Custom API development services design, build, secure and document an application programming interface made for one company's data and workflows, instead of adapting a generic connector. The work covers the API contract, endpoints and business logic, authentication, testing, documentation, and the versioning that keeps every consuming app working as the API changes.
Our API development services cover the whole build, from the first spec to long-term support:
The API contract comes first, because in custom API development the contract is what other teams build against. Senior engineers working with AI tooling write the specs, code and tests.
Typical projects open platform data to partners, put a gateway in front of an older system, or join several outside providers into one product.
Most API projects mix styles. We pick one per consumer: who calls the API, how often, and how much data each call needs.
| API style | Best fit | Trade-off to plan for | Where we used it |
|---|---|---|---|
| REST | Public and partner APIs, resource-based operations, anything a third party calls with standard HTTP tools | Clients may over-fetch or need several calls for one screen, so versioning and pagination rules must be set early | Fleet Management Platform (partner access to vehicles, events, geozones and routes, specified in OpenAPI) and Personal Loans Application |
| GraphQL | Front ends with many views that each need a different slice of the same data | Caching and query-cost limits need deliberate design, and per-request rate limiting is harder | Personal Loans Application |
| gRPC | Internal service-to-service calls where speed and strict contracts matter | Browsers need a proxy, and every team must share the same schema files | Personal Loans Application (calls between services) |
Some flows don't fit a request and a reply at all. Vehicle location updates, for example, are a stream of events, so we carry them with event streaming over Apache Kafka, as on the Fleet Management Platform and the Personal Loans Application.
Custom API development services earn their cost when the data or workflow is yours and nobody sells an endpoint for it. Partners need controlled access to your platform, several apps need the same records, or an older system holds data that newer products can't reach. If you only need to connect one application to a few SaaS tools that already publish APIs, configuration usually beats code, and our software and app-level integration team handles that with ready-made connectors. We'll tell you which side of that line your project sits on during the first call.
Every API has consumers: your own apps, other internal teams, partners. Our custom API development services design for them before any code is written.
This avoids the costliest rework there is, which is changing an endpoint after other apps depend on it. On the Fleet Management Platform the specification was OpenAPI, and on the Personal Loans Application the API was documented in Swagger and Postman.
We build endpoints and the business logic behind them in Java and Spring Boot or .NET Core, and choose the style per consumer with the comparison above:
If your API sits inside a larger platform build, our back-end development team works from the same plan. We also take on smaller web API development services for a single product or internal tool.
Your API rarely works alone. It calls identity checks, credit bureaus, messaging and map providers, and each one returns its own format and fails in its own way. Our custom API integration services wrap those providers behind your API:
On the Personal Loans Application that meant Plaid, GIACT, TALX, Equifax, Oscilar and LoanPro. On the Fleet Management Platform it meant Twilio, SendGrid, Google Maps and OpenStreetMap.
Teams comparing API development and integration services for payments can start with our guide to payment APIs, and our system integration team leads projects connecting ERP, CRM and legacy systems.
A good custom API development company treats the API as a contract other teams depend on. It agrees the spec first, secures every endpoint and keeps the same engineers on the API as it versions. Saigon Technology delivers custom API development services that way for US companies and global clients, at a published senior-led rate.
Senior Engineers With AI, at a Published Rate
Senior engineers lead every API build and use AI tooling for the repetitive work: boilerplate endpoints, test scaffolding, first-draft docs. A smaller team carries the same scope with fewer rebuilds. Our rate is published at $22-$46/hr, US teams get 10-12 hours of daily overlap, and Vietnam-based developers work your US business hours on request. Price pressure lands on the unglamorous parts of custom API development.
At premium onshore rates, hardening, versioning rules and documentation are usually the first items cut, and those are what keep consumers working a year later. We keep them in scope.
Security Designed Into Every Endpoint
Every endpoint is a door into your data, so one weak authentication rule can leak records to every consumer that calls it. We design access into the first spec: OAuth 2.1 and OpenID Connect, API keys with rate limits, role-based access, encryption and an audit trail for user actions. GDPR and PDPA requirements are followed, and HIPAA and PCI-DSS requirements are met where the workload needs them.
