PPR causes unwanted ISR behavior on Vercel

[chungweileong94]

Summary

So recently I enabled PPR on one of my small CRUD dashboard app, where it contains the following routes:

• /~/items - Item Listing
• /~/items/add - Add Item
• /~/items/[id] - Item Details
• /~/items/[id]/edit - Edit Item

Pretty much all the routes are marked as partial prerendering. However, I noticed that the app is causing ISR Writes on Vercel, which is unexpected, because I never opt into any ISR feature, nor calling revalidatePath & revalidateTag. Most of the ISR Writes (if not all writes) are coming from /~/items/[id]. After checking the related logs, they are all coming from a "Background Revalidation", which got me wondering that does the static shell in PPR come with a default time-based revalidation or something?

Additional information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:51 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T8112
  Available memory (MB): 16384
  Available CPU cores: 8
Binaries:
  Node: 22.18.0
  npm: 10.9.3
  Yarn: N/A
  pnpm: 10.14.0
Relevant Packages:
  next: 15.5.1-canary.14 // There is a newer canary version (15.5.1-canary.28) available, please upgrade!
  eslint-config-next: N/A
  react: 19.1.1
  react-dom: 19.1.1
  typescript: 5.9.2
Next.js Config:
  output: standalone

Example

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