Specified basePath should not end with /, found "${result.ba

Error message

Specified basePath should not end with /, found "${result.basePath}"

What it means

Next.js validates that a non-root basePath must start with '/' but must NOT end with '/'. This check runs in normalizeConfig during build/dev startup (config.ts:631-636) because a trailing slash on basePath silently breaks route matching, asset URLs, and proxy normalization. The framework treats the trailing slash as a structural error rather than silently stripping it, so routing stays unambiguous.

Source

Thrown at packages/next/src/server/config.ts:633

    )
  }

  if (result.basePath !== '') {
    if (result.basePath === '/') {
      throw new Error(
        `Specified basePath /. basePath has to be either an empty string or a path prefix"`
      )
    }

    if (!result.basePath.startsWith('/')) {
      throw new Error(
        `Specified basePath has to start with a /, found "${result.basePath}"`
      )
    }

    if (result.basePath !== '/') {
      if (result.basePath.endsWith('/')) {
        throw new Error(
          `Specified basePath should not end with /, found "${result.basePath}"`
        )
      }

      if (result.assetPrefix === '') {
        result.assetPrefix = result.basePath
      }
    }
  }

  if (result?.images) {
    const images: ImageConfig = result.images

    if (typeof images !== 'object') {
      throw new Error(
        `Specified images should be an object received ${typeof images}.\nSee more info here: https://nextjs.org/docs/messages/invalid-images-config`
      )
    }

View on GitHub (pinned to 0ae8c72462)

Solutions

Remove the trailing slash from basePath, e.g. change `basePath: '/docs/'` to `basePath: '/docs'`.
If you need a trailing slash in URLs, use the separate `trailingSlash: true` option instead of baking it into basePath.
Re-run `next build` or `next dev` to confirm the config loads.

Example fix

// before
module.exports = { basePath: '/docs/' }
// after
module.exports = { basePath: '/docs', trailingSlash: true }

Defensive patterns

Strategy: validation

Validate before calling

const cfg = require('./next.config'); const bp = cfg.basePath ?? '';
if (bp && bp !== '/' && bp.endsWith('/')) {
  throw new Error('basePath must not end with /. Fix: ' + bp.replace(/\/+$/, ''));
}

Type guard

function isValidBasePath(bp: unknown): bp is string {
  return typeof bp === 'string' && (bp === '' || (bp.startsWith('/') && !bp.endsWith('/')));
}

Prevention

Add a unit test asserting basePath never ends with '/' across all environment configs.
Use a config factory that normalizes basePath.trim().replace(/\/+$/, '') before returning.

When it happens

Trigger: Setting `basePath: '/docs/'` (or any value ending in '/') in next.config.js/next.config.ts. The guard only fires when basePath !== '' and basePath !== '/', so `basePath: '/'` fails earlier with a different message and `basePath: '/docs'` passes.

Common situations: Copy-pasting a deployment sub-path from a URL or nginx location block into basePath (e.g. reverse-proxy config usually shows `/docs/`). Migrating from another framework that tolerates trailing slashes. Editing basePath by hand without realizing the constraint.

Related errors

AI-assisted analysis of vercel/next.js@0ae8c72462 (2026-08-06). Data as JSON: /api/errors/25be961168fce56f. Report an issue: GitHub.