Treevoxa offers API development services for companies whose systems need to talk to each other, and for software teams that need a reliable back end built or rescued. We design, build, document and test REST APIs, and we build the integrations that connect payment gateways, accounting software, email, cloud storage, biometric devices and AI services to the systems you already run. As a custom API development company, our web API development services cover design, build, documentation and support.
Every product we sell runs on an API we built and still maintain, so we design APIs the way we would want to depend on them ourselves.
API development services: REST APIs and API integration services
REST API development is most of what we do; API integration services connect what already exists.
- New APIs for a web or mobile app, a SaaS product or a partner programme.
- Integrations between your systems: CRM to accounting, store to stock, HR to payroll, website to back office.
- Payment and billing integrations, including subscriptions, invoices and webhooks.
- Device and hardware integrations, such as fingerprint and face terminals that feed attendance.
- AI integrations that let a model read your documents or data through a controlled API, with limits and logs.
- API rescue: documenting, testing, securing and speeding up an API someone else built.
APIs we run in production
API software companies should show the APIs they run, not only describe them. Ours:
- MatrixHR runs on a NestJS API with OpenAPI (Swagger) documentation, JWT authentication, rate limiting and role-based access. A small agent on the customer's network brings ZKTeco punches into the API, and payroll exports a bank file and a balanced QuickBooks journal.
- PharmaFlow runs on an Express API on PostgreSQL, with request validation, rate limiting and security headers. The counter, the online store and supplier ordering all use the same API and the same stock.
- RE-ERP connects Stripe billing, two-factor sign-in and AI features built on the OpenAI and Anthropic APIs.
- DealWisp runs on a Python FastAPI service with PostgreSQL, file storage on Amazon S3 and transactional email.
Tech stack for API projects
| Part | What we use |
|---|---|
| Frameworks | Node.js with NestJS or Express; Python with FastAPI |
| Contracts and docs | OpenAPI (Swagger), JSON schema validation (Zod, class-validator, Pydantic) |
| Data | PostgreSQL, Prisma, SQL migrations |
| Security | JWT and session auth, role-based access, rate limiting, two-factor sign-in |
| Integrations | Stripe, QuickBooks journals, Amazon S3, email delivery, OpenAI and Anthropic APIs, ZKTeco devices |
See Node.js development, NestJS development and FastAPI development for more on each back-end stack, or hire dedicated developers for ongoing API work.
How an API project runs
- Map the systems. Every system involved, what data moves, in which direction, how often, and who is allowed to see it.
- Design the contract. Endpoints, data shapes, errors and versioning, written as an OpenAPI document you approve before code.
- Build and test. Automated tests for every endpoint, including the failures: bad input, missing permissions, timeouts.
- Harden. Authentication, limits, logging and monitoring.
- Hand over. Documentation, example requests and a test suite, plus support terms agreed in writing.
Engagement models for API work
- Fixed scope for a defined integration or API, priced per milestone.
- Dedicated developer for ongoing back-end work, billed monthly. See dedicated software development team.
- Hourly for audits, fixes and small integrations.
What an API costs
An integration's price depends on how many systems are involved, whether they have usable APIs, how much data must be cleaned and moved, and how close to real time it must run. Connecting two systems with good APIs is a small project; connecting a system that has no API, or migrating years of messy data, is not. Our custom software development cost guide explains the drivers, and our developer rates page compares what teams charge. We quote in US dollars, in writing.
Time-zone overlap
APIs fail at inconvenient times, so we agree the support hours before launch. During the build we keep a daily overlap window inside your working hours, usually your morning for US clients and the afternoon for UK and European clients, for reviews and decisions.
Who needs API development services
- Companies with several systems that do not talk to each other, so staff retype orders, invoices or attendance.
- Software teams launching a mobile app or a partner programme that needs a stable, documented API.
- SaaS founders whose product must integrate with their customers' tools to sell.
- Businesses adding AI that need a safe, limited way for a model to read and act on their data.
What to send us for an API quote
A list of the systems involved, what data must move between them and how often, any API documentation you already have, and the problem the integration must solve. Sample files or screenshots of today's manual process help more than anything else.