kubernetes/kops · error

unable to marshal JSON: %v

Error message

unable to marshal JSON: %v

What it means

RunGetAssets uses encoding/json.Marshal on the AssetResult for `kops get assets -o json` and wraps marshal failures with this message. AssetResult contains only pointer-to-struct slices of strings, so failures typically indicate a programming/data error rather than user misconfiguration.

Source

Thrown at cmd/kops/get_assets.go:169

	switch options.Output {
	case OutputTable:
		if err = imageOutputTable(result.Images, out); err != nil {
			return err
		}
		return fileOutputTable(result.Files, out)
	case OutputYaml:
		y, err := yaml.Marshal(result)
		if err != nil {
			return fmt.Errorf("unable to marshal YAML: %v", err)
		}
		if _, err := out.Write(y); err != nil {
			return fmt.Errorf("error writing to output: %v", err)
		}
	case OutputJSON:
		j, err := json.Marshal(result)
		if err != nil {
			return fmt.Errorf("unable to marshal JSON: %v", err)
		}
		if _, err := out.Write(j); err != nil {
			return fmt.Errorf("error writing to output: %v", err)
		}
	default:
		return fmt.Errorf("unsupported output format: %q", options.Output)
	}

	return nil
}

func imageOutputTable(images []*Image, out io.Writer) error {
	fmt.Println("")
	t := &tables.Table{}
	t.AddColumn("CANONICAL", func(i *Image) string {
		return i.Canonical
	})
	t.AddColumn("DOWNLOAD", func(i *Image) string {

View on GitHub (pinned to 4c8573c808)

Solutions

1. Read the wrapped inner error to find the offending field
2. Ensure all AssetResult fields are JSON-marshalable (plain data types)
3. Retry with -o yaml or default table output to confirm the data itself is fine
4. Rebuild from upstream kops if running a patched binary

Example fix

// before
type AssetResult struct {
    Images []*Image
    Extra  chan string // unmarshalable
}
// after
type AssetResult struct {
    Images []*Image `json:"images,omitempty"`
    Extra  string   `json:"extra,omitempty"`
}

// ensure result contains only JSON-marshalable data before calling
if err := json.Valid(mustBytes(json.Marshal(result))); err != nil {
    return fmt.Errorf("asset result not serializable")
}

j, err := json.Marshal(result)
if err != nil {
    return fmt.Errorf("unable to marshal JSON: %v", err)
}

When it happens

Trigger: `kops get assets -o json` where json.Marshal(result) errors — practically only possible if the result structure contains unsupported types (channels, funcs, cycles) as in modified builds.

Common situations: Custom/patched kops builds with new AssetResult fields of unmarshalable types; nil map/interface oddities introduced by plugins or forks.

Understand the failure class

Background: json.Marshal / "failed to marshal" errors in Go: why "unsupported type" happens and how to fix it — this error's family across 22 libraries.

Related errors

• error marshaling json: %v
• error writing json to stdout: %v
• unable to marshal YAML: %v
• unable to marshal JSON: %v
• unable to marshal JSON: %v

AI-assisted analysis of kubernetes/kops@4c8573c808 (2026-09-05). Data as JSON: /api/errors/9be18e30d5e88021. Report an issue: GitHub.