These controls run inside our ISO 27001 certified management system, and an outside body, BSI, audits that system.
Stable Teams for APIs That Version for Years
An API outlives its first release. Consumers keep calling v1 while v2 ships, and the engineers who wrote the original contract know which changes are safe. Keeping them is a technical decision, and we treat it as one.
Saigon Technology ranked #10 in the Medium category of the Southeast Asia Best Workplaces™ in Technology 2026 list from Great Place To Work, and was named one of the Fortune 100 Best Companies to Work For™ in Southeast Asia 2025. On the Personal Loans Application, an engagement of 3+ years, 20 senior engineers were onboarded in the first three months.
Architecture Advice From Day One, Including When Not to Build
Sometimes you don't need us. An existing connector or a vendor's own API already does the job. We'll say so on the first call, so you only pay to build the endpoints nobody sells.
14+ Years · 400+ Developers · 850+ Projects · 350+ Clients · 4.8★
Delivered from three development centers in Ho Chi Minh City and Da Nang. The case studies above are larger platforms in which the API layer did the heavy lifting. We'd rather show verifiable adjacent work than invent an API metric.
ISO 9001 and ISO 27001 (BSI, UK), Microsoft Solutions Partner
BSI audits both standards, which gives your security review an outside report to check instead of our word. That matters most when your API carries customer or financial data.
Two-Week Risk-Free Trial and Flexible Engagement
Start with a two-week risk-free trial and judge the work on your own API. After that, choose staff augmentation, a dedicated team, fixed price or an offshore development center, once you have interviewed each proposed engineer.
Custom APIs fit companies in any industry. What differs is the starting point, and most clients come to us from one of these three. When the API is one part of a larger product, our custom software development team works on the same plan.
eCommerce, education, manufacturing and real estate teams use the same API discipline. See all the industries we serve.
Ship a public API your customers can build on, with OpenAPI docs, a sandbox and versioning that lets you release without breaking integrations already in production. We build it alongside your SaaS application development roadmap.
Give partners your platform's data through one controlled API instead of a custom export per partner. API keys, rate limits and per-resource endpoints keep every partner to its own data, as on the Fleet Management Platform. A dedicated team can run the API after launch.
Let new apps reach an older system without rewriting it: put an API layer and gateway in front, as we did for the Merit Logistics ERP. When replacement comes, our legacy application modernization team plans it. HIPAA and PCI-DSS requirements are met where the workload needs them.
Custom API development services pay off in daily operations more than on an architecture diagram. Partners connect faster. Internal teams reuse the same data, and outages at outside providers stop reaching your customers. Clients notice it fast.
| Benefit | Proof on a live project |
|---|---|
| Controlled partner access to platform data | Fleet Management Platform: API keys, rate limits and secured endpoints per resource |
| One gateway in front of an older system | Merit Logistics: Ocelot first, then a custom-built gateway |
| Provider failures handled before they reach users | Personal Loans Application: fallback routes and background retries |
| One audit trail across connected providers | Personal Loans Application: verification, underwriting and servicing |
Partners onboard without custom work each time
One documented API and a sandbox replace one-off exports and file drops, so adding a partner becomes configuration work.
One controlled door to your data
Keys, rate limits and role-based access decide who reads what, which improves interoperability without loosening security.
New apps reuse the same endpoints
Web, mobile and partner apps call one API, so business rules live in one place and productivity rises with each new app.
Provider outages stay behind the API
Fallback routes and background retries keep requests moving when a third-party API slows or fails, which protects customer satisfaction.
Changes ship without breaking existing consumers
Versioning and a published deprecation schedule give each team time to move, and microservice-based architectures let one service scale or change without the others.
A clear record of who called what
Audit trails and API analytics show usage, errors and system performance per consumer, which supports compliance reviews and capacity planning.
Requirements gathering starts with who will call the API: your apps, internal teams and partners. We list the data each consumer needs, the access each one gets and the compliance scope, then agree project scope, priorities and expectations in writing. Exit criterion: every consumer and data owner named.
We write the OpenAPI spec, the error format and the versioning rule, and choose the API style for each consumer.
