grafana/k6 · error

group() does not support async functions as arguments, pleas

Error message

group() does not support async functions as arguments, please see https://grafana.com/docs/k6/latest/javascript-api/k6/group/ for more info

What it means

k6's execution model does not support async lifecycle callbacks. `k6.group` explicitly rejects async functions (internal/js/modules/k6/k6.go:102) because the group scope tags would be popped before the awaited continuation runs, corrupting metric tagging. The error points to the k6 group docs for alternatives.

Source

Thrown at internal/js/modules/k6/k6.go:103

	mi.vu.Runtime().SetRandSource(randSource)
}

// Group wraps a function call and executes it within the provided group name.
func (mi *K6) Group(name string, val sobek.Value) (sobek.Value, error) {
	state := mi.vu.State()
	if state == nil {
		return nil, ErrGroupInInitContext
	}

	if common.IsNullish(val) {
		return nil, errors.New("group() requires a callback as a second argument")
	}
	fn, ok := sobek.AssertFunction(val)
	if !ok {
		return nil, errors.New("group() requires a callback as a second argument")
	}
	if common.IsAsyncFunction(mi.vu.Runtime(), val) {
		return sobek.Undefined(), errors.New("group() does not support async functions as arguments, " +
			"please see https://grafana.com/docs/k6/latest/javascript-api/k6/group/ for more info")
	}
	oldGroupName, _ := state.Tags.GetCurrentValues().Tags.Get(metrics.TagGroup.String())
	// TODO: what are we doing if group is not tagged
	newGroupName, err := lib.NewGroupPath(oldGroupName, name)
	if err != nil {
		return sobek.Undefined(), err
	}

	shouldUpdateTag := state.Options.SystemTags.Has(metrics.TagGroup)
	if shouldUpdateTag {
		state.Tags.Modify(func(tagsAndMeta *metrics.TagsAndMeta) {
			tagsAndMeta.SetSystemTagOrMeta(metrics.TagGroup, newGroupName)
		})
	}
	defer func() {
		if shouldUpdateTag {
			state.Tags.Modify(func(tagsAndMeta *metrics.TagsAndMeta) {

View on GitHub (pinned to 93accf6570)

Solutions

1. Convert the callback to a synchronous function: do the awaits before group, or chain .then() inside if it does not cross the group boundary
2. Restructure so async work happens outside group() and group() only wraps synchronous assertions
3. Keep using plain functions for group/check/callbacks — async is only supported for dedicated APIs in k6

Example fix

// before
k6.group('async-flow', async () => {
  const res = await doAsyncRequest();
  check(res, { ok: (r) => r.status === 200 });
});

// after
const res = doAsyncRequest(); // schedule/perform outside group
k6.group('async-flow', () => {
  check(res, { ok: (r) => r.status === 200 });
});

const AsyncFunction = (async () => {}).constructor;
if (fn instanceof AsyncFunction) { throw new TypeError('group() does not accept async functions'); }

const isAsyncFunction = (fn) => fn instanceof (async () => {}).constructor;

try {
  k6.group('phase', fn);
} catch (e) {
  if (String(e.message).includes('does not support async functions')) throw new Error('rewrite the group callback as a synchronous function (see k6 group docs)');
  throw e;
}

When it happens

Trigger: `k6.group('g', async () => { ... })` — an async function, an async arrow, or a function that returns a promise passed where the callback is expected (async detection is done on the function itself, so plain functions returning promises are not caught here).

Common situations: Writing modern async/await style scripts (e.g. with fetch or async httpx-style helpers) and wrapping them in group; converting working code to async during a refactor; k6 v0.36+ where async support grew for some APIs but not group.

Related errors

• the built-in check() does not support async functions as arg
• Uncaught (in promise) ${value}
• predicate function is not callable
• empty gRPC client
• not a gRPC client

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/73078652c0f0f92e. Report an issue: GitHub.