GoogleContainerTools/skaffold · error

invalid output type: %q. Must be "plain" or "json"

Error message

invalid output type: %q. Must be "plain" or "json"

What it means

The `skaffold schema list` command supports exactly two `--output` formats: `plain` and `json`. Any other value passed to the output flag causes list() to return this error instead of rendering the schema version list. It is a strict input-validation guard on the output flag.

Source

Thrown at cmd/skaffold/app/cmd/schema/list.go:42

	"github.com/GoogleContainerTools/skaffold/v2/pkg/skaffold/schema"
)

var OutputType string

// List prints to `out` all supported schema versions.
func List(_ context.Context, out io.Writer) error {
	return list(out, OutputType)
}

func list(out io.Writer, outputType string) error {
	switch outputType {
	case "json":
		return printJSON(out)
	case "plain":
		return printPlain(out)
	default:
		return fmt.Errorf(`invalid output type: %q. Must be "plain" or "json"`, outputType)
	}
}

type schemaList struct {
	Versions []string `json:"versions"`
}

func printJSON(out io.Writer) error {
	return json.NewEncoder(out).Encode(schemaList{
		Versions: versions(),
	})
}

func printPlain(out io.Writer) error {
	for _, version := range versions() {
		fmt.Fprintln(out, version)
	}

View on GitHub (pinned to a1189de023)

Solutions

1. Use `--output json` or `--output plain` (lowercase, exact spelling).
2. Omit the --output flag if a default format is acceptable.
3. Check the flag value in your script/CI with `echo` before invoking skaffold.

Example fix

// before
skaffold schema list --output yaml
// after
skaffold schema list --output json

out := os.Getenv("SKAFFOLD_OUTPUT")
if out != "" && out != "plain" && out != "json" {
    return fmt.Errorf("unsupported --output value %q; use \"plain\" or \"json\"", out)
}

func validSchemaOutput(v string) bool { return v == "plain" || v == "json" }

if err := listCmd.Run(); err != nil {
    if strings.Contains(err.Error(), "invalid output type") {
        // fall back to default output
    }
}

When it happens

Trigger: Running `skaffold schema list --output yaml`, `--output table`, `--output ''`, or any misspelled value such as `--output Json` (matching is case-sensitive via the switch).

Common situations: Users familiar with other CLI tools (kubectl uses -o yaml/json) assume more formats exist; typos in scripts; shell variables expanding to an empty or wrong value.

Understand the failure class

Background: "unknown output mode", "invalid value for flag", "expects true/false": fixing invalid flag value errors in CLI tools — this error's family across 24 libraries.

Related errors

• `apply` requires at least one manifest argument
• `exec` requires exactly one action to execute
• `config-dependencies add` requires exactly one file path arg
• `jobManifestPaths modify` requires exactly one manifest file
• `inspect namespaces list` requires exactly one manifest file

AI-assisted analysis of GoogleContainerTools/skaffold@a1189de023 (2026-09-05). Data as JSON: /api/errors/e288e90022445177. Report an issue: GitHub.