Full definition
The User-Agent header is a string your HTTP client sends with every request, like `Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 ... Chrome/121.0.0.0 Safari/537.36`. It's historically meant to identify the browser so a server can serve compatible content — but in practice it's mostly a fingerprint axis.
Default User-Agents from common scraping libraries are dead giveaways: `python-requests/2.31.0`, `Go-http-client/1.1`, `node-fetch/1.0`. Anti-bot systems block these instantly. The first move when scraping is to set a realistic browser User-Agent.
Don't stop there. Modern anti-bot also checks JA3 (TLS fingerprint), header order, header values consistency (does your `Accept-Language` match your claimed locale?), and behavioral signals. Spoofing User-Agent alone is necessary but very far from sufficient.