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Comment: We just shipped support for the ugliest part of HTTP: Vary

Simon Willison's Weblog ·

A commentary by Simon Willison on Cloudflare's newly added support for the HTTP Vary header, noting his longstanding desire for it, describing prior caching limitations where Cloudflare ignored Vary for non-image responses, and stating his personal preference for URL-based content negotiation over Accept-header–based approaches. Read 3 viewpoints with supporting evidence and source links.

Simon Willison
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  1. Longstanding desire for Vary header support

    Simon Willison has wanted Cloudflare to support the HTTP Vary header for years.

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    Original excerpt

    I've been wanting this from Cloudflare for years .

    Simon Willison · Paragraph 2

    Comment: We just shipped support for the ugliest part of HTTP: Vary · Sep 23, 2026

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    HTTP caching infrastructure →

  2. Cloudflare previously ignored Vary for non-image responses

    Before the new feature, Cloudflare ignored the Vary header on responses other than images, risking incorrect cache sharing—e.g., caching a JSON response and serving it to a client expecting HTML.

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    Original excerpt

    This used to be impossible to deploy behind Cloudflare caching, because they ignored the Vary header on anything other than images - so you risked caching the JSON version and then serving it up to someone who was expecting HTML.

    Simon Willison · Paragraph 4

    Comment: We just shipped support for the ugliest part of HTTP: Vary · Sep 23, 2026

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    CDN caching limitations →

  3. Preference for URL-based content negotiation over Accept headers

    Simon Willison independently chose not to use Accept-header–based content negotiation, preferring predictable URLs—for example, using a '.json' suffix to serve JSON.

    Supporting evidence 1

    Original excerpt

    (Independent of the Cloudflare feature I ended up deciding never to use that pattern, because I prefer having URL that predictably returns HTML or JSON - I add a .json suffix to my apps to serve JSON instead.)

    Simon Willison · Paragraph 5

    Comment: We just shipped support for the ugliest part of HTTP: Vary · Sep 23, 2026

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    API design preference →

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Paragraph 5 (opens in a new tab)
API design preference

Preference for URL-based content negotiation over Accept headers

Original excerpt

(Independent of the Cloudflare feature I ended up deciding never to use that pattern, because I prefer having URL that predictably returns HTML or JSON - I add a .json suffix to my apps to serve JSON instead.)
Simon Willison·
Paragraph 4 (opens in a new tab)
CDN caching limitations

Cloudflare previously ignored Vary for non-image responses

Original excerpt

This used to be impossible to deploy behind Cloudflare caching, because they ignored the Vary header on anything other than images - so you risked caching the JSON version and then serving it up to someone who was expecting HTML.
Simon Willison·

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Simon Willison states he has wanted Cloudflare to support the HTTP Vary header for years.

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