What are REST API Headers? A Complete Guide

When working with REST APIs, headers play a critical role in how clients and servers exchange information. They are not part of the message body but still define how the request or response should be handled.

This article explains REST API headers in depth, including their types, common examples, advanced use cases, and how Requestly helps test and debug them effectively.

What Are REST API Headers?

REST API headers are additional pieces of information that accompany an HTTP request or response. Unlike the body, which carries the actual data being sent or received, headers provide metadata that helps define the rules of communication.

REST API headers ensure that the client and server know how to interpret the message, how to authenticate the request, and how to optimize the exchange of data.

Headers follow a simple key-value format, such as:

Content-Type: application/json

Authorization: Bearer <token>

The key tells the server or client what the information is about, and the value carries the actual instruction or detail. Together, they provide the context required for the API call to work correctly.

To understand REST API headers more clearly, it helps to break down what they control and why they matter. Below are the key aspects of how headers function:

• Separation of data and metadata: The body carries actual content like a JSON object, while headers specify how that content should be treated.
• Control over communication: Headers define rules such as content format, caching, and security, which prevent miscommunication between client and server.
• Flexibility and extensibility: APIs can use both standard and custom headers, allowing developers to extend functionality without changing the API endpoints.
• Consistency across systems: Since headers follow HTTP standards, they create a consistent way for APIs to work across different languages, platforms, and frameworks.

Why Are REST API Headers Important?

Headers are important as they affect performance, security, compatibility, and overall reliability of an API. Below are the main reasons why they are essential:

• Enable authentication and authorization: Headers carry tokens, API keys, or credentials that verify the identity of the client and control what resources can be accessed. Without headers, secure communication would not be possible.
• Ensure correct data handling: Headers specify the content type and encoding of requests and responses. For example, Content-Type: application/json ensures the receiving side interprets the message as JSON.
• Support caching strategies: Headers like Cache-Control or ETag define how long responses can be reused and when they should be refreshed. This reduces server load and improves response times.
• Enable cross-origin communication: Headers such as Access-Control-Allow-Origin handle CORS (Cross-Origin Resource Sharing), which allows APIs to be accessed securely across different domains.
• Improve debugging and monitoring: Response headers often carry diagnostic details like rate limits, request IDs, or server information that help developers troubleshoot and monitor API usage.

Types of REST API Headers

REST API headers fall into four main categories. Each category serves a different purpose, and seeing them in real request–response examples makes their role clearer.

1. General headers

These apply to both requests and responses and control aspects of communication that are not specific to the body.

Example:

GET /products HTTP/1.1

Host: api.example.com

Cache-Control: no-cache

Connection: keep-alive

Cache-Control tells the server not to serve cached data, while Connection: keep-alive asks the server to reuse the TCP connection. When testing APIs, it is common to check whether cache headers are respected and whether persistent connections behave correctly under load.

2. Request headers

These are sent by the client to provide information about the request or client environment.

Example (with Requestly rule):

{

  "rules": [

    {

      "ruleType": "ModifyHeaders",

      "requestHeaders": [

        { "header": "Authorization", "value": "Bearer <token>" },

        { "header": "Accept", "value": "application/json" },

        { "header": "User-Agent", "value": "TestingClient/1.0" }

      ]

    }

  ]

}

Authorization carries a token for authentication, Accept: application/json requests the response in JSON format, and User-Agent identifies the client. Testers often modify these headers to simulate different clients, check authentication flows, and verify how the API handles unsupported formats.

3. Response headers

These are added by the server to describe the response or influence client behavior.

Example:

HTTP/1.1 200 OK

Content-Type: application/json

Content-Length: 349

Set-Cookie: sessionId=abc123; HttpOnly

Server: nginx/1.23.4

Content-Type confirms the response format, Set-Cookie issues a session token, and Server identifies the software used. In testing, these headers are checked to ensure correct content types are returned, cookies are issued with proper security flags, and unnecessary server details are not leaked.

4. Entity headers

These describe the body of a request or response so the receiver can process it correctly.

