vitest-dev/vitest · error · SyntaxError

`bench.compare()` requires at least 2 benchmarks, received

Error message

`bench.compare()` requires at least 2 benchmarks, received ${registrations.length} instead. ${registrations.length === 1 ? 'Consider calling `bench().run()`. ' : 'Define benchmarks by calling `bench()`. '}See https://vitest.dev/guide/benchmarking#comparing-benchmarks

What it means

`bench.compare(...)` requires at least two benchmark registrations — comparison is meaningless with fewer. This `SyntaxError` fires when zero or one registrations are passed (after stripping a trailing options object). The error message dynamically hints: with 1 registration it suggests `bench().run()`; with 0 it suggests defining benchmarks via `bench()`.

Solutions

Pass at least two registrations: `await bench.compare(regA, regB)`.
If you have a single benchmark, call `await reg.run()` or `await bench('name', fn).run()` instead of `bench.compare`.
If passing run options, ensure they are the LAST argument and at least two registrations precede them: `bench.compare(regA, regB, { iterations: 100 })`.

Example fix

// before
const a = bench('sort', sortFn)
await bench.compare(a) // only one

// after
await a.run() // single benchmark
// or
const b = bench('sort2', sort2Fn)
await bench.compare(a, b)

Defensive patterns

Strategy: validation

Validate before calling

if (registrations.length < 2) throw new Error(`need >=2 registrations, got ${registrations.length}`)

Type guard

function hasMinTwo<T>(arr: T[]): arr is [T, T, ...T[]] { return arr.length >= 2 }

Try / catch

null

Prevention

Use `bench().run()` for single benchmarks instead of `bench.compare`.
When building args dynamically, assert `regs.length >= 2` before calling.
Place trailing run options only after at least two registrations.

When it happens

Trigger: Calling `bench.compare(singleReg)` (1 arg), `bench.compare()` (0 args), or `bench.compare(singleReg, optionsObj)` where the trailing object is detected as `BenchRunOptions` and stripped, leaving one registration. Detection at benchmark.ts:460-462 pops the last arg when it is an object without the `kRegistration` symbol.

Common situations: Forgetting to register a second baseline; passing the options object as the only non-registration argument; conditionally building the args array and ending with one element; refactoring `bench().run()` into `bench.compare()` without adding a second benchmark.

Related errors

AI-assisted analysis of vitest-dev/vitest@1fa9837ec2 (2026-08-11). Data as JSON: /api/errors/2bb8a6f3802fb704. Report an issue: GitHub.

Appendix: source

Thrown at packages/vitest/src/runtime/benchmark.ts:475

    validateBenchmarkProject(config)

    // extract optional trailing BenchRunOptions argument
    const lastArg = args.at(-1)
    const isOptions = lastArg != null && typeof lastArg === 'object' && !(kRegistration in lastArg)
    const benchOptions = isOptions ? args.pop() as BenchRunOptions : undefined
    const registrations = args as BenchRegistration<any>[]

    // Mark every passed-in registration as consumed before validation so a
    // throwing `bench.compare()` (wrong arity, wrong shape) doesn't also
    // trigger the unrun-bench warning — the user's intent was to consume them.
    for (const reg of registrations) {
      if (reg != null && typeof reg === 'object' && kRegistration in reg) {
        pending.delete(reg)
      }
    }

    if (registrations.length < 2) {
      throw new SyntaxError(`\`bench.compare()\` requires at least 2 benchmarks, received ${registrations.length} instead. ${registrations.length === 1 ? 'Consider calling `bench().run()`. ' : 'Define benchmarks by calling `bench()`. '}See https://vitest.dev/guide/benchmarking#comparing-benchmarks`)
    }
    for (const reg of registrations) {
      if (reg == null || typeof reg !== 'object' || !(kRegistration in reg)) {
        throw new SyntaxError('`bench.compare()` expects every argument to be the return value of `bench` or `bench.from`.')
      }
    }

    const runnable: RunnableRegistration<any>[] = []
    const fromEntries: FromRegistration<any>[] = []
    for (const reg of registrations) {
      if (isFromRegistration(reg)) {
        fromEntries.push(reg)
      }
      else {
        runnable.push(reg as RunnableRegistration<any>)
      }
    }

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