Custom API development for businesses that need more than an off-the-shelf connector. We design and build bespoke APIs that let your systems, apps and partners exchange data reliably: from a REST API behind your web application or mobile app to real-time WebSocket feeds and integrations with ERP, CRM or e-commerce platforms.
- Design first: an OpenAPI contract you can review before a single line of code is written.
- The right stack: built in Rust, C# or PHP, depending on what your project needs.
- Secure from day one: OAuth2 and JWT, rate limiting and audit logging.
- Fully documented: so your own team or partners can integrate without hand-holding.
Only need to connect existing software? See our API integration services. Want to work out the architecture first? Then API strategy & design is the better starting point.
Why a Custom API?
An API is the contract between systems. Whether you are opening your platform to third-party developers, connecting internal services, letting an app talk to your backend or delivering data feeds to customers, the quality of that contract determines how reliably everything built on top of it works.
A custom API is the right choice when off-the-shelf solutions don't fit your data model, your performance requirements, your security constraints or your integration needs. Typical situations:
- Your platform needs to expose functionality to partners or external developers.
- Your web or mobile app needs a backend shaped around the data it actually uses.
- Internal systems need to share data without being tightly coupled.
- You deliver data feeds to multiple consumers with different access levels and limits.
- You are building a real-time product, such as a trading platform, a collaboration tool or a live dashboard.
Is your team spending serious effort working around the limitations of an existing API? Then an API built to your own requirements is often the cheaper option in the long run.
How We Build Your API
- Discovery. We discuss what the API needs to do, who will use it and which systems it has to talk to.
- API design. Endpoints, data models, authentication and error handling are captured in an OpenAPI specification. You and your team review the design before development starts.
- Short development sprints. A staging environment lets you follow progress and course-correct early.
- Delivery. With automated tests, documentation and integration guides, so consumers can integrate on their own.
- Operations and evolution. Hosting, monitoring, security updates and new versions, if you want us to.
REST API Development
REST is still the default for APIs: it maps naturally to how the web works, every HTTP client supports it, and its interfaces are easy to understand and document. But a REST API that holds up in production takes more than a few endpoints returning JSON. Decisions about resources, versioning, authentication, errors and pagination ripple through every application built on the API, and get more expensive the later you change them. So we make them upfront.
Resources and URL structure. We model the API around the concepts in your domain, with predictable URLs. HTTP methods mean what they should: GET to retrieve, POST to create, PUT and PATCH to update, DELETE to remove.
Versioning. APIs change, but the applications that depend on them can't always change at the same time. We pick a versioning strategy upfront (URL path, headers or content negotiation) so a new version never breaks existing consumers.
Authentication and authorisation. API keys for server-to-server integrations, OAuth2 with PKCE when users delegate access, JWT bearer tokens for stateless authentication, and HMAC request signing where the integrity of every request must be provable. Permissions are checked per resource, not just per endpoint: users only see and change what they are allowed to.
Validation and error responses. Every request is validated before it is processed. When something goes wrong, the consumer gets a structured error with the right HTTP status code, a machine-readable error code and a human-readable explanation. Consistently, across the whole API.
Pagination, filtering and sorting. Cursor pagination for large or fast-changing datasets, offset pagination where simplicity matters more. With sensible defaults, so no single request accidentally scans an entire table.
Rate limiting. APIs exposed to external consumers get rate limits per key, user or endpoint, with headers in every response so consumers can pace their own requests.
WebSocket API Development
A WebSocket connection is the right choice when communication is not request-response but continuous and bidirectional: real-time dashboards, live price feeds, chat, notifications and streaming data. A production WebSocket service has to solve problems a REST API never faces.
Thousands of concurrent connections. The server keeps an open connection with every client. That takes low memory overhead per connection, clean teardown and graceful handling of clients that reconnect.
A clear message protocol. With REST, HTTP defines most of the rules; with WebSockets we design them: typed messages, routing, acknowledgements where delivery must be guaranteed, and consistent error messages.
Channels and subscriptions. Clients subscribe only to the data streams they need, instead of receiving everything the server sends.
