grafana/k6 · error

writing form field key %q and value %q: %w

Error message

writing form field key %q and value %q: %w

What it means

Raised in newFileUploadRequest when multipart.Writer.WriteField fails while copying a form field (key '%q', value '%q') received in the presigned-URL response into the multipart form buffer. The inputs at fault come from the remote service's FormFields map. Failure is a local encoding/buffer-write error — essentially only possible if a field value contains invalid UTF-8 that the multipart writer rejects.

Source

Thrown at internal/js/modules/k6/browser/storage/file_persister.go:217

}

// newFileUploadRequest creates a new HTTP request to upload a file as a multipart
// form to the presigned URL received from the server.
func newFileUploadRequest(
	ctx context.Context,
	resp PresignedURLResponse,
	data io.Reader,
) (*http.Request, error) {
	// we don't support multiple presigned URLs at the moment.
	psu := resp.URLs[0]

	// copy all form fields received from a presigned URL
	// response to the multipart form fields.
	var form bytes.Buffer
	fw := multipart.NewWriter(&form)
	for k, v := range psu.FormFields {
		if err := fw.WriteField(k, v); err != nil {
			return nil, fmt.Errorf("writing form field key %q and value %q: %w", k, v, err)
		}
	}
	// attach the file data to the form.
	ff, err := fw.CreateFormFile("file", psu.Name)
	if err != nil {
		return nil, fmt.Errorf("creating multipart form file: %w", err)
	}
	if _, err := io.Copy(ff, data); err != nil {
		return nil, fmt.Errorf("copying file data to multipart form: %w", err)
	}
	if err := fw.Close(); err != nil {
		return nil, fmt.Errorf("closing multipart form writer: %w", err)
	}

	req, err := http.NewRequestWithContext(
		ctx,
		psu.Method,
		psu.PresignedURL,

View on GitHub (pinned to 01ffac6f24)

Solutions

1. Retry the persist to get a fresh presigned response with new form fields
2. Report as a bug if the cloud service consistently returns problematic field values
3. Verify no man-in-the-middle proxy is mangling the presigned response

When it happens

Trigger: Thrown at internal/js/modules/k6/browser/storage/file_persister.go:217 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/05477e6d335303f9. Report an issue: GitHub.