Seeds

Seeds, essentially random strings of characters, are used as the basis for all decision making in the game. By inputting a seed manually, one can guarantee that most randomly-generated decisions will occur the same every time. For instance, two players using the same seed will find the same items in the same locations.

Notes Regarding Seed Entry

 * Except for some special seeds, inputting a seed will cause the 'no trophies' mark to appear. This prevents any achievements from being obtained on that particular run.
 * Special seeds do not affect level generation.
 * All alphanumeric characters are accepted except for 5, I, O, and U. On the PC version pressing the I, O, and U keys during seed entry will replace them with 1, 0, and V respectively.
 * Once a special seed has been entered, it will be permanently unlocked without needing to enter the code again. Pressing Q / Left Bumper during character selection will bring up the Easter Egg Section, in which any previously unlocked game mode can be selected. Selecting multiple modes at once is also possible.
 * On the special seed selection menu, it is possible to remove a discovered seed by focusing it and pressing Q+E+Tab on keyboard or LB+RB+X on controller.

Notes Regarding Seed Effects

 * While the BRTL B0NS seed is enabled, getting hit will not result in death if Isaac possesses Red hearts as well as Soul hearts or Black hearts. Instead, the Soul hearts and/or Black hearts will be completely depleted, leaving Isaac with only his Red hearts. You can also prevent death with Bone Hearts, as they will get depleted without destroying the bone heart and protecting the rest of the hearts, and can be refilled with Red Hearts. The soul hearts in front of them will get destroyed though.
 * If the ICES KATE seed is enabled, purple champion monsters will be much more dangerous, as escaping their gravitational pull is almost impossible.
 * When playing with the KAPP A seed enabled, certain items will override the grayscale, such as.
 * Some seeds can not be enabled alongside other certain seeds. For example, the PTCH BLCK seed cannot be enabled alongside THEB LANK.

Trivia

 * Many of the special seeds can be found by using either the Fortune Teller or the Rules Card.
 * Some special seeds were shared with the community by the developers.
 * If an item that would appear on a seed isn't unlocked yet, it will be replaced by another item that is unlocked.
 * The "DYSL EXIA" seed is a reference to dyslexia, a disorder that causes difficulties in spelling, reading, and writing.
 * The seeds "ANDA NTE", "LARG HET0", "ALLE GR0", and "PRES T0" refer to andante, larghetto, allegro, and presto, respectively. These are names of musical tempos, which are the speeds of given pieces of music.
 * The "KAPP A" seed is a reference to a commonly used Twitch emote "KAPPA".
 * The "ALM1 GHTY" seed's description is, "Dog Mode". This is referencing the term "God mode", which is used to describe someone or something in a game that's invincible, usually through the use of cheats.
 * "Dog Mode" is also a reference to Rise of the Triad, which has a power-up of the same name.
 * The "FEAR M1NT" seed is a reference to spearmint, a popular chewing gum flavor.
 * The "SVPE RH0T" seed is a reference to the game SUPERHOT.
 * "M0DE SEVN" i.e. Mode 7 is a graphics mode on the Super Nintendo Entertainment System console that often results in pronounced pixelation.