saver. If you're a JavaScript developer, Hono on Cloudflare Workers is an excellent serverless alternative.

Step-by-Step Plan

Phase 1: Foundation (Week 1-2)

Design your API before you code it. Write down the endpoints you need, what data they accept, and what they return. I use a simple markdown document for this. Nothing fancy. Just a list of routes with example request/response payloads.

Set up your project structure with clear separation between routes, business logic, and database queries. Configure your database with proper migrations from day one. Set up API key authentication. This is non-negotiable for a public API. Every request needs to be authenticated and rate-limited.

Phase 2: Core Features (Week 3)

Build your 5-10 most important endpoints. For each one, handle the happy path first, then add error handling, input validation, and edge cases. FastAPI makes validation almost free with Pydantic models, so there's no excuse for accepting bad data.

Implement rate limiting per API key. This protects you from abuse and becomes a natural pricing lever later. I use a simple Redis-based counter per key per minute. Start with generous limits (100 requests/minute) and tighten them for different plan tiers.

Add logging for every request. You'll need this for debugging, billing, and understanding how developers use your API. Log the endpoint, response time, status code, and API key (but never the full request body if it contains sensitive data).

Phase 3: Polish & Launch (Week 4)

Write documentation. This is not optional. Your documentation IS your product when you're selling an API. FastAPI generates Swagger/OpenAPI docs automatically, but you also need a getting-started guide, code examples in multiple languages, and a clear explanation of error codes.

Set up a developer portal. This can be a simple Next.js site with documentation pages and a signup flow where developers get their API keys. Stripe handles billing for different usage tiers.

Deploy to Railway or Render. Both support automatic deployments from Git and include managed PostgreSQL. Set up monitoring so you know when your API goes down before your customers do.

Monetization Strategy

API businesses have beautiful economics. Usage-based pricing means revenue scales with adoption, and margins are high because serving API requests is cheap.

The standard model is tiered pricing. A free tier with 1,000 requests/month to attract developers, a starter tier at $29/month for 50,000 requests, and a pro tier at $99/month for 500,000 requests. Charge per additional request over the limit.

What I've found works best is making the free tier generous enough that developers build real integrations before hitting the paywall. A developer who has already integrated your API into their product will pay happily rather than rip it out and switch to a competitor.

Common Mistakes to Avoid

Bad documentation. I cannot stress this enough. Developers will evaluate your API by reading the docs first. If the docs are confusing, incomplete, or outdated, they'll move to a competitor without ever making a request. Keep docs in sync with your code using auto-generation tools.

Breaking changes without versioning. Once developers depend on your API, breaking their integration is the fastest way to lose them. Version your API from day one (v1, v2) and never change existing endpoint behavior. Add new endpoints instead.

No rate limiting. Without rate limits, one bad actor or buggy integration can take down your entire API. Rate limit by default and treat unlimited access as a premium feature.

Ignoring latency. API response time matters enormously. A 200ms response is fine. A 2-second response is unusable for many applications. Profile your endpoints, optimize database queries, and add caching for frequently accessed data.

Is This Worth Building?

Definitely, if you have access to unique data or can provide a useful transformation that developers need. The API economy is massive, and solo developers can compete effectively because developer experience (good docs, fast responses, reliable uptime) matters more than brand name.

The best API businesses serve a specific niche. Weather data for a specific region, sentiment analysis for a specific language, or data aggregation for a specific industry. Find something developers need, wrap it in a clean API, and charge for access. The recurring revenue model makes this one of the best solo developer businesses you can build.