API Server & Integration
REST API, Payments, Auth, AI
Backend API servers, payment/SMS/social login integration, JWT/OAuth authentication, Swagger docs, and AWS deployment — built end to end.
8 Types of API Work We Deliver
From standalone API servers to adding endpoints and integrations to your existing backend.
REST API Server
Backend API server design & development
External API Integration
Payment, SMS, maps & social login integration
AI / GPT API
GPT/Claude-powered features, chatbots & automation
Data Collection APIs
Serve crawled/scraped data through APIs
Auth & Security
JWT, OAuth2, social login & role design
DB Design & Tuning
Schema design, query tuning & migration
API Documentation
Swagger/OpenAPI docs & collaboration specs
Deploy & Operate
AWS deployment, CI/CD, monitoring & scaling
Technology We Use
* Technology choices are tailored to each project's requirements and scale.
What Affects Timeline & Cost
Number of endpoints
Endpoint count is the base unit of development effort.
External integrations
Each external service (payment, SMS, maps) expands scope.
Auth complexity
From simple API keys to OAuth2 and role-based access.
Data volume & performance
High-volume processing and latency requirements (caching, queues) raise design complexity.
Docs & testing level
Swagger docs, automated tests, and load testing add effort.
Operations scope
Monitoring, incident response, and versioning affect ongoing cost.
What You Receive
Full source code
API server & DB schema delivered via Git repository.
API docs (Swagger)
Request/response specs for every endpoint.
Server deployment
Deployed to production (AWS, etc.) and ready to use.
Operations guide
Deployment procedures, env vars & incident response guide.
Budget & Timeline Reference
Indicative ranges based on past projects. Actual quotes depend on your requirements.
Simple Integration
Examples: 1-2 external APIs, ~5 endpoints
Mid-size API Server
Examples: Backend with auth, payment/notification integration, docs
Large / High-perf Backend
Examples: High-volume processing, microservices, realtime, AI integration
* All prices in KRW. VAT not included. Free preliminary consultation available.
Get an accurate quote for your project →Recent API Projects
Booking & Payment Backend + PG
Real-time booking API server with Toss Payments integration and automated SMS/email confirmation APIs.
Multi-store Management API Platform
REST APIs for stores, menus & orders with role-based auth (HQ/owner/staff).
Data Collection → Reporting API
Automated data collection, cleaning & aggregation served via APIs with daily reports and alert integration.
What Clients Say
Real reviews from verified marketplace transactions — check them yourself on Kmong.
Extremely satisfied. It was a data crawling request with many complex conditions, but they understood the requirements accurately and kindly from the first consultation. The data was delivered in a clean, organized format and was highly usable.
They handled the requested data crawling accurately and quickly. The data was cleanly organized and ready to use right away. For difficult, complex crawling — this is the expert. I will definitely come back.
It got complicated midway and took a bit longer, but I am satisfied! I want to entrust other work too!
They quickly produced exactly the information I wanted — perfectly usable for marketing. Thank you.
Thank you for the thorough, diligent work!
Frequently Asked Questions
Can you build only the API server while we handle the frontend?
Yes. We provide Swagger/OpenAPI documentation so your frontend team can integrate immediately. Full-stack development including frontend is also available.
Can you add endpoints to an existing server?
Yes. We analyze your existing codebase and add features following its structure and conventions. We have experience extending Node.js, Python, and Java (Spring) projects.
What payment integrations do you support?
We integrate Korean PGs (Toss Payments, PortOne, NicePay) and global providers (Stripe, PayPal), covering one-time payments, subscriptions/billing, partial refunds, and webhook-based settlement.
What about API performance guarantees?
We agree on performance targets based on expected traffic during design, and use caching (Redis), async queues, and DB index optimization. Load test results can be included in deliverables.
How is authentication implemented?
We propose JWT, OAuth2, API keys, or sessions based on your service. Social login (Kakao, Naver, Google, Apple) and role-based access control (RBAC) are available.
How is documentation delivered?
Swagger (OpenAPI) auto-generated docs are included by default — with request/response examples, error code definitions, and auth instructions, ready for any developer to integrate.
Who operates the server after completion?
We handle initial deployment (AWS, etc.). Afterwards you can operate it yourself post-handover, or sign a monthly maintenance contract covering monitoring, incident response, and feature additions.
Who owns the source code?
After completion and final payment, full source code ownership transfers to you, including Git repository transfer and complete technical documentation.
Start Your API Project
Describe your requirements and get an AI-powered estimate in 30 seconds. Questions? Chat with us in real time — bottom right.
Freesi is a platform that supports the entire outsourcing process — from entering requirements, receiving AI-powered estimates, to contract and milestone management — for web development, RPA, crawling, API integration, and GPT chatbot projects.