In what scenarios are cache headers forced to "public, max-age=0, must-revalidate" by Vercel?

[good-idea]

Our project is using Next.js 14.2.5 + app router, deployed to Vercel.

We are trying to debug why pages on our site - the initial GET request to /page - are all being returned with a cache-control: public, max-age=0, must-revalidate response header. We would expect and are hoping for it to be max-age=600 stale-while-revalidate.

Running a production build locally (not using vercel) gives us the results we are expecting.

The pages all have:

export const dynamic = 'force-static';
export const revalidate = 600; 

And use async React components to load the data. (Some of them go on to render other components which may contain their own async React components)

We have explicitly set cache: 'force-cache' on all requests made to generate the page. Some of these are GET, some are POST. These requests are given tags.

The response includes a x-vercel-cache: HIT heading - does this refer to the CDN cache or the Next.js data cache?

We are using middleware for localized routing (i.e. /some-page, /en-US/some-page)

We have a check for draftMode().isEnabled on every request - but we tried disabling this and doing so had no effect.

The documentation is not clear on when the dynamic = 'force-static' setting will be overridden. I understand that making a fetch request with cache: 'no-store', reading cookies, or reading headers will force a page to be dynamic - but we are not doing any of this.

We see that a fresh project created by create-next-app and deployed to vercel is showing the same behavior.

What else could be happening that would be overriding force-static?

[leerob]

You're seeing SSR-like caching headers because you're turning on draft mode for every request. I was not able to reproduce with a fresh project (seeing x-vercel-cache: HIT, meaning it is a fully static page).

Are there any minimal reproductions?

[x0good]

We don't use draft mode but also in Vercel all nextjs static pages return public, max-age=0, must-revalidate

access-control-allow-origin:
*
age:
384962
cache-control:
public, max-age=0, must-revalidate
content-disposition:
inline
content-encoding:
br
....
content-type:
text/html; charset=utf-8
date:
Mon, 17 Feb 2025 16:22:26 GMT
server:
Vercel
strict-transport-security:
max-age=63072000
vary:
RSC, Next-Router-State-Tree, Next-Router-Prefetch, Next-Router-Segment-Prefetch
x-matched-path:
/mission

x-nextjs-prerender:
1
x-nextjs-stale-time:
4294967294
x-vercel-cache:
HIT
x-vercel-id:
arn1::48khl-1739809346046-0719a57e9b6b

locally with npm run build && npm run start static page cache response Headers >

cache-control:
s-maxage=43200, stale-while-revalidate=31492800

and second question >
even locally when I open nextjs static page in browser: it's still waiting for server and never show in dev tools that page downloaded from browser cache. Is it expected behaviour of nextjs ?

for me it's confusing, I expect if "next build" show that page url is static -> it should be static and cached by browser automatically or at least not trigger any vercel functions during refresh page.
Or when we see public, max-age=0, must-revalidate + HIT in response header it means that NO Vercel Functions triggered and Vercel cache still works properly ?

@leerob could you explain please, I read official doc and see your videos in youtube but still not understand how test caching of static generated pages in vercel