Example:

POST /upload HTTP/1.1

Host: api.example.com

Content-Type: application/json

Content-Length: 82

{

  "fileName": "report.pdf",

  "size": 24576,

  "uploadedBy": "tester01"

}

Content-Type identifies the body format as JSON, and Content-Length specifies its size. In real-world testing, entity headers are used to verify that payload sizes are consistent and that compressed or encoded data (for example, Content-Encoding: gzip) is processed correctly by both client and server.

Common REST API Headers

While there are many HTTP headers available, some appear far more frequently in REST API communication. These headers handle core aspects like authentication, content negotiation, caching, and security.

1. Authorization

Carries credentials or tokens that verify the identity of the client. Most modern APIs use formats like Bearer tokens, API keys, or Basic authentication.

Example (Bearer token):

 curl -X GET "https://api.example.com/accounts" \

  -H "Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..."

This header is critical for secure communication. Testers often check whether invalid or expired tokens are correctly rejected and whether endpoints without authentication return proper error codes.

2. Content-Type

The Content-Type header specifies the media type of the request or response body so that the receiver can interpret the data correctly.

Example:

Content-Type: application/json

Common values include application/json, application/xml, or multipart/form-data for file uploads. Incorrect values often lead to parsing errors, which testers use to confirm error handling.

3. Accept

Tells the server which content types the client is able to handle in the response.

Example:

Accept: application/json

If the server cannot return the requested type, it should respond with 406 Not Acceptable. This allows testers to verify proper negotiation between client and server.

4. Cache-Control

Defines caching rules for requests and responses. It influences how long a response can be stored and when it should be refreshed.

Example:

Cache-Control: no-store

Common values include no-cache, no-store, and max-age=3600. Testers validate these headers to ensure that sensitive responses are not cached and that cached data refreshes as expected.

5. User-Agent

Identifies the client making the request. This can be a browser, mobile app, or API client.

Example:

User-Agent: MyApp/2.5 (Linux; x64)

APIs may behave differently depending on the client. By modifying this header, testers can simulate requests from different platforms or devices.

6. Set-Cookie

Used by the server to create or update cookies on the client side. Cookies often manage sessions, preferences, or authentication states.

Example:

Set-Cookie: sessionId=abc123; HttpOnly; Secure

Testers typically check that cookies include security flags like HttpOnly and Secure to protect against cross-site scripting and man-in-the-middle attacks.

7. Access-Control-Allow-Origin

Defines which domains are permitted to access the resource through CORS (Cross-Origin Resource Sharing).

Example:

Access-Control-Allow-Origin: https://client.example.com

This header is critical when APIs are consumed by web applications hosted on different domains. Testers validate that CORS rules are not overly permissive (for example, using * in production).

How to Set and Read REST API Headers

Working with REST API headers involves two main tasks: setting headers when making requests and reading headers when processing responses. Both are essential for developers and testers because headers often control authentication, caching, and the format of the data being exchanged.

Setting headers in requests

When sending a request to an API, headers are typically added to provide authentication details, specify content formats, or define client capabilities. Tools like Requestly make this process easier by letting testers modify headers dynamically without changing the code.

Example with Requestly (Modify Headers Rule):

1. Open Requestly and create a new rule.

2. Choose Modify Headers as the rule type.

3. Configure the request headers you want to add:

• Authorization: Bearer <token>
• Content-Type: application/json

Once this rule is active, every request that matches your defined conditions will automatically include these headers. This is useful for testing APIs without repeatedly editing code or scripts.

Example with JavaScript (fetch API):

fetch("https://api.example.com/orders", {

  method: "POST",

  headers: {

    "Authorization": "Bearer <token>",

    "Content-Type": "application/json"

  },

  body: JSON.stringify({ itemId: 123, quantity: 2 })

})

Here, headers are explicitly set before sending the request, ensuring that the server knows how to interpret the message and authenticate the client.

Reading headers in responses

When receiving a response, headers often contain just as much useful information as the body itself. Developers and testers commonly read response headers to validate content types, handle authentication tokens, or manage caching rules.

