Request negotiation · Request header

User-Agent

The client software identifying itself.

What is User-Agent?

The User-Agent string names the browser, engine, OS, and often the device. It is famously convoluted for historical reasons.

Typical usage

Set automatically by browsers and HTTP libraries. Read for analytics and rare compatibility branching.

Examples

User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36

Common gotchas

User-Agent can be trivially spoofed and is being reduced by modern browsers. Do not use it as a security control.

Frequently asked questions

What is the User-Agent HTTP header?

The User-Agent string names the browser, engine, OS, and often the device. It is famously convoluted for historical reasons.

Is User-Agent a request or response header?

User-Agent is a request header — the client sends it to the server. Set automatically by browsers and HTTP libraries. Read for analytics and rare compatibility branching.

What does a User-Agent header look like?

A typical User-Agent header looks like: User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36.

What are common mistakes with User-Agent?

User-Agent can be trivially spoofed and is being reduced by modern browsers. Do not use it as a security control.

Related headers

Which media types the client can process.

Accept-Encoding

Which content encodings (compressions) the client accepts.

Accept-Language

Preferred human languages for the response.

Host

The requested host name and port.

Referer

The URL of the previous page that led to this request.

Work with this header

User-Agent Parser →Parse the User-Agent request header.cURL Converter →Turn a curl command into fetch / code and back.URL Parser →Break a URL into scheme, host, path and query.