What Are REST API Calls and How Do They Work? A Guide

REST API calls are the backbone of modern web communication, but understanding their mechanics goes beyond knowing that they're HTTP requests. Whether you're building your first integration or architecting enterprise systems, grasping how these calls work—from request structure to response handling—directly impacts your ability to design reliable, maintainable applications.

This guide breaks down REST API calls from the ground up: what they contain, how different HTTP methods map to actions, what responses look like, and how tools like SDKs and AI agents can streamline the entire process. You'll learn the practical details that matter when you're actually building with APIs, not just the theory.

What are REST API calls?

A REST API call is an HTTP request sent from a client application to a server to access or manipulate a resource. These calls are the fundamental way applications communicate over the internet, following a set of architectural constraints known as Representational State Transfer (REST). Think of it as a structured conversation where a client asks a server to perform an action, and the server responds.

The term REST refers to the architectural style, while the “call” is the actual implementation of a request and response. For example, a client making a GET request to /users/42 is performing a REST API call to retrieve data for a specific user.

How do REST API calls work?

REST API calls operate on a simple request-response model between a client and a server. The client initiates a request to a specific server endpoint, and the server processes that request and sends back a response. This entire exchange happens over the HTTP protocol.

A core principle of REST is statelessness. This means each API call is a self-contained, independent transaction. The server does not store any information about the client between requests, so every call must include all the information needed for the server to fulfill it, such as authentication details.

What does a REST API request contain?

A well-formed REST API request is composed of several distinct parts that work together to tell the server exactly what the client wants to do. Understanding these components is key to both making and debugging API calls.

Endpoint structure

The endpoint is the URL that identifies the resource you want to interact with. It combines the base URL of the API with a specific path and optional query parameters to refine the request.

• Base URL: The starting address for the API, like https://api.yourcompany.com/v1.

• Path: Specifies the resource, such as /users/123 to target a user with the ID 123.

• Query Parameters: Key-value pairs added to the end of the URL after a ? to filter or sort results, like ?status=active&sort=createdAt.

Header list

Headers provide metadata about the request itself. They are key-value pairs that can specify the format of the data being sent, provide authentication credentials, or control caching behavior.

• Content-Type: application/json tells the server the request body is formatted as JSON.

• Authorization: Bearer <your-api-key> passes a security token to authenticate the request.

• Well-designed SDKs often add useful headers for analytics, helping API providers understand which languages and runtimes their users prefer.

Body format

The body contains the data you want to send to the server, typically used with POST, PUT, and PATCH requests. While JSON is the most common format for its human-readability and machine-parsability, other formats are used for specific cases like file uploads.

Which HTTP method fits each action?

The HTTP method, or verb, specifies the action you want to perform on the resource. Using the correct method is essential for a predictable and well-behaved REST API.

GET method usage

Use GET to retrieve data from a resource. It is a read-only operation and should never modify data on the server. GET requests are considered safe and idempotent, meaning multiple identical requests will have the same effect as a single one.

POST method usage

Use POST to create a new resource. For example, sending a POST request to /users with user data in the body would create a new user. POST is not idempotent, as calling it multiple times will create multiple new resources.

PUT and PATCH method usage

Use PUT or PATCH to update an existing resource. PUT typically replaces the entire resource with the data provided in the request body, while PATCH applies a partial update. Both methods are idempotent.

DELETE method usage

Use DELETE to remove a resource. A successful DELETE request permanently removes the targeted resource from the server. Like GET and PUT, DELETE is idempotent.

What does a REST API response include?

After processing a request, the server sends back a response containing a status code, headers, and an optional body. This response tells the client whether the request was successful and provides any requested data.

Status code map

HTTP status codes are three-digit numbers that provide a quick summary of the request's outcome. They are grouped into classes:

• 2xx (Success): The request was successfully received, understood, and accepted. (e.g., 200 OK, 201 Created)

• 4xx (Client Error): The request contains bad syntax or cannot be fulfilled. (e.g., 400 Bad Request, 401 Unauthorized, 404 Not Found)

• 5xx (Server Error): The server failed to fulfill an apparently valid request. (e.g., 500 Internal Server Error)

Response header set

Similar to request headers, response headers provide metadata about the response. This can include information like the format of the response body (Content-Type) or caching instructions. Some APIs also use headers like Retry-After to guide clients on handling rate limits.

