What causes "sizes defined as strings must end in K, M, G, T" in nats-io/nats-server?

Change the string to end with a single uppercase K, M, G, or T, e.g. "10M"

sizes defined as strings must end in K, M, G, T

Error message

sizes defined as strings must end in K, M, G, T

What it means

getStorageSize() accepts string sizes only when they end in one of the unit suffixes K, M, G, or T (shifted by 10/20/30/40 bits). Any other final character (or a bare number string) causes this error. Bare numeric strings are rejected because there is no default unit.

Source

Thrown at server/opts.go:2521

		return 0, fmt.Errorf("must be int64 or string")
	}

	if s == _EMPTY_ {
		return 0, nil
	}

	suffix := s[len(s)-1:]
	prefix := s[:len(s)-1]
	num, err := strconv.ParseInt(prefix, 10, 64)
	if err != nil {
		return 0, err
	}

	suffixMap := map[string]int64{"K": 10, "M": 20, "G": 30, "T": 40}

	mult, ok := suffixMap[suffix]
	if !ok {
		return 0, fmt.Errorf("sizes defined as strings must end in K, M, G, T")
	}
	num *= 1 << mult

	return num, nil
}

// Parse enablement of jetstream for a server.
func parseJetStreamLimits(v any, opts *Options, errors *[]error) error {
	var lt token
	tk, v := unwrapValue(v, &lt)

	opts.JetStreamLimits = JSLimitOpts{}

	vv, ok := v.(map[string]any)
	if !ok {
		return &configErr{tk, fmt.Sprintf("Expected a map to define JetStreamLimits, got %T", v)}
	}
	for mk, mv := range vv {

View on GitHub (pinned to 3a66a489d2)

Solutions

Change the string to end with a single uppercase K, M, G, or T, e.g. "10M"
Remove a trailing 'B' ("10MB" -> "10M")
Uppercase lowercase suffixes ("1kb" -> "1K")
Add an explicit unit to a bare numeric string ("1024" -> "1K" or use int64 bytes)

Example fix

// before
limits["max_memory"] = "10MB"
// after
limits["max_memory"] = "10M"

Defensive patterns

Strategy: validation

Validate before calling

func validSizeString(s string) bool {
	if len(s) < 2 { return false }
	if _, err := strconv.ParseInt(s[:len(s)-1], 10, 64); err != nil { return false }
	return strings.ContainsRune("KMGT", rune(s[len(s)-1]))
}

Try / catch

if err := checkSize(s); err != nil {
	log.Fatalf("config: %v (use e.g. 10K, 5M, 2G, 1T)", err)
}

Prevention

Use single uppercase unit letters without 'B' (M not MB)
Never write bare numeric strings — add a unit or use an int
Add a config linter that checks all size fields against ^[0-9]+[KMGT]$

When it happens

Trigger: Configuring a JetStream storage/memory limit as a string whose last character is not K, M, G, or T, e.g. "1024", "10MB", "1kb" (lowercase), or "5B".

Common situations: Writing "10MB" or "1gb" instead of "10M"/"1G" (no B allowed, uppercase only); leaving a plain number string without a unit; copy-pasting sizes from other tools that use KB/MB notation.

Related errors

AI-assisted analysis of nats-io/nats-server@3a66a489d2 (2026-09-02). Data as JSON: /api/errors/a86f05a4fa7d9a8a. Report an issue: GitHub.