nats-io/nats-server · error

processLeafMsgArgs Bad or Missing Size: '%s'

Error message

processLeafMsgArgs Bad or Missing Size: '%s'

What it means

Returned by processLeafMsgArgs when the size token of a leaf-node MSG protocol line parses to a negative number (parseSize failed on the last argument, e.g. a truncated or malformed payload length). The raw args slice is included so the offending control line can be inspected.

Source

Thrown at server/leafnode.go:3291

			c.pa.reply = args[2]
		case '|':
			c.pa.reply = nil
		default:
			return fmt.Errorf("processLeafMsgArgs Bad or Missing Reply Indicator: '%s'", args[1])
		}
		// Grab size.
		c.pa.szb = args[len(args)-1]
		c.pa.size = parseSize(c.pa.szb)

		// Grab queue names.
		if c.pa.reply != nil {
			c.pa.queues = args[3 : len(args)-1]
		} else {
			c.pa.queues = args[2 : len(args)-1]
		}
	}
	if c.pa.size < 0 {
		return fmt.Errorf("processLeafMsgArgs Bad or Missing Size: '%s'", args)
	}
	maxPayload := atomic.LoadInt32(&c.mpay)
	if maxPayload != jwt.NoLimit && int64(c.pa.size) > int64(maxPayload) {
		c.maxPayloadViolation(c.pa.size, maxPayload)
		return ErrMaxPayload
	}

	// Common ones processed after check for arg length
	c.pa.subject = args[0]

	return nil
}

// processInboundLeafMsg is called to process an inbound msg from a leaf node.
func (c *client) processInboundLeafMsg(msg []byte) {
	// Update statistics
	// The msg includes the CR_LF, so pull back out for accounting.
	c.in.msgs++

View on GitHub (pinned to 3a66a489d2)

Solutions

1. Fix the leaf node operator's client/server so it sends a well-formed MSG proto with a non-negative size token
2. Capture the full control line from the error and check for proxy or protocol corruption between the leaf node and the cluster
3. Verify max_payload negotiation is not truncating the protocol line

When it happens

Trigger: Thrown at server/leafnode.go:3291 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of nats-io/nats-server@3a66a489d2 (2026-09-02). Data as JSON: /api/errors/6a5ec415ad878f32. Report an issue: GitHub.