Content negotiation
HTTP mechanism by which the client indicates preferred response format (e.g. Accept: text/markdown) and the server returns the matching format.
Definition
Content negotiation is the process where the client sends preferences (e.g. Accept: text/markdown or Accept: text/html) and the server responds with the appropriate format. For GEO, agents can request Accept: text/markdown to receive Markdown instead of HTML, reducing tokens and improving parsing. If the server returns different content based on Accept, it must send Vary: Accept so caches (CDNs, proxies) do not serve the wrong version to another client.
Relevance to GEO
Content negotiation allows a single URL to serve HTML to browsers and Markdown to agents. Implementing it (or using CDN-level Markdown for Agents) is an optional but high-impact GEO improvement.