Binding of Isaac: Rebirth Wiki

Added in Afterbirth

GB Bug is an unlockable passive item added in The Binding of Isaac: Afterbirth.


  • Removed in Repentance Spawns a familiar that bounces diagonally around the room, deals 8 damage per tick or 120 damage per second and applies a random status effect to enemies it comes in contact with. The familiar also has a chance to re-roll pickups that it passes over into other pickups, Chest Chests, or items, in an uncleared room only (with at least one enemy alive).
  • Added in Repentance Spawns a familiar that follows Isaac. Double tapping the fire button launches it forward, causing it to reroll the first monster, trinket, or pickup it hits. It respawns after clearing a room.
    • If it cannot reroll an enemy (e.g. a boss), it will deal 8 damage instead.
    • Items and opened chests cannot be rerolled.
    • Pickups have a 10% chance of becoming chests (see the algorithm).
      • If a chest is rerolled, it has a 28% chance of staying as some kind of a chest.


  • Removed in Repentance GB Bug can block projectiles.
  • Removed in Repentance GB Bug does more damage per second than The Peeper The Peeper, with the added random status chance.
  • Added in Repentance GB Bug is able to reroll Throwable Bomb Throwable Bombs into normal consumables. After doing so, the corresponding Bomb Grimace Bomb Grimace will generate another Throwable Bomb, making it able to generate a potentially large amount of free consumables.
  • Added in Repentance Rerolled champion enemies will retain their color, unless they reroll into a monster that cannot naturally spawn as a champion.


  • Added in RepentanceThe Twins The Twins: The familiar can be duplicated even if it hasn’t respawned; by constantly moving to new rooms, GB Bug can constantly respawn, allowing pickups to be rerolled indefinitely.

In-game footage[]

In Rebirth Removed in Repentance[]

In Repentance Added in Repentance[]


A reverse-engineered algorithm[1] used by GB Bug to determine its rerolls is available:
local chestPickupVariants = {
   PickupVariant.PICKUP_CHEST, -- 50
   PickupVariant.PICKUP_BOMBCHEST -- 51
   PickupVariant.PICKUP_SPIKEDCHEST -- 52
   PickupVariant.PICKUP_ETERNALCHEST -- 53
   PickupVariant.PICKUP_MIMICCHEST, -- 54
   PickupVariant.PICKUP_OLDCHEST, -- 55
   PickupVariant.PICKUP_WOODENCHEST, -- 56
   PickupVariant.PICKUP_MEGACHEST, -- 57
   PickupVariant.PICKUP_HAUNTEDCHEST, -- 58
   PickupVariant.PICKUP_LOCKEDCHEST, -- 60
   PickupVariant.PICKUP_REDCHEST, -- 360
   PickupVariant.PICKUP_MOMSCHEST, -- 390
 if (
   (contains(chestPickupVariants, pickupVariant) and (pickupSeed:Next() % 5) == 0)
   or (pickupSeed:Next() % 10) == 0
 ) then
   local chestVariant
   if (pickupSeed:Next() & 3) == 0 then
     chestVariant = PickupVariant.PICKUP_LOCKEDCHEST
     chestVariant = PickupVariant.PICKUP_CHEST
   pickup:Morph(pickup.Type, chestVariant, 0, true, false, false)
   pickup:Morph(pickup.Type, 0, 1, true, false, false)
  • An explanation of the code: rerolling something has a 10% chance of becoming a chest and 90% chance of becoming a random pickup.
    • A chest of any kind has an independent 20% extra chance of staying a chest, for a total 28% chance.
    • If something is rerolled into a chest, it is a random chest according to standard spawning rules 75% of the time, and a locked chest 25% of the time.


  • Before Repentance, GB Bug's pickup quote was "Game breaking bug, right away!"
    • This is a reference to a quote by Phil Fish in Indie Game: The Movie, where he says the same words when a bug is discovered in the game "FEZ" on the first day it is being shown at PAX[2].
  • This item's in-game appearance may be a reference to "Missingno.", a famous bug occurring in the first Pokémon games.


PC Removed in Afterbirth † Hard mode only 9B7E 3G67 (Treasure Room adjacent to spawn, curse of the labyrinth)

PC Removed in Afterbirth † Normal mode only GZWT H4LE (First floor Treasure Room)

PC Removed in Repentance Normal and hard modes only 44W2 XHJ8 (Treasure Room adjacent to spawn)

PC Added in Repentance Normal and hard modes only OCML SLQN (First floor Treasure Room)