User-Agent

Identifies the client software and usually its operating system.

Use it for diagnostics and narrowly scoped compatibility work. Prefer feature detection over browser sniffing, and expect reduced or customized strings.

httpheaderprivacy

Reference (http)

User-Agent: ExampleClient/1.0

Use it for diagnostics and narrowly scoped compatibility work. Prefer feature detection over browser sniffing, and expect reduced or customized strings.

Common mistakes

Using it as an authentication factor or blocking browsers based on fragile version parsing.

IANA registry: http-fields/field-names

Registry reference: RFC 9110, Section 10.1.5: HTTP Semantics

Sources

MDN HTTP Reference
IANA HTTP Field Name Registry