nats-io/nats-server · error

processLeafHeaderMsgArgs Parse Error: '%s'

Error message

processLeafHeaderMsgArgs Parse Error: '%s'

What it means

Leaf-node protocol parse guard: a message-with-headers protocol line split into fewer than 3 argument tokens, which cannot contain the required header-size and total-size fields.

Source

Thrown at server/leafnode.go:3167

		case ' ', '\t', '\r', '\n':
			if start >= 0 {
				args = append(args, arg[start:i])
				start = -1
			}
		default:
			if start < 0 {
				start = i
			}
		}
	}
	if start >= 0 {
		args = append(args, arg[start:])
	}

	c.pa.arg = arg
	switch len(args) {
	case 0, 1, 2:
		return fmt.Errorf("processLeafHeaderMsgArgs Parse Error: '%s'", args)
	case 3:
		c.pa.reply = nil
		c.pa.queues = nil
		c.pa.hdb = args[1]
		c.pa.hdr = parseSize(args[1])
		c.pa.szb = args[2]
		c.pa.size = parseSize(args[2])
	case 4:
		c.pa.reply = args[1]
		c.pa.queues = nil
		c.pa.hdb = args[2]
		c.pa.hdr = parseSize(args[2])
		c.pa.szb = args[3]
		c.pa.size = parseSize(args[3])
	default:
		// args[1] is our reply indicator. Should be + or | normally.
		if len(args[1]) != 1 {
			return fmt.Errorf("processLeafHeaderMsgArgs Bad or Missing Reply Indicator: '%s'", args[1])

View on GitHub (pinned to 3a66a489d2)

Solutions

1. Verify protocol compatibility between leaf peers
2. Check for a proxy mangling the protocol
3. Capture the offending line with trace logging and fix the emitter

When it happens

Trigger: Thrown at server/leafnode.go:3167 when the library encounters an invalid state.

Common situations: See trigger scenarios.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

AI-assisted analysis of nats-io/nats-server@3a66a489d2 (2026-09-02). Data as JSON: /api/errors/a2fbaa4e1e82ea22. Report an issue: GitHub.