The User-Agent header is a text label sent with every HTTP request that names the client, for example a browser and its version or a scraper and its version. Sites use it for analytics, bot detection and rate limiting, and many ask automated clients to include a bot name and an operator URL so the owner can be reached. It is self-declared and easy to forge, and putting a personal email in it exposes that address to every server and log that sees the request. In October 2026, public Claude Code bug reports described agents copying the signed-in user's email into this header.