RFC 9110 — HTTP Semantics (content negotiation)
Discovery · IETF · Updated 2026-07-19
Status: Internet Standard (IETF, 2022) — the HTTP core specification · Authoritative source
The oldest agent-readiness mechanism on this list: proactive content negotiation, where the same URL serves HTML to browsers and markdown or JSON to agents that ask — no new protocol required.
RFC 9110 defines proactive negotiation: the client states preferences in the Accept header with quality weights, and the server picks the representation. For agents this is the highest-leverage low-tech signal on the web — an agent that sends Accept: text/markdown and receives clean markdown ingests your content in one pass, no scraping, no rendering.
The subtleties are where sites fail: honoring the leading preference rather than serving any non-HTML type, labeling the response’s Content-Type from what was negotiated, and varying caches correctly. Fewer than 1% of top sites pass basic negotiation checks — which makes it the cheapest differentiation available. The content-negotiation guide covers the mechanics and the common bugs.
One URL, two audiences
GET /pricing HTTP/1.1
Accept: text/markdown, text/html;q=0.8
HTTP/1.1 200 OK
Content-Type: text/markdown; charset=utf-8
# Pricing
| Plan | Price | …How AgentGrade checks it
The scanner probes the same URLs with different leading Accept preferences (markdown, plain text, JSON) and verifies both the body and the declared Content-Type match what was asked for — the strict version of the check, not just “anything non-HTML.” Scan your site to see where you stand.
Related
All standards
RFC 9421 · Web Bot Auth · RFC 8615 · MCP · A2A · x402 · WebMCP · llms.txt · RFC 9309 · RFC 9110 · Index