Vercel Services-Mode Deploy Fails When Next.js Middleware Is Present

What this error means

An unexpected error occurred when running this build. This may be a transient issue, please try rebuilding your project. is a Vercel failure pattern reported for developers trying to fix vercel deployment failure in services-mode (experimentalservices) when next.js middleware.ts is present. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

GitHub issue vercel/vercel#16296 (May 11, 2026): With experimentalServices enabled, any middleware.ts triggers deterministic deploy failure at ‘Deploying outputs…’ stage. Removing middleware.ts fixes deploy. Root cause: service-route-ownership.ts rewrites middleware route src with negative lookaheads but doesn’t exempt middleware routes. Category: Deployment (Vercel deployment pipeline).

Common causes

• GitHub issue vercel/vercel#16296 (May 11, 2026): With experimentalServices enabled, any middleware.ts triggers deterministic deploy failure at ‘Deploying outputs…’ stage. Removing middleware.ts fixes deploy. Root cause: service-route-ownership.ts rewrites middleware route src with negative lookaheads but doesn’t exempt middleware routes. Category: Deployment (Vercel deployment pipeline).

Quick fixes

1. Confirm the exact error signature matches An unexpected error occurred when running this build. This may be a transient issue, please try rebuilding your project..
2. Check the Vercel 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.

Sources checked

• https://github.com/vercel/vercel/issues/16296

Evidence note: GitHub issue vercel/vercel#16296 (May 11, 2026): With experimentalServices enabled, any middleware.ts triggers deterministic deploy failure at ‘Deploying outputs…’ stage. Removing middleware.ts fixes deploy. Root cause: service-route-ownership.ts rewrites middleware route src with negative lookaheads but doesn’t exempt middleware routes. Category: Deployment (Vercel deployment pipeline).

Related errors

• Deployment

FAQ

What should I check first?

Start with the exact An unexpected error occurred when running this build. This may be a transient issue, please try rebuilding your project. text and the smallest action that reproduces it.

Can I ignore this error?

No. Treat it as a failed Vercel workflow until the root cause is understood.

Is this guaranteed to have one fix?

No. The imported evidence supports the troubleshooting path above, but tool behavior can vary by account, plan, version, provider, and local configuration.

How do I know the fix worked?

Rerun the same command, editor action, or request. The fix is working when that action completes without An unexpected error occurred when running this build. This may be a transient issue, please try rebuilding your project..