grafana/k6 · error

marshaling request body: %w

Error message

marshaling request body: %w

What it means

Raised in buildPresignedRequestBody when json.Marshal fails on the internal presign request struct (service, operation, files). Because the struct contains only plain strings, a marshal failure is effectively impossible under normal operation — encountering this error signals an internal inconsistency or unsupported string content, not a user-input problem at the file level.

Source

Thrown at internal/js/modules/k6/browser/storage/file_persister.go:179

		Operation string `json:"operation"`
		Files     []struct {
			Name string `json:"name"`
		} `json:"files"`
	}{
		Service:   "aws_s3",
		Operation: "upload_post",
		Files: []struct {
			Name string `json:"name"`
		}{
			{
				Name: filepath.ToSlash(filepath.Join(basePath, path)),
			},
		},
	}

	bb, err := json.Marshal(b)
	if err != nil {
		return nil, fmt.Errorf("marshaling request body: %w", err)
	}

	return bb, nil
}

func readPresignedURLResponse(resp *http.Response) (PresignedURLResponse, error) {
	var rb PresignedURLResponse

	decoder := json.NewDecoder(resp.Body)
	err := decoder.Decode(&rb)
	if err != nil {
		return PresignedURLResponse{}, fmt.Errorf("decoding response body: %w", err)
	}

	if len(rb.URLs) == 0 {
		return PresignedURLResponse{}, errors.New("missing presigned url in response body")
	}

View on GitHub (pinned to 01ffac6f24)

Solutions

1. Report as a k6 internal bug with the artifact path and configuration that triggered it
2. Upgrade to the latest k6 version in case the payload construction changed
3. Double-check basePath/path values for unusual content (invalid UTF-8)

When it happens

Trigger: Thrown at internal/js/modules/k6/browser/storage/file_persister.go:179 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/d39e0f7fbf802cd2. Report an issue: GitHub.