What this error means

Vercel deploy fails with 'An unexpected error occurred when running this build' with middleware + experimentalServices is a Vercel / Next.js failure pattern reported for developers trying to fix vercel deploy error 'an unexpected error occurred when running this build' with middleware. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

Isolation matrix confirms: middleware.ts is the trigger regardless of content. 5-file minimal repro provided. Suspected root cause in scopeRouteSourceToOwnership() prepending ownership guard to middleware routes.

Common causes

  • Deterministic deploy failure when combining Next.js middleware with Vercel services-mode. Removing middleware.ts is the only workaround. Breaks multi-service deployments.
  • Isolation matrix confirms: middleware.ts is the trigger regardless of content. 5-file minimal repro provided. Suspected root cause in scopeRouteSourceToOwnership() prepending ownership guard to middleware routes.

Quick fixes

  1. Confirm the exact error signature matches Vercel deploy fails with 'An unexpected error occurred when running this build' with middleware + experimentalServices.
  2. Check the Vercel / Next.js account, local tool state, and provider configuration involved in the failing workflow.
  3. Check the build output, project root, and deployment platform configuration before redeploying.

Platform/tool-specific checks

  • Verify the command, editor, extension, or API client that produced the error.
  • Compare local settings with CI, deployment, or editor-level settings when the error appears in only one environment.
  • Avoid deleting credentials, local model data, or project settings until the failing scope is clear.

Step-by-step troubleshooting

  1. Capture the exact error message and the command, editor action, or request that triggered it.
  2. Check whether the failure is account/auth, quota/rate, model/provider, local runtime, or deployment configuration.
  3. Review the source evidence below and compare it with your environment.
  4. Apply one change at a time and rerun the smallest failing action.
  5. Keep the working fix documented for the team or deployment environment.

How to prevent it

  • Keep provider/tool configuration documented.
  • Record non-secret diagnostics such as tool version, provider name, model name, and command path.
  • Add a lightweight check before CI or production workflows depend on the tool.