Client Hints are a set of HTTP Header fields and a JavaScript web application programming interface (API) for proactive content negotiation in the Hypertext Transfer Protocol (HTTP). The client can advertise information about itself through these fields so the server can determine which resources should be included in its response. Initially proposed in 2013 by engineers at Google, Client Hints were presented as a privacy-preserving alternative to user-agent header strings as part of Google's Privacy Sandbox initiative. The initial design of Client Hints faced pushback from browser vendors due to various privacy concerns. As of May 2024, over 75% of all internet traffic supports Client Hints. Despite this widespread adoption, privacy researchers have raised concerns that Client Hints are primarily being used by tracking scripts.