A Beginner’s Guide to Understanding HTTP Message Body

HTTP messages are the foundation of communication between clients and servers. The message body carries the actual data exchanged in these communications.

This article covers the definition, importance, structure, and types of HTTP message bodies.

What is an HTTP Message Body?

The HTTP message body is the part of an HTTP request or response that carries the actual data being transmitted. While headers provide metadata such as content type, length, or authentication information, the message body holds the core content. This content can vary from simple text or JSON objects to complex file uploads.

In requests, the message body often contains data sent from the client to the server, such as form inputs, JSON payloads, or files. In responses, it carries the data returned by the server, such as HTML pages, JSON responses from APIs, or binary files.

Why is HTTP Message Body Important?

The HTTP message body is critical because it directly impacts how data is transmitted, received, and processed by web servers and clients. Handling it correctly ensures that APIs and applications function as expected and that testing, debugging, and security processes are accurate.

Below are the main reasons it matters:

• Data Transmission Accuracy: Ensures that the data sent in requests or received in responses reaches its destination without alteration or corruption.
• API Testing Effectiveness: Enables testers to simulate realistic scenarios, validate responses, and confirm that APIs handle data correctly.
• Content Formatting Control: Knowing how to structure the body and set the correct Content-Type ensures that servers and clients interpret the data properly.

• Security and Compliance: Correct handling prevents vulnerabilities such as injection attacks, data leaks, or non-compliance with standards like PCI DSS or GDPR.
• Debugging and Troubleshooting: Access to the message body allows developers and testers to identify errors in requests or responses faster and more accurately.
• Support for Complex Data Types: Many applications send structured data like JSON arrays or files. Understanding the body ensures that such data is processed correctly.

Structure of HTTP Message Body

The HTTP message body does not have a fixed format by itself, but its structure depends on the type of data being sent and the HTTP method used. While headers describe the data, the body carries it in a raw or structured form.

Below are the key aspects of HTTP message body structure:

• Raw Data: The simplest form, where the body contains plain text, JSON, XML, or other serialized formats. For example, an API request sending user details might include a JSON object with fields like name, email, and id.
• Empty Body: Some HTTP methods, like GET or HEAD, do not send a message body. However, headers still accompany the request or response.

• Binary Data: This includes files, images, or other non-text data. Binary bodies require proper encoding and handling to ensure the data is received correctly.
• Multipart Data: Used when a request or response carries multiple pieces of data together, such as files with form fields. Each part has its own headers and content, separated by boundary markers.
• Encoding and Transfer Details: Depending on the size or type of data, bodies may use chunked transfer encoding, gzip compression, or other mechanisms to optimize transmission.

Types of HTTP Message Body

HTTP message bodies vary depending on the HTTP method, purpose of the request or response, and the type of data being exchanged. Understanding these types helps testers and developers handle data correctly, avoid errors, and design more effective API tests.

Below are the main types of HTTP message bodies:

• No Body (GET, HEAD, etc.): Certain HTTP methods, such as GET and HEAD, typically do not include a message body. GET requests retrieve data, and HEAD requests only retrieve headers. Even though the body is absent, headers still provide metadata, such as content type and length.
• Raw Data (Text, JSON, XML, etc.): This type of body carries structured or unstructured data in formats like plain text, JSON objects, or XML. For example, an API POST request sending user registration details often uses a JSON body with fields like username, email, and password.
• File/Data Uploads: The body can carry binary data, such as images, PDFs, or other files. Proper content-type headers and encoding are required to ensure the server interprets the files correctly. This is common in APIs that support file upload or media sharing.
• Multipart Data: Multipart bodies combine multiple pieces of data in a single request or response. Each part can have its own headers and content. This is typically used in forms where text fields and file uploads are sent together. Boundaries separate each part to ensure correct parsing.

HTTP Message Body in Requests vs. Responses

The HTTP message body carries data in both requests and responses, but its purpose and handling differ. Requests send data to the server for processing, while responses return data or results to the client.

Here is a table highlighting the key differences between the HTTP message body in requests vs. responses.