Presence and shared state. For products where users need to see each other's state, such as collaboration tools and live trading screens.
Horizontal scaling. A single server can only hold so many connections. With Redis pub/sub or a similar message bus, we spread connections across multiple servers without losing messages.
Fallbacks. Not every network keeps a WebSocket connection open reliably. Server-Sent Events or long polling keep the application working when it doesn't.
GraphQL
GraphQL lets the client decide which data it fetches in a single request. That shines when many different screens or apps need the same data, each in their own shape. For simple integrations, public APIs and highly cacheable data, REST is usually the better choice: easier to secure, cache and monitor. We advise per project and combine both where it makes sense.
REST and WebSocket Combined
Many products use both: REST for login, creating resources, configuration and bulk operations, WebSocket for real-time data and notifications. We design such a hybrid API as one coherent whole, with clear conventions for which transport does what and shared authentication across both.
Security
Security is not something you bolt onto an API afterwards; it lives in the decisions made during design and development.
Encrypted transport. All APIs are served over TLS, with properly configured certificates, cipher suites and HSTS.
All input is suspect. SQL injection, command injection, path traversal and XXE attacks are stopped where input enters the system.
Sensitive data. Passwords, tokens, personal data and financial data never end up readable in logs or error responses, and are encrypted at rest.
Audit logging. For every authenticated request we record who did what, and with which result. Essential for incident investigation, compliance and troubleshooting.
CORS. Configured explicitly per allowed origin, never a wildcard left over from testing.
Documentation
An API without documentation is not finished. Every REST API ships with a complete OpenAPI (Swagger) specification: every endpoint, parameter, response and error code. Consumers can use it to generate client SDKs and validate requests automatically. WebSocket protocols are documented with the same rigour. On top of that we write integration guides for the most common tasks, such as authentication, pagination and handling errors and limits.
Performance and Scalability
A slow API makes every application built on it slow. So we set response-time targets upfront and test against them during development.
Caching in the right places: HTTP cache headers, application-level caching for expensive computations, and an invalidation strategy that keeps stale data out.
Asynchronous processing for work that takes too long for a single request, such as reports, bulk processing or slow external systems: via a job queue with status polling, or a webhook once the work is done.
Database design that matches how the API queries its data, with the right indexes and connection pooling.
Technologies We Build APIs With
- Rust / Axum: maximum performance, for latency-critical services and high-concurrency WebSocket servers
- C# / ASP.NET Core: enterprise APIs, Microsoft ecosystem integration, complex business logic
- PHP / Laravel: fast-to-build APIs and CRUD services, CMS-adjacent backends
- Next.js API Routes: frontend-adjacent APIs (BFF pattern), serverless deployments
- PostgreSQL, MySQL and SQLite: relational data storage
- Redis: caching, sessions and pub/sub for WebSockets
- Auth0, JWT and OAuth2: authentication and authorisation
- OpenAPI / Swagger: specification and documentation
- Docker: the same environment from development to production
Frequently Asked Questions about Custom API Development
How long does it take to build an API?
That varies a lot. An API with a handful of endpoints for one app is a very different project from a public API with real-time feeds and dozens of consumers, so quoting a standard lead time would be misleading. Once the design is done, it is clear which parts are needed and we draw up a realistic plan.
REST, GraphQL or WebSocket: which do I need?
REST for most integrations and public APIs. GraphQL when many different clients need the same data in their own shape. WebSocket when data has to flow continuously in real time. Often it is a combination; we decide together during the design phase.
What is the difference between building an API and integrating one?
Building an API means creating the interface your own system offers to others. Integrating means connecting systems through APIs that already exist, for example your web shop with your accounting software. For the latter, see our API integration services.
Do I own the source code?
You can. Ownership of the source code is set out in the contract, and the code can be fully yours, including documentation and tests.
Can you extend or improve an existing API?
Yes. We start with an assessment of the existing API: design, security, performance and documentation. Then we extend it, or build a new version alongside the old one so existing consumers keep working undisturbed.
Let's Design Your API
Good API design is one of the highest-return investments a software project can make: the interface you define today will be used by apps, partners and other systems for years. We help you get it right the first time.