vercel/next.js · error

Warning: 'dev' is not a boolean which could introduce unexpe

Error message

Warning: 'dev' is not a boolean which could introduce unexpected behavior. https://nextjs.org/docs/messages/invalid-server-options

What it means

createServer validates its options: when the dev option is present but not a boolean (e.g. the string 'false', which is truthy), it warns that this could introduce unexpected behavior since truthiness checks will misinterpret the value.

Source

Thrown at packages/next/src/server/next.ts:673

    ) as unknown as NextWrapperServer
  }

  if (options == null) {
    throw new Error(
      'The server has not been instantiated properly. https://nextjs.org/docs/messages/invalid-server-options'
    )
  }

  if (
    !('isNextDevCommand' in options) &&
    process.env.NODE_ENV &&
    !['production', 'development', 'test'].includes(process.env.NODE_ENV)
  ) {
    log.warn(NON_STANDARD_NODE_ENV)
  }

  if (options.dev && typeof options.dev !== 'boolean') {
    console.warn(
      "Warning: 'dev' is not a boolean which could introduce unexpected behavior. https://nextjs.org/docs/messages/invalid-server-options"
    )
  }

  // When the caller is a custom server (using next()).
  if (options.customServer !== false) {
    const dir = path.resolve(/* turbopackIgnore: true */ options.dir || '.')

    return new NextCustomServer({
      ...options,
      dir,
    })
  }

  // When the caller is Next.js internals (i.e. render worker, start server, etc)
  return new NextServer(options)
}

View on GitHub (pinned to 0eb3775416)

Solutions

1. Pass `dev` as a boolean in the Next.js server options.

When it happens

Trigger: Thrown at packages/next/src/server/next.ts:673 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of vercel/next.js@0eb3775416 (2026-08-19). Data as JSON: /api/errors/1893eab9a7e7fcc0. Report an issue: GitHub.