This came up in a game today and both my opponent and I were stumped about whether or not it was legal.

Scenario: Sleuth Scout engages an objective alone (steps 1 and 2 for resolving an engagement). The DS has no units 2 or less to defend with and so passes (step 3). Then after he declares no defenders, the DS then uses an action in one of the action windows after step 3 to drop Backstabber (cost 3) into the battle to defend. Legal or no?

Sleuth Scout's text reads: "While this unit is attacking alone, enemy units with printed cost 3 or higher cannot be declared as defenders."

The argument for yes is that Sleuth Scout's ability only applies during engagement step 3 (Declare Defenders). Therefore units like Backstabber and Human Replica Droid, which come in after step 3, are allowed to engage Sleuth Scout.

The argument for no is that only units with a cost of 2 and less can defend against these units. Since Backstabber and HRD cost 3, they can not legally enter the battle because they cost too much.

I think yes, it's legal.

Sweet moves from Backstabber! Yes, perfectly legal. DS did not declare defenders with costs 3 or higher. After declaring defenders step passed, he then added a unit to participate which in this case is legal because Backstabber says so.

this same thing can work with human replica droid. defend with a 2 cost unit, win edge, deploy human replica droid.