Formatting and Content-Type of HTTP Message Body

The HTTP message body can carry different types of data, and the Content-Type header tells the server or client how to interpret it. Correct formatting ensures the data is processed accurately, prevents errors, and supports proper validation.

Below is a detailed overview of common content types and their typical use cases:

Common Examples of HTTP Message Body

HTTP message bodies vary depending on request type, data format, and use case. Understanding these examples helps testers and developers anticipate how servers parse data, identify validation issues, and simulate realistic API interactions effectively.

Below are detailed examples:

1. JSON Payloads in API Requests

JSON is the most widely used format for API requests. It allows sending structured data with nested objects and arrays. Testers should ensure that the server correctly validates types, handles optional or missing fields, and processes arrays or null values without errors.

The code below shows a JSON payload for creating a user, sending structured data, including roles and metadata, to the server:

{

  "username": "jdoe",

  "email": "jdoe@example.com",

  "roles": ["admin", "editor"],

  "metadata": {

    "signupDate": "2025-08-25",

    "referrer": null

  }

}

2. Form Data Submission (application/x-www-form-urlencoded)

Form data is commonly sent from HTML forms as URL-encoded key-value pairs. Testers should validate proper encoding, boundary values, maximum lengths, and how the server parses missing or extra fields.

The code below shows URL-encoded form data submitted to the server:

username=jdoe&email=jdoe%40example.com&age=30

3. File Uploads with Multipart Data (multipart/form-data)

Multipart data combines files and text fields in a single request. Each part has its own headers and content, separated by boundaries. Testing should include file size limits, multiple uploads, boundary handling, and server processing of both files and associated metadata.

The code below shows a simplified multipart file upload combining a profile picture and a username field:

--boundary

Content-Disposition: form-data; name="profilePic"; filename="photo.jpg"

Content-Type: image/jpeg

[binary file data]

--boundary

Content-Disposition: form-data; name="username"

jdoe

--boundary--

4. XML Payloads (application/xml)

XML is still used in some APIs or legacy systems for structured data. Testers must check schema validation, correct nesting, required versus optional fields, and parsing behavior on the server.

The code below shows an XML payload for submitting user information:

<user>

  <username>jdoe</username>

  <email>jdoe@example.com</email>

</user>

5. Binary Data (application/octet-stream)

Some APIs transfer raw binary files like images, PDFs, or media. Testers need to validate file integrity, encoding, size, and server handling.

The following example represents sending a PDF invoice via API:

[binary PDF data]

6. Empty Body

Certain requests, such as GET or HEAD, typically do not include a body. Testers must ensure that all required information is passed via headers or URL parameters and that the server handles empty bodies gracefully.

The code below shows a GET request with no body:

GET /api/users/123 HTTP/1.1

Host: example.com

How Requestly Helps Modify HTTP Message Bodies

Requestly by BrowserStack is a tool that allows testers and developers to intercept, inspect, and modify HTTP requests and responses without changing backend code. It is particularly useful for modifying HTTP message bodies during testing, debugging, or frontend development when APIs are not fully ready.

Below are the key ways Requestly helps:

• Modify Request Body: Capture outgoing requests and modify the body in real-time to test different input scenarios.
• Modify Responses: Alter API responses before they reach the client to simulate backend behavior or errors.
• Support Multiple Formats: Works with JSON, XML, form data, and binary files.
• Simulate Errors: Test how the application handles invalid, missing, or malformed data.
• No Backend Changes Needed: All modifications are done client-side, keeping production APIs safe.
• Reusable Rules: Save modifications as rules to run repeatable tests across sessions.

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Conclusion

Effective testing of HTTP message bodies requires verifying payload formats, validating required and optional fields, checking nested structures, and testing edge cases such as large or malformed inputs. This ensures servers handle requests correctly, maintain data integrity, and respond predictably under different scenarios.

Requestly enhances this process by allowing testers to intercept, inspect, and modify request and response bodies without changing backend code. It allows you to simulate different payloads, test error scenarios, and validate application behavior under varying conditions.

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