vercel/next.js · error · Error

Segment names may not start or end with extra brackets ('${s

Error message

Segment names may not start or end with extra brackets ('${segmentName}').

What it means

After stripping the outer brackets (and optional catch-all brackets) from a dynamic segment, _insert throws if the remaining name still starts with '[' or ends with ']' — e.g. [[foo]] used outside a catch-all or unbalanced brackets — because nested/extra brackets are not a valid parameter name.

Source

Thrown at packages/next/src/shared/lib/router/utils/sorted-routes.ts:110

        // Strip optional `[` and `]`, leaving only `something`
        segmentName = segmentName.slice(1, -1)
        isOptional = true
      }

      if (segmentName.startsWith('…')) {
        throw new Error(
          `Detected a three-dot character ('…') at ('${segmentName}'). Did you mean ('...')?`
        )
      }

      if (segmentName.startsWith('...')) {
        // Strip `...`, leaving only `something`
        segmentName = segmentName.substring(3)
        isCatchAll = true
      }

      if (segmentName.startsWith('[') || segmentName.endsWith(']')) {
        throw new Error(
          `Segment names may not start or end with extra brackets ('${segmentName}').`
        )
      }

      if (segmentName.startsWith('.')) {
        throw new Error(
          `Segment names may not start with erroneous periods ('${segmentName}').`
        )
      }

      function handleSlug(previousSlug: string | null, nextSlug: string) {
        if (previousSlug !== null) {
          // If the specific segment already has a slug but the slug is not `something`
          // This prevents collisions like:
          // pages/[post]/index.js
          // pages/[id]/index.js
          // Because currently multiple dynamic params on the same segment level are not supported
          if (previousSlug !== nextSlug) {

View on GitHub (pinned to 0eb3775416)

Solutions

1. Remove the extra leading/trailing brackets from the segment name.

When it happens

Trigger: A route segment name starts or ends with extra brackets.

Common situations: Folder names like '[[slug]' or '[slug]]' with mismatched brackets.

AI-assisted analysis of vercel/next.js@0eb3775416 (2026-08-19). Data as JSON: /api/errors/c62a37bbcf67455d. Report an issue: GitHub.