cloudflare/cloudflared · error

Unknown output format '%s'

Error message

Unknown output format '%s'

What it means

renderOutput serializes command results to stdout in the format chosen by the --output flag. Only 'json' and 'yaml' are supported; any other value produces 'Unknown output format %s'. This is used by create, list, info, routes, and vnet list commands.

Source

Thrown at cmd/cloudflared/tunnel/subcommands.go:712

	tunnelIDs, err := sc.findIDs(c.Args().Slice())
	if err != nil {
		return err
	}

	return sc.delete(tunnelIDs)
}

func renderOutput(format string, v interface{}) error {
	switch format {
	case "json":
		encoder := json.NewEncoder(os.Stdout)
		encoder.SetIndent("", "  ")
		return encoder.Encode(v)
	case "yaml":
		return yaml.NewEncoder(os.Stdout).Encode(v)
	default:
		return errors.Errorf("Unknown output format '%s'", format)
	}
}

func buildRunCommand() *cli.Command {
	//nolint: prealloc
	cliFlags := []cli.Flag{
		credentialsFileFlag,
		credentialsContentsFlag,
		postQuantumFlag,
		selectProtocolFlag,
		featuresFlag,
		tunnelTokenFlag,
		tunnelTokenFileFlag,
		icmpv4SrcFlag,
		icmpv6SrcFlag,
		maxActiveFlowsFlag,
		dnsResolverAddrsFlag,
	}

View on GitHub (pinned to 2253eeeb25)

Solutions

1. Use --output json or --output yaml only (lowercase)
2. Omit the --output flag entirely for the default human-readable output
3. Check `cloudflared tunnel list --help` for the exact accepted values on your version
4. Fix case in scripts — the format string is matched exactly

Example fix

// before
cloudflared tunnel list --output table
// after
cloudflared tunnel list --output json   # or omit --output

var supportedFormats = map[string]bool{"json": true, "yaml": true}
if format != "" && !supportedFormats[strings.ToLower(format)] {
	return fmt.Errorf("unsupported --output %q; use json or yaml", format)
}

func isSupportedOutputFormat(f string) bool {
	return f == "" || f == "json" || f == "yaml"
}

if err := renderOutput(format, v); err != nil {
	if strings.HasPrefix(err.Error(), "Unknown output format") {
		// fall back to default text output
	}
	return err
}

When it happens

Trigger: Passing an unsupported --output value, e.g. `cloudflared tunnel list --output table`, --output JSON (case-sensitive), or --output xml.

Common situations: Users assuming table/text output is supported; capitalization mistakes like JSON/YAML; older cloudflared versions where --output handling or supported formats differ; scripts carrying flags over from other CLIs.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

• Couldn't parse UUID from %s
• %s is not a valid tunnel ID
• not a valid host
• no input provided
• failed to parse Host

AI-assisted analysis of cloudflare/cloudflared@2253eeeb25 (2026-09-06). Data as JSON: /api/errors/40edd31a344ac091. Report an issue: GitHub.