Content Negotiation

An HTTP mechanism where the same URL returns different representations based on request headers such as Accept.

term: Content Negotiation
category: protocols
short_def: An HTTP mechanism where the same URL returns different representations based on request headers such as Accept.
long_def: Standardized in HTTP/1.1 (RFC 9110, HTTP Semantics), it is the clean way to serve HTML to browsers and markdown to agents from one URL, advertised with a Vary: Accept response header so caches behave.
see_also: markdown-twin
etymology_origin: Specified by the IETF as part of HTTP semantics; current normative reference is RFC 9110 'HTTP Semantics' (June 2022), which obsoletes the earlier RFC 7231 / RFC 2616 definitions.
related_to: markdown-twin robots-txt
contrast_with: Unlike URL-based routing, which serves different content from different paths, content negotiation serves different representations from ONE URL based on request headers (Accept, Accept-Language).
example: A server sets Vary: Accept and returns text/markdown to an agent and text/html to a browser from the same URL, as defined in RFC 9110 (June 2022).
source: https://www.rfc-editor.org/rfc/rfc9110.html#name-content-negotiation
status: active
why_it_matters: Content negotiation is the standards-compliant mechanism that makes markdown twins possible without breaking caches, SEO or canonical URLs.
sameAs: https://en.wikipedia.org/wiki/Content_negotiation
bridge_entity: agent-readiness
last_verified: 2026-06-15
md_twin: /glossary/content-negotiation.md