What a seed controls
A seed is eight characters that lock the run’s random events. Enter the same seed and you face the same floors, the same item pedestals, and the same bosses as anyone else using it.
Seeds are not made of every character. The letters I, O and U and the digit 5 cannot be typed, and the game swaps them for 1, 0, V and S. Most eight-character strings are not valid either, because part of the string is a check value. If the game refuses your seed, that is the reason.
Running a seeded run
Type the eight characters into the seed field on the character select screen. The run then plays out on that fixed sequence.
Here is the part that catches people out: a seeded run is flagged as non-progression. Achievements, completion marks, and character unlocks do not count for it.
If you are in the middle of grinding unlocks, seeds cost you the run. If you already have what you want and are learning the game, they cost you nothing.
Easter Egg seeds are a separate system
Some strings are not ordinary seeds at all. Enter one and it permanently unlocks an Easter Egg, a named run modifier you can switch on whenever you like.
The egg list lives on the character select screen. Open it with Q on a keyboard or LB on a gamepad. Once an egg is discovered it stays in that list, so you type the string once and never again.
You can enable more than one egg at a time as long as they do not conflict. Eggs change effects rather than the random sequence, so an egg laid over a seed still leaves the floor layout identical.
Which eggs keep achievements running
Ordinary seeded runs kill your progress for that run. Easter Eggs are not ordinary seeds, and most of them leave achievements and unlocks running normally.
The exceptions are marked rather than hidden. The egg list flags the eggs that block achievements, and the game shows the same no-achievements marker when a run will not count.
Check that marker before you commit. A run that will not count looks exactly like one that will, right up until you finish and nothing registers.
Seeds respect your save, not someone else’s
This is the most common reason a shared seed disappoints.
A seed fixes the random rolls, but it still works within your own unlocks. If a seed calls for an item you have not unlocked yet, the game substitutes a different item that you do own.
So a seed shared by a player with a full save will not reproduce on a fresh one. Two people on the same seed with different unlock progress get different runs.
When a seed is worth it
Seeds are worth using for a few specific jobs. Practise a boss you keep losing to, or test whether two items combine the way you think. You can also learn a floor before you gamble a serious run on it.
They are not worth using when the goal is progress. Every seeded run is a run that does not count.
Daily runs and shared seeds
Daily runs use one seed per day, shared by every player. Three achievements are tied to them, and those runs have to be played online and without mods. See Isaac online co-op for what the online mode allows.
Racing and community events also fix a single seed so every runner faces identical ground. When everyone starts from the same layout, the result measures decisions rather than luck.
Where to find seeds worth trying
The r/bindingofisaac community pins verified seeds, and the game’s own egg list is the only place worth trusting for Easter Eggs. Third-party seed lists are frequently out of date, because a patch can change what a seed contains.
For the run itself, how a run works covers the floor structure a seed locks into place, and unlocking progression explains what carries over.
Floor routing decides how much of a fixed layout you see, and common Isaac mistakes covers the errors that seeds will not fix.
More on the The Binding of Isaac guides hub.