"On API projects, most expensive rework traces back to the contract. We agree the OpenAPI spec, error format and versioning rule with every consuming team before we build, because changing an endpoint after other apps depend on it costs more than getting it right." Quy Truong, Senior Solution Architect, Saigon Technology
Exit criterion: contract reviewed by every consuming team.
Engineers code endpoints in two-week sprints against the agreed spec. Nobody waits for us. Consumers start early using mock servers generated from it. Exit criterion: endpoints passing contract tests.
We test authentication, rate limits and permission boundaries, run load and stress tests at expected peak traffic, and simulate slow or failing providers. Our software QA and testing team owns this stage. Exit criterion: auth, rate-limit and failure cases tested.
Swagger or Postman docs, a sandbox for partners and a versioned release go live together, so the first outside developer can start without a support call. Docs ship with the code. Exit criterion: docs and sandbox ready before go-live.
API monitoring, alerting and performance tracking run from day one, inside the pipelines our DevOps services team sets up. Every breaking change gets a new version and a retirement date for the old one. Exit criterion: deprecation schedule published for every breaking change.
The cost of custom API development services depends on the number of endpoints and consumers, how many outside providers the API calls, the security and compliance scope, and whether partners need docs and a sandbox. Scope drives price. Our published senior-led rate is $22-$46 per hour, and projects start from $10,000. Fixed price suits a well-defined spec, while staff augmentation, dedicated teams and ODCs suit work that keeps evolving.
A timeline for custom API development services depends on the number of consumers and providers, the compliance scope and how settled the requirements are. An underdeveloped project specification or ongoing changes in project requirements stretch any plan, which is why we fix the contract first. You see a working prototype from 48 hours. The Fleet Management Platform in our case studies took three months, a reference point for scale rather than a quote.
Both paths work. Keep it in-house when the API is your core product and your team already handles versioning, security and on-call support. Outsource when you need that experience quickly, or when API work would pull engineers off your roadmap. Outsourced custom API development services also suit fixed-scope builds. If you only need to connect existing SaaS tools, you may not need a custom API at all.
Security is part of the spec from the start. That means OAuth 2.1 with OpenID Connect, JWT, API keys with rate limits, and role-based access. Data is encrypted, user actions write to an audit trail, and secrets live in a managed store. GDPR and PDPA requirements are followed, HIPAA and PCI-DSS requirements are met where the workload needs them, and clinical data gets HL7 and FHIR interoperability support, all under our ISO 27001 certified management system.
Choose by consumer. REST suits public and partner APIs that third parties call with standard tools. GraphQL suits front ends with many screens that need different slices of the same data. gRPC suits fast, strictly typed calls between your own services. Many systems use two or all three, as the Personal Loans Application does, and the comparison table above sets out the trade-offs.
An API development company designs, builds and runs the interfaces that let software systems share data and functions. The work covers consumer discovery, API design and specification, endpoint development, security, testing, documentation, deployment, and the versioning and monitoring that keep an API usable after launch. Many also work as an API integration company, connecting the API to the outside providers it depends on.
Each API development service we run uses tools matched to the stack you already have. These are the areas we work across most often.
| Area | What we work with |
|---|---|
| API styles and specs | REST, GraphQL, gRPC, OpenAPI / Swagger, Postman |
| Back end | .NET Core, Java and Spring Boot, Python |
| Gateways and messaging | Ocelot, custom-built gateways, Consul, Fabio, Apache Kafka, Apache Camel |
| Identity and access | OAuth 2.1, OpenID Connect, JWT, Keycloak, AWS Cognito |
| Data | PostgreSQL, MongoDB, Microsoft SQL Server, DynamoDB, Redis |
| Cloud and DevOps | AWS, Docker, Kubernetes, Terraform, GitHub Actions, Grafana |
Yes. You can hire API development specialists through staff augmentation, a dedicated team or an offshore development center, full-time or part-time. We send a shortlist of matched profiles within 1-2 working days of your role brief, you interview each engineer, and a two-week risk-free trial lets you check the fit on your own codebase.
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Nov 11, 2025 
