What an API developer does
An API developer builds the connections between software systems. That’s two related jobs: building an API that lets other applications talk to yours, or integrating your systems with existing third-party APIs — payment processors, shipping carriers, CRMs, whatever you rely on. The deliverable is working, secure connective code plus, non-negotiably, documentation so the next developer can use the interface without reverse-engineering it.
This is close cousin to broader back-end work — many projects that need an API also need a backend developer, and it’s sometimes the same person — but the API is a narrower, sharper specialty. It’s often a smaller, well-defined job than building a whole system, which is worth knowing when you scope it: connecting two existing systems is usually cheaper than building new ones from scratch.
What it costs
Independent API developers typically charge $2,000–$15,000 per project. A straightforward integration with a well-documented third-party service sits near the bottom; a custom API built for scale, or an integration with a messy legacy system, sits at the top. Three things move the price:
- Build versus integrate — designing a public API to serve many clients is more involved than wiring up one known integration.
- Complexity of the other side — a clean, documented third-party API is easy; an old or poorly documented one is slow, careful work.
- Non-functional demands — high volume, strict security, and versioning for many consumers all add real engineering.
See the full breakdown in our API developer cost guide.
How to evaluate one
Make documentation a hard requirement and ask to see an example of theirs. An API without documentation is worthless to everyone but the person who wrote it, so a pro treats docs as part of the deliverable, not an extra. Then probe failure and change: what happens when a third-party dependency changes its response format or goes down, and how do they version an API so an update doesn’t silently break existing users? These questions separate people who’ve run APIs in production from people who’ve only built demos.
Get the interface and its guarantees in writing. A statement of work for API work should name the endpoints or integrations, the authentication approach, and the documentation deliverable. Because “build the API” can mean very different sizes of job, our guide to comparing quotes that aren’t comparable is especially useful here.
What to prepare before you start
Gather the documentation and credentials for every system involved. If you’re integrating with a third-party service, the developer needs API access, keys, and the service’s own docs before they can estimate accurately — half the cost of integration work is dealing with the other side’s quirks.
Be clear about who will consume this API and how it might change. If it’s internal and stable, the job is simpler; if outside developers or partners will build on it, versioning and documentation matter enormously and are worth paying for. Deciding that up front prevents an expensive redesign the first time you need to change something without breaking everyone downstream.
Questions to ask before you hire
- Are you building an API for others to use, or integrating with existing third-party APIs?
- How will you document the API so another developer can use it without calling you?
- How do you handle authentication, rate limits, and versioning as the API changes?
- What happens when a third-party API I depend on goes down or changes its response?
- Have you built or integrated an API at my scale, and can you show it?
Red flags
- No documentation deliverable. An undocumented API is a locked room only they have the key to.
- No answer for what happens when a dependency changes or fails. Integrations break for external reasons constantly.
- Ignores versioning, so a future change will silently break everything already using the API.
- Vague on authentication and security, which is where APIs leak data.