Vercel Services-Mode Deploy Fails with Middleware Routes in Deploying Outputs Stage

What this error means

An unexpected error occurred when running this build — Deploying outputs stage failure with middleware.ts in services-mode is a Vercel failure pattern reported for developers trying to fix vercel deploy failing at ‘deploying outputs’ with middleware and services-mode. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

GitHub issue reported 2026-05-11. When experimentalServices is used with middleware.ts in the web service, deploys fail at ‘Deploying outputs’ stage with generic error. Affects Next.js monorepo deployments on Vercel.

Common causes

• Vercel users with experimentalServices (services-mode) experience build failures at the ‘Deploying outputs’ stage when middleware.ts is present. The error message is generic (‘unexpected error’), making debugging difficult.
• GitHub issue reported 2026-05-11. When experimentalServices is used with middleware.ts in the web service, deploys fail at ‘Deploying outputs’ stage with generic error. Affects Next.js monorepo deployments on Vercel.

Quick fixes

1. Confirm the exact error signature matches An unexpected error occurred when running this build — Deploying outputs stage failure with middleware.ts in services-mode.
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 reported 2026-05-11. When experimentalServices is used with middleware.ts in the web service, deploys fail at ‘Deploying outputs’ stage with generic error. Affects Next.js monorepo deployments on Vercel.

Related errors

• Vercel build command failed with Next.js middleware
• Vercel services-mode deployment error

FAQ

What should I check first?

Start with the exact An unexpected error occurred when running this build — Deploying outputs stage failure with middleware.ts in services-mode 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 — Deploying outputs stage failure with middleware.ts in services-mode.