slimtoolkit/slim · error

unknown log-format %q

Error message

unknown log-format %q

What it means

setLogFormat maps the configured format string to a logrus formatter: 'text' uses a colorless TextFormatter, 'json' uses JSONFormatter. Any other value falls into the default branch and returns this error, which configureLogger surfaces as 'failed to set log format'.

Source

Thrown at pkg/app/sensor/logger.go:52

		f, err := os.OpenFile(logFile, os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0644)
		if err != nil {
			return fmt.Errorf("failed to set log output destination to %q: %w", logFile, err)
		}

		log.SetOutput(f)
	}

	return nil
}

func setLogFormat(format string) error {
	switch format {
	case "text":
		log.SetFormatter(&log.TextFormatter{DisableColors: true})
	case "json":
		log.SetFormatter(&log.JSONFormatter{})
	default:
		return fmt.Errorf("unknown log-format %q", format)
	}

	return nil
}

func setLogLevel(enableDebug bool, levelName string) error {
	if enableDebug {
		log.SetLevel(log.DebugLevel)
		return nil
	}

	var logLevel log.Level

	switch levelName {
	case "trace":
		logLevel = log.TraceLevel
	case "debug":
		logLevel = log.DebugLevel

View on GitHub (pinned to 81940d17fa)

Solutions

1. Use exactly 'text' or 'json' (lowercase) for --log-format.
2. If the value comes from an env var or template, validate/normalize it (strings.TrimSpace, lowercase) before passing it.
3. Omit the flag entirely to keep the default formatter.

Example fix

// before
if format == "" { format = "JSON" } // wrong casing/empty
// after
format = strings.ToLower(strings.TrimSpace(format))
if format != "text" && format != "json" { format = "text" }

func normalizeFormat(f string) string {
    f = strings.ToLower(strings.TrimSpace(f))
    if f != "text" && f != "json" { return "text" }
    return f
}

func isUnknownFormatError(err error) bool {
    return err != nil && strings.Contains(err.Error(), "unknown log-format")
}

if err := Run(ctx); err != nil && strings.Contains(err.Error(), "unknown log-format") {
    fmt.Fprintf(os.Stderr, "unsupported format, must be text or json: %v\n", err)
    os.Exit(2)
}

When it happens

Trigger: Passing a --log-format value other than 'text' or 'json' to the sensor, including empty strings from unset environment variables or templating mistakes.

Common situations: Users try formatter names from other logging libraries ('console', 'cli', 'logfmt'); CI injects an unsupported default; a typo such as 'jsn' or 'JSON' (case-sensitive switch).

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

• failed to set log-level: %v
• failed to set log format: %v
• unknown log-level %q
• failed to set log output destination to %q, touch failed wit
• No global params

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/2a0a138bfd903c28. Report an issue: GitHub.