nats-io/nats-server · error

processLeafHeaderMsgArgs Bad or Missing Size: '%s'

Error message

processLeafHeaderMsgArgs Bad or Missing Size: '%s'

What it means

Leaf-node protocol parse guard: parseSize returned a negative total-size (payload size) token — missing, non-numeric, or an int64 overflow from an absurdly large length.

Source

Thrown at server/leafnode.go:3214

		c.pa.hdb = args[len(args)-2]
		c.pa.hdr = parseSize(c.pa.hdb)

		// 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)-2]
		} else {
			c.pa.queues = args[2 : len(args)-2]
		}
	}
	if c.pa.hdr < 0 {
		return fmt.Errorf("processLeafHeaderMsgArgs Bad or Missing Header Size: '%s'", arg)
	}
	if c.pa.size < 0 {
		return fmt.Errorf("processLeafHeaderMsgArgs Bad or Missing Size: '%s'", args)
	}
	if c.pa.hdr > c.pa.size {
		return fmt.Errorf("processLeafHeaderMsgArgs Header Size larger then TotalSize: '%s'", arg)
	}
	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
}

func (c *client) processLeafMsgArgs(arg []byte) error {
	// Unroll splitArgs to avoid runtime/heap issues

View on GitHub (pinned to 3a66a489d2)

Solutions

1. Verify the sender encodes the total size correctly
2. Check for corrupted length prefixes
3. Reject/limit messages beyond max payload settings

When it happens

Trigger: Thrown at server/leafnode.go:3214 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/b4693c65c52166a4. Report an issue: GitHub.