Hello Vercel team,
I'm experiencing repeated Preview deployment failures with my Vite application.
The project builds successfully locally, but both Vercel Preview deployments fail during npm run build.
What happens
Vite successfully compiles all 1,617 modules.
The failure occurs afterward in our custom prerender routing-consistency validation.
Our script compares vercel.json against a generated routing configuration.
All four expected fields (cleanUrls, trailingSlash, redirects, headers) match correctly.
However, during the cloud build, the script detects an additional top-level property in vercel.json that is absent from our uploaded source configuration.
Build error
Error: vercel.json is out of sync with the authoritative route manifest.
First difference at $[redacted-property]: unexpected property.
[BUILD_UTILS_SPAWN_1] Command "npm run build" exited with 1
Environment
Vite 6.4.3
Node.js 24.21.0
Vercel CLI 64.0.2
Vercel Hobby plan
Preview deployment with Standard Protection
We verified that the uploaded vercel.json matches our local source. The project also passes local builds and routing regression tests.
The additional property's identity and origin remain unknown. Our diagnostics intentionally redact unknown property names.
Questions
Can Vercel's build process, configuration normalization or restored build cache introduce additional properties into vercel.json?
How can we safely identify the unexpected build-time property?
What is the recommended way to validate routing configuration without rejecting legitimate platform-generated properties or weakening checks for redirects, headers and security rules?
I've attached a screenshot of the relevant build error.
I can provide the deployment IDs privately if needed.
Any guidance would be greatly appreciated.
Thank you!