API Development Services

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.

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 planning figures, 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.

Integ — System integrations. Connect your stack..

Partner — Partner APIs. Open your product..

Legacy — Legacy wrappers. Modern APIs on old systems..

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.

Key features and components

What separates a useful build from a costly one.

API development: key components
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.

Common integrations
System	What we connect
CRM	Salesforce, HubSpot
ERP / accounting	NetSuite, QuickBooks
Payments	Stripe and processors
Marketing	Email, ads, analytics
Identity	OAuth providers
Data	Warehouses

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.

How we deliver API development — Designing the contract before code prevents breaking changes for API consumers.

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.

How much does API development cost?

As a planning range, 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+.

API development cost ranges (planning figures)
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.

Technology we typically use
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

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.

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.

API styles compared
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 planning estimate.

Get a proposal

Getting found in search

AI, AEO and what is changing

Paid media and lead generation

Websites and design

Choosing and working with an agency

Software and app development

Web development, platforms and hosting

Social, content and brand

By industry and by situation

Frequently asked questions

What is API development?

Designing and building interfaces that let software systems communicate securely.

How much does API development cost?

As a planning range, $15,000-$40,000 for an integration and $40,000-$100,000 for a custom product API.

How long does API development take?

Commonly 3-6 weeks for an integration.

REST or GraphQL?

REST for most integrations; GraphQL when clients need flexible queries.

What is API integration?

Connecting existing systems through their APIs.

Can you integrate our CRM and ERP?

Yes.

How do you secure APIs?

OAuth, scoped tokens, rate limits, validation and monitoring.

What is OpenAPI?

A specification for describing REST APIs.

Do you build webhooks?

Yes.

Can you build a public API for our product?

Yes, with a developer portal.

Can you wrap a legacy system with an API?

Yes.

What is API versioning?

Managing changes without breaking existing consumers.

What tech stack do you use?

Node.js or Python, OpenAPI, OAuth and PostgreSQL.

How much maintenance is required?

Monitoring, security updates and new versions.

What should I look for in an API developer?

Contract-first design, security and documentation.

Can you fix a failing integration?

Yes.

What is idempotency?

Safe retries without duplicate effects.

Do you use iPaaS tools?

Where they fit; custom code where they do not.

Who owns the API?

You do.

Where is Progression Agency located?

New York City, serving clients across the United States and worldwide.

What does an API development company deliver?

An API development company delivers a documented, versioned and secured interface, usually with an OpenAPI specification, automated tests, authentication and monitoring, plus the integrations that consume it.

When do I need an API development service?

You need an API development service when systems must exchange data reliably, when partners need access to your product, or when a mobile or web app needs a backend built for it.

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 software proposal

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.

API Development Services: Custom API Development & Integration Company

What is API development?