113 APIs · REST and MCP

Reliable APIs your code and AI agents can call instead of guessing

Validation, date math, finance calculators, DNS and web lookups, and reference data behind one API key. Exact, deterministic answers over REST, or as MCP tools for your agents.

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APIs: 113
MCP servers: 19
Free calls / month: 1,000

curl -X POST https://apistack.online/api/finance/loan-calculator/v1 \
  -H "x-api-key: YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"principal":25000,"annualRate":6.5,"termYears":5}'

200 OK · response

{
  "success": true,
  "data": {
    "paymentCalculation": {
      "monthlyPayment": 489.15,
      "totalAmount": 29349.22,
      "totalInterest": 4349.22,
      "interestPercentage": 14.82,
      "firstPayment": { "principal": 353.74, "interest": 135.42 }
    }
  }
}

See it in action

APIStack in 2 minutes

Find an API, run it live with your own key, test anything in the Playground and connect your AI agent.

All 8 video tutorials

Popular

Start with the tools developers use most

Exact answers for the things software, and language models, often get wrong.

View all 113 APIs

Utilities

Email Address Validator

Comprehensive email validation and verification API with syntax validation, domain verification, disposable email…

Utilities

Phone Number Validation & Lookup

Comprehensive phone number validation, formatting, and lookup API supporting international phone numbers with country…

Utilities

Business Days Calculator

Calculate business days between dates, add/subtract business days, excluding weekends and holidays with support for…

Utilities

Domain and DNS Info Checker

Comprehensive domain and DNS information retrieval API with WHOIS, DNS records, SSL verification, and availability…

Finance

Mortgage Calculator

Professional mortgage calculator with 4 specialized endpoints for payment calculations, affordability analysis,…

Reference data

World Universities Database

Comprehensive database of 1,050+ universities worldwide with rankings, locations, institutional details, and…

How it works

From sign-up to first call in a minute

1
Create a free account
No card needed. The free plan includes 1,000 calls a month.
2
Copy your API key
One key works for every API. Send it in the x-api-key header.
3
Call an API or connect an agent
Use REST from your code, or add an MCP server so Claude, ChatGPT or Cursor can call the same tools.

Full guide in the documentation, or import the OpenAPI spec.

For AI agents

Give your agent tools instead of guesses

Language models are unreliable at date arithmetic, validation, live lookups and financial math. MCP servers let Claude, ChatGPT, Cursor and other agents call APIStack tools and get exact answers.

Watch: connect Claude Code, Cursor or VS Code with one MCP URL (1:34).

Deterministic results
Business days, loan payments and validations are computed, not predicted.
Live data
DNS, SSL, phone and email checks run at call time.
One key, one quota
Agents use the same API key and plan as your code.
Browse 19 MCP servers

Catalog

Everything behind one key

Utilities

Validation, dates, conversion, encoding

Reference data

Countries, cities, universities, codes

Web & SEO

DNS, SSL, metadata, performance

Finance

Loans, mortgages, investments, markets

Developer tools

Formatting, schemas, mocking

Analytics

Readability, bandwidth, load time

AI text tools

Summaries, keywords, classification

Pricing

Simple plans, one quota across every API

Free

$0/month

1,000 calls / month
10/min rate limit
Community support

Pro

Most popular

$19.99/month

100,000 calls / month
100/min rate limit
Email Support

Enterprise

$79.99/month

1,000,000 calls / month
500/min rate limit
Priority Support

FAQ

Common questions

Do I need a credit card to start?

No. The free plan includes 1,000 API calls a month across the catalog. Upgrade when you need more.

How is usage counted?

Each successful request counts as one call against your monthly quota, shared across all APIs. Your dashboard shows usage, and we email you at 80% and 100%.

What happens if I hit my limit?

Requests return HTTP 429 until your quota resets or you upgrade. Pro and Enterprise plans can also buy top-ups.

Can AI agents use these APIs?

Yes. Many APIs are available as MCP servers that Claude, ChatGPT, Cursor and other MCP clients can call as tools, using the same key and quota.

Is there an OpenAPI spec?

Yes. /openapi.json covers every API, so you can import it into Postman, Insomnia or a code generator.

Can I cancel anytime?

Yes, from your dashboard. Your account moves to the Free plan and you keep your API key.

Make your first call today

Free plan, no credit card. 1,000 calls a month to start.

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