Vercel CDN stops caching responses with Vary: Cookie, and Speed Insights stops recording First Input Delay on 1 November 2026

Vercel CDN stops caching responses with Vary: Cookie, and Speed Insights stops recording First Input Delay on 1 November 2026

30 September 2026 — Vercel published two changelog entries on 30 September and 1 October 2026 that affect sites hosted on its platform. One changes caching behaviour for responses that vary by cookie. The other retires a performance metric from Speed Insights.

Not yet independently verified. Both sources are Vercel's own changelog. The Vary: Cookie entry gives no effective date beyond its 30 September publication and does not say whether the change is already live for all projects. Cost or latency effects on any particular site were not measured. We will update this when it can be confirmed, and remove this note.

What changed

Vercel's CDN no longer caches origin responses when the Vary header includes Cookie. The response is still served normally but is not stored for later requests. Vercel explains that cookies often hold visitor-specific values, so varying by Cookie can create many cache entries that are rarely reused. Caching behaviour for other supported Vary headers is unchanged.

Affected responses can be found by checking the x-vercel-cache header, which is set to MISS, and in Runtime Logs, which show vary_key_denied:cookie as the cache reason.

Separately, Speed Insights is deprecating First Input Delay (FID) and will stop recording new measurements on 1 November 2026. Vercel states that Speed Insights already uses Interaction to Next Paint (INP), the Core Web Vital that replaced FID, in its Real Experience Score. The deprecation does not require changes, does not affect the score, and does not remove existing FID data from view.

Who is affected

Teams running sites or APIs on Vercel whose origin sets Vary: Cookie are affected by the caching change. This can include applications that return the same content to every visitor but send a blanket Vary header from a framework, a proxy or a middleware layer. Such routes will now show cache misses, which may raise origin load, function invocations and response times.

The Speed Insights change affects only teams that export or report on FID specifically. Dashboards or contracts that cite FID as a measure will stop receiving new data from 1 November.

What to do

Vercel gives two courses of action for the caching change:

• If the response is the same regardless of cookies, remove Cookie from Vary so the CDN can cache it, provided it meets the other caching requirements.
• If the response depends on cookies, keep Cookie in Vary and use Cache-Control: private so personalised responses are not stored in the shared CDN cache.

Teams should review Runtime Logs for vary_key_denied:cookie on routes they expect to be cached, and compare cache hit rates and usage charges before and after. For Speed Insights, replace any FID targets in internal reports or service-level documents with INP before 1 November.

Why it matters

Caching defaults can change the cost and speed of a production site without a code change on the customer side. A route that was cached before may now call the origin on each request, which shows up in compute usage and in page speed. Buyers comparing hosting platforms may want to note that cache keys and metric definitions differ between providers and can change with short notice. The fixes Vercel describes are small, but they depend on teams noticing the change.

This summary rests on Vercel's own changelog entries. TrustList has not measured effects on any particular deployment.

Sources

• Vercel changelog: CDN no longer caches responses with Vary: Cookie — 30 September 2026
• Vercel changelog: Speed Insights deprecates First Input Delay on November 1st — 1 October 2026