Example response (raw HTTP):

HTTP/1.1 200 OK

Content-Type: application/json

Content-Length: 187

Cache-Control: no-store

Set-Cookie: sessionId=xyz789; HttpOnly; Secure

Here:

• Content-Type confirms the response body is JSON.
• Cache-Control indicates that the response should not be cached.
• Set-Cookie issues a secure session cookie.

Example with Python (requests library):

import requests

response = requests.get("https://api.example.com/users")

print(response.headers["Content-Type"])

print(response.headers.get("Cache-Control"))

In this example, the headers dictionary provides access to individual response headers. This is especially useful when verifying that an API is returning the expected metadata.

Advanced Use Cases of REST API Headers

Beyond basic communication, REST API headers enable fine-grained control over performance, security, debugging, and compatibility. These use cases become critical when dealing with complex systems, distributed architectures, or enterprise-grade APIs.

1. Rate Limiting and Quotas

Many APIs enforce limits on how many requests a client can make within a given time window, and headers communicate these restrictions. For example:

X-RateLimit-Limit: 1000

X-RateLimit-Remaining: 250

X-RateLimit-Reset: 1693587200

These values tell the client its current quota and when it resets. Testers rely on them to validate that throttling is enforced correctly.

2. Content Negotiation and Versioning

Instead of embedding versions in the URL, headers can indicate version control, such as:

Accept: application/vnd.myapi.v2+json

This allows smooth upgrades without breaking existing clients, so testers must confirm that both old and new clients can function using version-based headers.

3. Security Enhancements

APIs often handle sensitive data such as personal information, payment details, or authentication tokens. Security headers act as an extra defense layer against common web threats. For example:

• Strict-Transport-Security – Forces browsers and clients to always use HTTPS, protecting against downgrade attacks.
• X-Content-Type-Options: nosniff – Prevents browsers from misinterpreting files, which stops attackers from tricking clients into executing malicious scripts.
• X-Frame-Options: DENY – Blocks the API from being loaded inside an iframe, which prevents clickjacking attacks.

These headers are important because they reduce the attack surface without requiring changes to the API’s logic. Testers validate them to ensure compliance with security standards like OWASP and to confirm that endpoints are not left exposed.

4. Cross-Origin Resource Sharing (CORS)

Modern applications often have a frontend running on one domain and an API hosted on another. By default, browsers block such cross-domain requests. CORS headers control when and how clients from different origins can interact with the API. For example,

Access-Control-Allow-Origin: https://example.com

Access-Control-Allow-Methods: GET, POST

Access-Control-Allow-Headers: Content-Type, Authorization

These headers are important because they balance usability and security. Without CORS, legitimate apps cannot access the API from the browser. If configured too loosely, any website could read sensitive responses. Testers check CORS headers to make sure they enable the right clients while keeping out unauthorized domains.

How Requestly Helps Test REST API Headers

Requestly is a browser-based tool that lets testers and developers intercept, modify, and monitor network requests in real time. Instead of repeatedly changing code or configurations, Requestly allows direct manipulation of request and response headers through simple rules. This makes it easier to validate APIs under different conditions with minimal setup.

With Requestly, you can:

• Add missing headers: Insert required headers like Authorization or Content-Type into every outgoing request automatically.
• Override existing headers: Modify values such as Accept-Language to test localization or switch API versions using Accept.
• Remove headers: Exclude headers like If-None-Match to confirm how the API responds without cache validation.
• Simulate authentication flows: Replace or rotate tokens in the Authorization header to verify handling of expired or invalid credentials.
• Debug CORS issues: Adjust Access-Control-Allow-Origin or similar headers to replicate client-side restrictions and troubleshoot failures.

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Conclusion

REST API headers define how requests and responses are shaped, interpreted, and secured. They control everything from authentication and caching to localization and content negotiation. For testers, headers help validate whether an API correctly enforces security, adheres to caching rules, or serves the right content under varying conditions.

Requestly simplifies this process by giving testers the ability to intercept, add, remove, or modify headers in real time. Instead of repeatedly changing code or working through complex environments, testers can replicate different scenarios instantly within their browser.

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