API
An API is a defined way for one software system to request something from another.
A request leaves one system, another system performs work or returns data, and a structured response comes back.
Tap Pay and watch the request travel
Zepto checkout example
Maggi ×2 ₹60 · Coca-Cola ₹45 · Delivery ₹29📱Zepto app
Creates payment request
→
⚡UPI API
Passes structured request
→
🏦Bank
Approves / declines
→
✅Confirmation
Returns to app
Ready. No API request has been sent yet.
The waiter analogy
📱 Your app
Knows what it wants.
🤵 API contract
Defines how to ask and what format comes back.
🏦 Other system
Performs the operation or returns data.
APIs are already everywhere
Zepto
Several systems cooperate behind one action.
UPI payment · bank confirmation · maps · notifications
Key insight: an API is the connection contract. Tool calling is the AI behaviour that decides when to use a connected capability.