HTTP: What is a user agent?

In a nutshell

An HTTP user agent is any client program that initiates a request. Examples include:

• Web browsers (e.g., Chrome or Firefox)
• Crawlers/spiders that traverse the web to build things like search indexes.
• Command-line tools like curl or wget
• Mobile apps
• IoT devices (e.g., a fridge or a light bulb).

Example

Run curl example.com -v from your terminal and you should see the a User-Agent request header:

> User-Agent: curl/7.79.1

If you visit example.com from Chrome, you can also see the User-Agent header set: