Markdown negotiation caching (Vary)

Without Vary: Accept, a cache may return the wrong response, sending browsers Markdown instead of HTML.

Category: Content accessibility
Standard: Recommended

2%Top 100 adoption2 of 99 sites implement it 4/9OverviewSpeculativeSpecified, but little evidence anything uses it yet.

Who supports it

No agent or browser has announced support yet.

27 others: no evidence yet. See every agent and browser

What it checks

When the page serves Markdown through content negotiation, the same URL returns two different bodies depending on the Accept header. The Vary response header tells caches and CDNs to store them separately.

This check reuses the request made by Markdown content negotiation. If Markdown was negotiated, it looks for Accept in the response’s Vary header. If Markdown was not negotiated, there is nothing to cache separately and the check does not apply.

Results

Status	When
Pass	Markdown is negotiated and the response includes `Vary: Accept`
Fail	Markdown is negotiated but `Vary` does not include `Accept`
N/A	The page does not negotiate Markdown

How to fix

Add Accept to the Vary header on every response from a URL that negotiates, both the HTML and the Markdown:

Vary: Accept, Accept-Encoding

Without it, a CDN can serve its cached HTML to an agent that asked for Markdown, or its cached Markdown to a browser.

Support by agent and browser

Announced means the vendor’s own docs or announcement say so. Published is the date on the vendor’s own page, where it has one. Checked is the last time our weekly research found that page still saying so.

No agent or browser has announced anything about this yet.

No evidence yet27

ChatGPT
Claude
Gemini
Perplexity
Copilot
Meta AI
ChatGPT Atlas
Comet
Dia
Gemini in Chrome
Edge Copilot Mode
Claude Code
Cursor
Codex
GitHub Copilot
Windsurf
CCBot
Applebot
Amazonbot
Bytespider
Cohere
Mistral
DuckAssist
Chrome
Edge
Firefox
Safari

Names and marks belong to their owners and are used only to identify products. None of these vendors is affiliated with Sitebulb. Spotted something out of date? Send a correction.

Adoption on the top 100 sites

2 of 99 measured sites implement it

Scanned 23 Oct 2026 with the Sitebulb Labs extension, against the Tranco list (tranco-56Z8N). 1 sites couldn’t be measured (bot challenge or timeout) and are left out of the share.

9 Oct 2026 2% → 23 Oct 2026 2%

Sites implementing it2

#2 cloudflare.com
#55 wordpress.org

Overview

4/9

Speculative

Specified, but little evidence anything uses it yet.

How this rating is worked out4 of 9 points

Factor	Points	Why
Vendor support	0 / 3	0 vendors supporting, 0 partly or planned
Top-100 adoption	1 / 3	2% of measured sites
Spec maturity	3 / 3	Published standard

8 or more points is Established, 5 to 7 is Gaining traction, 2 to 4 is Speculative and under 2 is Stalled. A feature is also Stalled when more vendors have said no than yes. Read the spec.

Community insights

No community insights for this feature yet.

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