Updated September 2026 · Written and maintained by the Progression Agency strategy team
Progression Agency provides API development and integration services: custom REST and GraphQL APIs for your products and partners, and integrations that connect CRMs, ERPs, payment processors, marketing platforms and legacy systems. As a custom API development company, we also deliver API integration services that connect those APIs to the systems you already run.
On this page · 12 sections
- What is API development?
- API development services
- Key features and components
- Common integrations
- API security
- Our API development process
- How much does API development cost?
- Technology we typically use
- Mistakes to avoid
- Why choose Progression Agency for API development?
- Related services
- REST vs GraphQL vs gRPC
The short answerAPI development is designing, building, documenting and securing interfaces that let software systems exchange data and trigger actions. API integration connects existing systems through their APIs. A custom API or integration commonly costs $15,000-$100,000.
Cost ranges are US market price bands, not quotes.
What is API development?
API development is designing and building application programming interfaces that allow software systems to communicate, share data and trigger actions securely.
Well-designed APIs are products in their own right: documented, versioned, secure and monitored.
API development services
Projects we are most often asked to build.
Custom REST APIs
Resource design, validation, pagination, errors and versioning.
GraphQL APIs
Flexible queries for web and mobile clients.
System integrations
CRM, ERP, payments, marketing and data platforms.
Public and partner APIs
Developer portals, keys, rate limits and documentation.
Webhooks and events
Real-time notifications between systems.
Legacy system APIs
Modern interfaces on older software.
Well-designed APIs are products in their own right: documented, versioned, secure and monitored.
REST and web API development services
REST API development services produce resource-based interfaces with clear URLs, predictable errors, pagination and versioning. Web API development services cover the same ground for any HTTP interface, including those that serve browsers, mobile apps and partner systems. In both cases the specification comes first, so every consumer can build against the same contract.
Key features and components
What separates a useful build from a costly one.
| Feature | Why it matters |
|---|---|
| OpenAPI documentation | Easy adoption |
| Authentication (OAuth 2.0) | Secure access |
| Rate limiting | Protects stability |
| Versioning | No breaking changes |
| Validation and errors | Predictable behavior |
| Automated tests | Reliable releases |
| Monitoring | Uptime and latency |
| Idempotency | Safe retries |
Common integrations
Where the value usually comes from.
| System | What we connect |
|---|---|
| CRM | Salesforce, HubSpot |
| ERP / accounting | NetSuite, QuickBooks |
| Payments | Stripe and processors |
| Marketing | Email, ads, analytics |
| Identity | OAuth providers |
| Data | Warehouses |
Custom API integration services
What is custom API integration? It is building the connection between two systems that were not designed to talk to each other, using their APIs, so that data moves without manual re-entry. Custom API integration services cover mapping the fields, handling authentication and rate limits, retrying failures and logging what moved, so the link keeps working when either side changes.
Third party API integration services focus on connecting your software to outside platforms such as payment processors, shipping carriers, marketing tools and data providers. The work is mostly about reading each provider’s documentation closely, testing against its sandbox and planning for the day it changes a version.
SAP API development
Searches for SAP API development usually mean making data in an SAP system available to other software through a modern API layer. If this is your case, ask any provider which SAP product and version they have worked with before you commit, since the interfaces differ widely between systems and releases.
API security
APIs are a common attack surface.
- OWASP API Security Top 10 practices.
- OAuth 2.0 and scoped tokens.
- Rate limiting and abuse detection.
- Audit logging and secrets management.
Our API development process
We identify API consumers and use cases, design the contract with OpenAPI, implement security and rate limits, build endpoints with automated tests, publish documentation, release with versioning and monitor usage.
API development best practice starts with a written contract, such as an OpenAPI specification, agreed before any code is written. API development and testing tools such as Postman or Insomnia help check behavior during the build, and automated contract tests in the build pipeline catch changes that would break existing consumers.
How much does API development cost?
As a price band, a single system integration commonly costs $15,000-$40,000, a custom product API $40,000-$100,000, and a public API with developer portal $100,000+.
| Scope | Typical range | Timeline |
|---|---|---|
| Single integration | $15,000-$40,000 | 3-6 weeks |
| Custom product API | $40,000-$100,000 | 2-4 months |
| Public API + developer portal | $100,000+ | 4-6 months |
Technology we typically use
Mainstream, maintainable choices.
| Layer | Choice |
|---|---|
| Languages | TypeScript (Node.js), Python |
| Specs | OpenAPI, GraphQL schema |
| Auth | OAuth 2.0, API keys |
| Data | PostgreSQL |
| Gateway | API gateway with rate limits |
| Hosting | Cloud |
API development Python work is a good fit when a team already uses Python for data, automation or machine learning. API development in Python commonly uses frameworks such as FastAPI and Django REST Framework, both of which can generate documentation from the code.
Mistakes to avoid
Common causes of overruns and low adoption.
- Coding before designing the contract.
- No versioning.
- Weak authentication.
- Missing documentation.
- No rate limits.
- Silent failures without monitoring.
Why choose Progression Agency for API development?
Engineering, design and go-to-market in one team.
- Contract-first API design.
- Integration experience across common business platforms.
- Documentation and developer experience.
- You own the code, data and accounts.
- Headquartered in New York City, working with clients nationwide and worldwide.
Related services
See backend development, custom software development and AI integration services.
REST vs GraphQL vs gRPC
The API style should follow the clients that consume it. We choose per project.
| Style | Strengths | Best for |
|---|---|---|
| REST | Simple, cacheable, universally supported | Public and partner APIs |
| GraphQL | Clients request exactly the data they need | Apps with many screens and data shapes |
| gRPC | Fast binary protocol with strict contracts | Internal service-to-service calls |
| Webhooks | Push events to other systems | Integrations and notifications |
Documentation as a deliverable
Every API ships with an OpenAPI specification and examples, so partners can integrate without a meeting.
Ready to scope it?
Tell us the users, workflow and systems involved. We will propose a first release and price estimate.
Getting found in search
AI, AEO and what is changing
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Frequently asked questions
What is API development?
How much does API development cost?
How long does API development take?
REST or GraphQL?
What is API integration?
Can you integrate our CRM and ERP?
How do you secure APIs?
What is OpenAPI?
Do you build webhooks?
Can you build a public API for our product?
Can you wrap a legacy system with an API?
What is API versioning?
What tech stack do you use?
How much maintenance is required?
What should I look for in an API developer?
Can you fix a failing integration?
What is idempotency?
Do you use iPaaS tools?
Who owns the API?
Where is Progression Agency located?
What does an API development company deliver?
When do I need an API development service?
Is API development backend or frontend?
What is a custom API?
Need software built?We build REST and GraphQL APIs and the integrations around them, documented and versioned so your partners can build on them.
Get a free marketing proposal
Tell us what you are trying to grow and we will come back with a plan, not a pitch deck. Same-day reply on weekdays.