JSON body example

For successful GET requests, the response body typically contains the requested data, most often in JSON format. For example, a request to GET /users/123 might return:

{
  "id": 123,
  "name": "Jane Doe",
  "email": "[email protected]",
  "status": "active"
}

How do you make REST API calls?

You can make REST API calls using various tools, from simple command-line utilities to full-featured programming libraries. A raw curl command shows the manual work involved in constructing a request, including setting the method, headers, and body.

curl -X POST https://api.example.com/v1/users \
  -H "Authorization: Bearer your_api_key" \
  -H "Content-Type: application/json" \
  -d '{"name": "John Doe", "email": "[email protected]"}'

Alternatively, you can use a Stainless-generated SDK to achieve the same result with far less boilerplate:

import { MyAPI } from "my-stainless-sdk";

const client = new MyAPI({
  apiKey: "your_api_key",
  // any other client options…
});

async function createUser() {
  const user = await client.users.create({
    name: "John Doe",
    email: "[email protected]",
  });
  console.log("Newly created user:", user);
}

In code, you would use a library like fetch in JavaScript or requests in Python to handle the HTTP communication. While more structured than curl, this still requires you to manually manage authentication, format the request body, and parse the response.

Debugging API calls often involves:

• Inspecting status codes to see if the request succeeded or failed.

• Using tools like curl -v to view the full request and response headers.

• Leveraging API clients like Postman or Insomnia for a graphical interface to build and test requests.

How can SDKs and AI agents simplify REST API calls?

While making manual API calls is feasible, it involves a lot of repetitive boilerplate for authentication, request formatting, and error handling. This is where SDKs provide a powerful layer of abstraction. Instead of building an HTTP request by hand, you can use a simple, idiomatic function call.

Here’s how the same call looks if you’re using a Stainless-generated SDK. Notice you only specify the data you want to send; authentication, headers, and JSON parsing are handled automatically:

import { MyAPI } from "my-stainless-sdk";

const client = new MyAPI({
  apiKey: "your_api_key",
  // any other client options…
});

async function createUser() {
  const user = await client.users.create({
    name: "John Doe",
    email: "[email protected]",
  });
  console.log("Newly created user:", user);
}

The SDK handles building the request, serializing the body, adding authentication headers, and even implementing best practices like automatic retries and pagination.

Taking this abstraction a step further, AI agents can interact with APIs using natural language. This is enabled by standards like MCP, which allows an LLM to discover and use an API's capabilities. An MCP server, which you can generate from an OpenAPI spec, acts as a translator between the AI and the REST API.

For example, a user could tell an AI assistant, "Create a new product called 'The Cube' for $1500." The AI, interacting with an MCP server, translates this command into a structured POST /products REST API call with the correct JSON body, completely hiding the underlying complexity.

Frequently asked questions about REST API calls

REST vs RESTful API meaning?

REST is the name of the architectural style, while RESTful is the adjective used to describe an API that adheres to that style; the terms are often used interchangeably.

Four primary REST API methods?

The four primary methods map to CRUD (Create, Read, Update, Delete) operations: POST (Create), GET (Read), PUT/PATCH (Update), and DELETE (Delete).

Example of an API call?

A simple example is a GET request to https://api.example.com/users/123, which asks the server to return data for the user with an ID of 123.

How to test API calls without code?

You can use dedicated API client applications like Postman or Insomnia, which provide a graphical user interface for building, sending, and inspecting REST API calls.

Can AI generate REST API calls?

Yes, AI agents can use protocols like MCP to understand an API's functions and generate the appropriate REST API calls to complete tasks based on natural language commands.

Ready to provide a first-class developer experience for your API? Stainless generates idiomatic, high-quality SDKs and MCP servers from your OpenAPI spec in minutes. Get started for free.