hashicorp/nomad · error

json format does not support template option.

Error message

json format does not support template option.

What it means

DataFormat in command/data_format.go returns this error when format is "json" but a non-empty template string is also supplied. The JSON formatter renders data as JSON and cannot accept a Go text/template, so the conflicting combination is rejected outright.

Source

Thrown at command/data_format.go:33

var (
	jsonHandlePretty = &codec.JsonHandle{
		HTMLCharsAsIs: true,
		Indent:        4,
	}
)

// DataFormatter is a transformer of the data.
type DataFormatter interface {
	// TransformData should return transformed string data.
	TransformData(any) (string, error)
}

// DataFormat returns the data formatter specified format.
func DataFormat(format, tmpl string) (DataFormatter, error) {
	switch format {
	case "json":
		if len(tmpl) > 0 {
			return nil, fmt.Errorf("json format does not support template option.")
		}
		return &JSONFormat{}, nil
	case "template":
		return &TemplateFormat{tmpl}, nil
	}
	return nil, fmt.Errorf("Unsupported format is specified.")
}

type JSONFormat struct{}

// TransformData returns JSON format string data.
func (p *JSONFormat) TransformData(data any) (string, error) {
	var buf bytes.Buffer
	err := codec.NewEncoder(&buf, jsonHandlePretty).Encode(data)
	if err != nil {
		return "", err
	}

View on GitHub (pinned to 482b49bf1a)

Solutions

1. Remove the -template flag when using json output.
2. Or switch to -format=template (or drop -json) so the template is honored.
3. If flags come from shared flag set, guard your script so only one of -json/-template is ever set.
4. Update code calling DataFormat directly to pass "" as tmpl for json format.

Example fix

// before
f, err := command.DataFormat("json", myTmpl)
// after
f, err := command.DataFormat("json", "") // or command.DataFormat("template", myTmpl)

if format == "json" && len(tmpl) > 0 {
    return errors.New("choose either json output or a template, not both")
}

null

f, err := command.DataFormat(format, tmpl)
if err != nil {
    if strings.Contains(err.Error(), "does not support template") {
        f, err = command.DataFormat("json", "")
    }
}

When it happens

Trigger: Calling command.DataFormat("json", tmpl) with len(tmpl) > 0, e.g. via the CLI when both -json (or format=json) and -template flags are set.

Common situations: Users passing both -json and -template flags to a Consul CLI command; scripts that hardcode format=json while a template flag is inherited from shared flag parsing code.

Related errors

• Both json and template formatting are not allowed
• format error: %v
• template needs to be specified in golang's text/template for
• default auth config text could not be deserialized: %v
• [✘] Role data could not be deserialized: %w

AI-assisted analysis of hashicorp/nomad@482b49bf1a (2026-09-04). Data as JSON: /api/errors/96a904643a7b8d16. Report an issue: GitHub.