vitest-dev/vitest · error · Error

Tag name " " is already defined in "test.tags". Tag names…

Error message

Tag name "${tag.name}" is already defined in "test.tags". Tag names must be unique.

What it means

Tag names within `test.tags` must be unique. Vitest accumulates names into a `Set` as it iterates and throws as soon as a duplicate `name` is encountered. Uniqueness is required because tags are referenced by name in test selection and filter expressions, where duplicates would be ambiguous.

Solutions

De-duplicate by name before exporting: `tags = [...new Map(tags.map(t => [t.name, t])).values()]`.
Move shared tags into a single config layer so they are not redefined per project.
If two tags genuinely differ, give them distinct names (e.g. `smoke-unit` vs `smoke-e2e`).

Example fix

// before
export default defineConfig({ test: { tags: [
  { name: 'smoke' }, { name: 'smoke', color: 'red' },
] } })

// after
export default defineConfig({ test: { tags: [{ name: 'smoke', color: 'red' }] } })

Defensive patterns

Strategy: validation

Validate before calling

function dedupeTags<T extends { name: string }>(tags: T[]): T[] {
  const seen = new Set<string>()
  const out: T[] = []
  for (const t of tags) {
    if (seen.has(t.name)) continue
    seen.add(t.name)
    out.push(t)
  }
  return out
}

export default defineConfig({ test: { tags: dedupeTags(rawTags) } })

Prevention

Centralise tag definitions in one config layer to avoid cross-file duplication.
Run a dedupe pass when merging tags from presets and project config.
Use a Map keyed by name as the single source of truth.

When it happens

Trigger: Two entries with the same `name` (`[{name:'smoke'},{name:'smoke'}]`); merging multiple tag sources (config preset + project config) that both define the same tag; case-sensitive duplicates that look distinct but resolve equal.

Common situations: Shared base config and per-project config both listing `smoke`; refactoring tags across packages without de-duplicating; CI matrix injecting tags that already exist.

Related errors

AI-assisted analysis of vitest-dev/vitest@1fa9837ec2 (2026-08-11). Data as JSON: /api/errors/3d4f9814e546e7ef. Report an issue: GitHub.

Appendix: source

Thrown at packages/vitest/src/node/config/resolveConfig.ts:293

  if ('workspace' in resolved) {
    throw new Error('The `test.workspace` option was removed in Vitest 4. Please, migrate to `test.projects` instead. See https://vitest.dev/guide/projects for examples.')
  }

  resolved.pool ??= 'forks'

  resolved.project = toArray(resolved.project)
  resolved.provide ??= {}

  // shallow copy tags array to avoid mutating user config
  resolved.tags = [...resolved.tags || []]
  const definedTags = new Set<string>()
  resolved.tags.forEach((tag) => {
    if (!tag.name || typeof tag.name !== 'string') {
      throw new Error(`Each tag defined in "test.tags" must have a "name" property, received: ${JSON.stringify(tag)}`)
    }
    if (definedTags.has(tag.name)) {
      throw new Error(`Tag name "${tag.name}" is already defined in "test.tags". Tag names must be unique.`)
    }
    if (/\s/.test(tag.name)) {
      throw new Error(`Tag name "${tag.name}" is invalid. Tag names cannot contain spaces.`)
    }
    if (/[!()*|&]/.test(tag.name)) {
      throw new Error(`Tag name "${tag.name}" is invalid. Tag names cannot contain "!", "*", "&", "|", "(", or ")".`)
    }
    if (/^\s*(?:and|or|not)\s*$/i.test(tag.name)) {
      throw new Error(`Tag name "${tag.name}" is invalid. Tag names cannot be a logical operator like "and", "or", "not".`)
    }
    if (typeof tag.retry === 'object' && typeof tag.retry.condition === 'function') {
      throw new TypeError(`Tag "${tag.name}": retry.condition function cannot be used inside a config file. Use a RegExp pattern instead, or define the function in your test file.`)
    }
    if (tag.priority != null && (typeof tag.priority !== 'number' || tag.priority < 0)) {
      throw new TypeError(`Tag "${tag.name}": priority must be a non-negative number.`)
    }
    definedTags.add(tag.name)
  })

View on GitHub (pinned to 1fa9837ec2)