Custom "Cache-Control" header seems to have stopped being set since May

In my web app, I aggressively cache some static assets in user’s browsers by setting headers like cache-control: public, max-age=2592000, stale-while-revalidate=31536000. However, I noticed that in my recent deployments, this header was lost. It fell back to the default cache-control: public, max-age=0, must-revalidate.

I checked my git history and made sure I never changed my deploy configs, so I suspect something changed on vercel’s side.

I always deploy from cli: vercel --prod --local-config .vercel/vercel.json. And my vercel.json is:

{
  "headers": [
    {
      "source": "/assets/:any",
      "headers": [
        {
          "key": "Cache-Control",
          "value": "public, max-age=2592000, stale-while-revalidate=31536000"
        }
      ]
    }
  ]
}

I looked though all my deployments over time, and found the last deployment that had the correct header was on 4/24, all deployments before that were good. The first deployment that lost the header was on 5/16, all deployments after that didn’t have the header. So, something changed between 4/24 and 5/16, maybe the vercel cli, or maybe the hosting server.

I may have upgraded my vercel cli during that time, but I can’t pinpoint the date.

[ZYinMD]

I noticed that this issues seems to have been fixed, the fix happened somewhere between 2022-07-22 and 2022-07-17. It seems to have happened on the hosting server and no upgrade to vercel CLI was needed.

[ZYinMD]

I found the answer - it has to do with the setting of dist folder.

Because I host a simple static website and I build locally, I don’t need vercel to build for me in the cloud, so I set build command to "", then set both Output Directory and Root Directory to "dist". It may look weird, but I think it makes sense, because vercel doesn’t need to worry about anything outside of my locally bundled dist folder.

project-root
  |---.vercel/
  |---src/
  |---dist/
  |     |---index.html
  |     |---assets/

This has been working fine until May, then I believe there had been a regression - it still deploys ok, but my headers config doesn’t work anymore.

Anyway, I found that if I change Root Directory to "./", this issue is fixed.