vitest-dev/vitest · error · Error

Benchmark provider loaded from

Error message

Benchmark provider loaded from "${provider}" did not have a default export.

What it means

After successfully importing a custom benchmark provider module, Vitest checks that `mod.default` is not null/undefined. A benchmark provider must be supplied as the module's default export. This fires when the module loads but exposes its provider only via a named export, or has no default at all.

Solutions

1. Add `export default { run(group) { ... } }` to the provider module.
2. If using CommonJS, ensure `module.exports = { default: { run: ... } }` or rely on the runner's interop by setting `module.exports.default = { run }`.
3. Confirm the provider object conforms to `BenchmarkProvider`: `{ run: (group: BenchmarkGroup) => Promise<BenchResult[]> }`.

Example fix

// before - provider.ts
export const myProvider = { async run(group) { return [] } }

// after
export default { async run(group) { return [] } }

const mod = await import(providerPath); if (mod.default == null) throw new Error('add `export default` to provider')

function hasDefault<T>(m: Record<string, any>): m is { default: T } { return m.default != null }

try { provider = await loadProviderModule(path, runner) } catch (e) { if (/default export/.test(e.message)) fixProviderDefault(); else throw e }

When it happens

Trigger: Provider module uses `export const provider = { run: ... }` or `export function run(...)` without `export default`. The check at benchmark.ts:113 (`if (mod.default == null)`) triggers after a successful import. Common with CJS-interop where the namespace object's `default` is undefined.

Common situations: Migrating a named-export provider to Vitest and forgetting the default export; provider authored as CommonJS (`module.exports = { run }`) but the interop layer does not synthesize a `default`; provider exports a class but as a named binding.

Related errors

• benchmark provider did not return a result for
• Failed to load benchmark provider from
• All browser instances within a project must use the same…
• `bench.compare()` expects every argument to be the return…
• `bench.compare()` requires at least 2 benchmarks, received

AI-assisted analysis of vitest-dev/vitest@1fa9837ec2 (2026-08-11). Data as JSON: /api/errors/199f83fe6a4695ab. Report an issue: GitHub.

Appendix: source

Thrown at packages/vitest/src/runtime/benchmark.ts:117

async function loadProviderModule(
  provider: string,
  moduleRunner: TestModuleRunner,
): Promise<BenchmarkProvider> {
  let mod: Record<string, any>
  try {
    mod = await moduleRunner.import(provider)
  }
  catch (error) {
    throw new Error(
      `Failed to load benchmark provider from "${provider}".`,
      { cause: error },
    )
  }
  if (mod.default == null) {
    throw new Error(
      `Benchmark provider loaded from "${provider}" did not have a default export.`,
    )
  }
  return mod.default
}

/**
 * Resolves the benchmark provider for the current worker, importing a custom
 * provider module on first use. The result is cached for the lifetime of the
 * worker so a custom provider is imported at most once.
 */
function resolveBenchmarkProvider(
  config: SerializedConfig,
  moduleRunner: TestModuleRunner,
): Promise<BenchmarkProvider> {
  if (!cachedProvider) {
    const provider = config.benchmark.provider
    cachedProvider = !provider
      ? Promise.resolve(createDefaultBenchmarkProvider(config))
      : loadProviderModule(provider, moduleRunner)
  }

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