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API Development Services: Custom API Development & Integration Company

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 · 13 sections
  1. What is API development?
  2. API development services
  3. Key features and components
  4. Common integrations
  5. API security
  6. Our API development process
  7. How much does API development cost?
  8. Technology we typically use
  9. Mistakes to avoid
  10. Why choose Progression Agency for API development?
  11. Related services
  12. REST vs GraphQL vs gRPC
  13. Video: building software

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.

API development searches
Companies search for API developers to connect systems and open their products to partners.

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.

REST — REST APIs. Standard web APIs..
GQL — GraphQL APIs. Flexible queries..
Integ — System integrations. Connect your stack..
Partner — Partner APIs. Open your product..
Hooks — Webhooks. Real-time events..
Legacy — Legacy wrappers. Modern APIs on old systems..

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.

Key features and components

What separates a useful build from a costly one.

API development: key components
FeatureWhy it matters
OpenAPI documentationEasy adoption
Authentication (OAuth 2.0)Secure access
Rate limitingProtects stability
VersioningNo breaking changes
Validation and errorsPredictable behavior
Automated testsReliable releases
MonitoringUptime and latency
IdempotencySafe retries

Common integrations

Where the value usually comes from.

Common integrations
SystemWhat we connect
CRMSalesforce, HubSpot
ERP / accountingNetSuite, QuickBooks
PaymentsStripe and processors
MarketingEmail, ads, analytics
IdentityOAuth providers
DataWarehouses

API security

APIs are a common attack surface.

  • OWASP API Security Top 10 practices (OWASP).
  • 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)
ScopeTypical rangeTimeline
Single integration$15,000-$40,0003-6 weeks
Custom product API$40,000-$100,0002-4 months
Public API + developer portal$100,000+4-6 months

Technology we typically use

Mainstream, maintainable choices.

Technology we typically use
LayerChoice
LanguagesTypeScript (Node.js), Python
SpecsOpenAPI, GraphQL schema
AuthOAuth 2.0, API keys
DataPostgreSQL
GatewayAPI gateway with rate limits
HostingCloud

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.
Contract first — API. Stable..
OAuth — API. Secure..
Versioning — API. No breakage..
Docs — API. Adoption..
Rate limits — API. Stability..
Monitoring — API. Reliability..

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 nationwide.

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
StyleStrengthsBest for
RESTSimple, cacheable, universally supportedPublic and partner APIs
GraphQLClients request exactly the data they needApps with many screens and data shapes
gRPCFast binary protocol with strict contractsInternal service-to-service calls
WebhooksPush events to other systemsIntegrations 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

Video: building software

Explainers on the practices and frameworks we use.

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

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 nationwide.

Sources and further reading

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  3. Google: intro to structured data
  4. Google: LocalBusiness structured data
  5. Google: FAQPage structured data
  6. Google: Article structured data
  7. Google: Product structured data
  8. Google: title links in search results
  9. Google: control your snippets
  10. Google: robots.txt introduction
  11. Google: sitemaps overview
  12. Google: consolidate duplicate URLs
  13. Google: redirects and Search
  14. Google: JavaScript SEO basics
  15. Google: multi-regional and multilingual sites
  16. Google Search Central Blog
  17. Google: get started with Search Console
  18. Google: how local search results are determined
  19. Google Business Profile: prohibited and restricted content
  20. Google Business Profile: address and service area guidelines
  21. Google Business Profile: review policy
  22. Google Business Profile: add or edit categories
  23. Google Ads: location targeting settings
  24. Google Ads: about negative keywords
  25. Google Ads: about Quality Score
  26. Google Ads: importing offline conversions
  27. Google Ads: about Smart Bidding
  28. Google Ads: about Performance Max
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  188. Similarweb
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  191. Australian OAIC
  192. European Data Protection Board
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  199. Smashing Magazine
  200. web.dev
  201. MDN: web media
  202. MDN: the video element
  203. ISO 21001 (reference)
  204. Buma/Stemra (Netherlands)
  205. STIM (Sweden)
  206. Teosto (Finland)
  207. Koda (Denmark)
  208. TONO (Norway)
  209. IMRO (Ireland)
  210. SGAE (Spain)
  211. ZAiKS (Poland)
  212. KOMCA (South Korea)
  213. MCSC (China)
  214. CISAC
  215. World Intellectual Property Organization
  216. TikTok: creating videos
  217. TikTok: exploring videos
  218. TikTok: privacy settings
  219. TikTok: growing your audience
  220. TikTok Creator Academy
  221. TikTok Effect House
  222. TikTok for small business
  223. Instagram: Reels help
  224. YouTube: Shorts best practice
  225. How YouTube recommends
  226. Pinterest Predicts
  227. Snapchat for Business
  228. Hootsuite blog
  229. Social Media Examiner
  230. Marketing Week
  231. Adweek
  232. OWASP API Security
  233. OpenAPI Initiative
  234. OAuth 2.0

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