grafana/k6 · error · TypeError

invalid value for metric tag '%s': only String, Boolean and

Error message

invalid value for metric tag '%s': only String, Boolean and Number types are accepted as a metric tag values

What it means

tagsDynamicObject.Set (backing execution.vu.tags[tag]=value) received a value that cannot be converted to a metric tag: only String, Boolean and Number are accepted. Objects, null, undefined, symbols, and functions are denied; the assignment raises an exception in the JS runtime.

Source

Thrown at internal/js/modules/k6/execution/execution.go:374

	state   *lib.State
}

// Get a property value for the key. May return nil if the property does not exist.
func (o *tagsDynamicObject) Get(key string) sobek.Value {
	tcv := o.state.Tags.GetCurrentValues()
	if tag, ok := tcv.Tags.Get(key); ok {
		return o.runtime.ToValue(tag)
	}
	return nil
}

// Set a property value for the key. It returns true if succeed. String, Boolean
// and Number types are implicitly converted to the Sobek's relative string
// representation. An exception is raised in case a denied type is provided.
func (o *tagsDynamicObject) Set(key string, val sobek.Value) bool {
	o.state.Tags.Modify(func(tagsAndMeta *metrics.TagsAndMeta) {
		if err := common.ApplyCustomUserTag(tagsAndMeta, key, val); err != nil {
			panic(o.runtime.NewTypeError(err.Error()))
		}
	})
	return true
}

// Has returns true if the property exists.
func (o *tagsDynamicObject) Has(key string) bool {
	ctv := o.state.Tags.GetCurrentValues()
	if _, ok := ctv.Tags.Get(key); ok {
		return true
	}
	return false
}

// Delete deletes the property for the key. It returns true on success (note,
// that includes missing property).
func (o *tagsDynamicObject) Delete(key string) bool {
	o.state.Tags.Modify(func(tagsAndMeta *metrics.TagsAndMeta) {

View on GitHub (pinned to 01ffac6f24)

Solutions

1. Assign only strings, booleans, or numbers to execution.vu.tags entries
2. Convert objects to strings explicitly (e.g. JSON.stringify or String(...)) before assigning
3. Remove null/undefined assignments; clear tags via k6 API if supported

When it happens

Trigger: Thrown at internal/js/modules/k6/execution/execution.go:374 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of grafana/k6@01ffac6f24 (2026-08-18). Data as JSON: /api/errors/3345bd17819a651b. Report an issue: GitHub.