What is an User-Agent header?
Identifies the client application, operating system, and version making the request.
User-Agent is how a website tells a real visitor's browser apart from a bot, a crawler, or an uptime monitor's own health-check requests, and it's commonly used to serve different content, block abusive scraping, or simply understand traffic composition in analytics. A well-behaved uptime monitor should identify itself clearly in its User-Agent string precisely so a website operator can distinguish routine health checks from actual visitor traffic in their logs.
Common use case
A website serves a simplified mobile layout, or blocks a known scraper, by matching patterns in the incoming User-Agent string.
Example
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0 Safari/537.36History
Part of the original HTTP/1.0 specification, RFC 1945 (1996).
Did you know?
Nearly every modern browser's User-Agent still contains the word "Mozilla" for historical compatibility, a holdover from the 1990s browser wars when sites checked for that exact string to decide whether to serve advanced features.