golangci/golangci-lint · error

unsupported format: %s

Error message

unsupported format: %s

What it means

`golangci-lint migrate --format` only accepts empty (default), yml, yaml, toml, or json (case-insensitive). Any other value is rejected by `preRunE` before the command runs. This is a strict input validation error protecting the config serializer.

Source

Thrown at pkg/commands/migrate.go:138

	err = saveNewConfiguration(newCfg, dstPath)
	if err != nil {
		return fmt.Errorf("saving configuration file: %w", err)
	}

	c.log.Infof("Migration done: %s", dstPath)

	callForAction(c.cmd)

	return nil
}

func (c *migrateCommand) preRunE(cmd *cobra.Command, _ []string) error {
	switch strings.ToLower(c.opts.format) {
	case "", "yml", "yaml", "toml", "json":
		// Valid format.
	default:
		return fmt.Errorf("unsupported format: %s", c.opts.format)
	}

	if c.cfg.Version != "" {
		return fmt.Errorf("configuration version is already set: %s", c.cfg.Version)
	}

	if c.opts.skipValidation {
		return nil
	}

	usedConfigFile := c.viper.ConfigFileUsed()
	if usedConfigFile == "" {
		c.log.Warnf("No config file detected")
		os.Exit(exitcodes.NoConfigFileDetected)
	}

	c.log.Infof("Validating v1 configuration file: %s", usedConfigFile)

View on GitHub (pinned to ed7a235d2d)

Solutions

1. Use one of the supported values: yml, yaml, toml, or json (case-insensitive).
2. Omit --format entirely to keep the source config's extension/default format.
3. Check `golangci-lint migrate --help` for the exact accepted values.

Example fix

// before
golangci-lint migrate --format ini
// after
golangci-lint migrate --format toml

// validate the format flag before invoking the command
var allowed = map[string]bool{"": true, "yml": true, "yaml": true, "toml": true, "json": true}
if !allowed[strings.ToLower(format)] {
    return fmt.Errorf("unsupported format: %s (use yml, yaml, toml, json)", format)
}

if err := cmd.Execute(); err != nil {
    if strings.Contains(err.Error(), "unsupported format") {
        fmt.Fprintln(os.Stderr, "hint: --format accepts yml, yaml, toml, json")
    }
    os.Exit(1)
}

When it happens

Trigger: Passing an unsupported value to the migrate command's format flag, e.g. `golangci-lint migrate --format ini` or a typo like `--format yaml.` or `--format YAMLX`.

Common situations: Users guessing at available formats (trying 'xml', 'ini', 'properties'); typos such as 'yamll' or 'tomm'; copying a flag value from another tool.

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

• saving configuration file: %w
• configuration version is already set: %s
• writing file %s: %w
• can't load config: %w
• can't get enabled linters: %w

AI-assisted analysis of golangci/golangci-lint@ed7a235d2d (2026-09-02). Data as JSON: /api/errors/5538eebe7ad4027f. Report an issue: GitHub.