Here are the differences between the difficulty levels:
Passive
Bots will stay in their lanes the entire game.
Bots will not use any active abilities.
Easy
Reaction times for ability/item usage range from 200-300ms.
Last-hitting has a 400ms window of random timing uncertainty.
Bots do not try to avoid overlapping stuns.
Bots do not deny creeps.
Bots do not harass.
Bots do not roam or gank.
Bots do not try to interrupt channels.
Bots will not use Force Staff, Orchid, or Phase Boots.
Bots do not try to avoid linear projectiles.
Bots never figure out that a hero is an illusion.
Bots will try to avoid forming groups of larger than 3.
Medium
Reaction times for ability/item usage range from 100-150ms.
Last-hitting has a 200ms window of random timing uncertainty.
Bots do not try to avoid overlapping stuns.
Bots do not deny creeps.
Bots do not harass.
Bots do not roam or gank.
Bots do not try to interrupt channels.
Bots do not try to avoid linear projectiles.
Bots will try to avoid forming groups of larger than 4.
Hard
Reaction times for ability/item usage range from 50-75ms.
The baseline difficulty.
All features are enabled.
Bots do not cheat (with a couple minor exceptions).
Unfair
Bots have no reaction time delay.
All bots get a 25% bonus to both gold and XP.
Bots will not disable themselves to balance teams when an opposing player disconnects.
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More stuff will be hooked up over time, I'll try to keep this updated.
Passive
Bots will stay in their lanes the entire game.
Bots will not use any active abilities.
Easy
Reaction times for ability/item usage range from 200-300ms.
Last-hitting has a 400ms window of random timing uncertainty.
Bots do not try to avoid overlapping stuns.
Bots do not deny creeps.
Bots do not harass.
Bots do not roam or gank.
Bots do not try to interrupt channels.
Bots will not use Force Staff, Orchid, or Phase Boots.
Bots do not try to avoid linear projectiles.
Bots never figure out that a hero is an illusion.
Bots will try to avoid forming groups of larger than 3.
Medium
Reaction times for ability/item usage range from 100-150ms.
Last-hitting has a 200ms window of random timing uncertainty.
Bots do not try to avoid overlapping stuns.
Bots do not deny creeps.
Bots do not harass.
Bots do not roam or gank.
Bots do not try to interrupt channels.
Bots do not try to avoid linear projectiles.
Bots will try to avoid forming groups of larger than 4.
Hard
Reaction times for ability/item usage range from 50-75ms.
The baseline difficulty.
All features are enabled.
Bots do not cheat (with a couple minor exceptions).
Unfair
Bots have no reaction time delay.
All bots get a 25% bonus to both gold and XP.
Bots will not disable themselves to balance teams when an opposing player disconnects.
---
More stuff will be hooked up over time, I'll try to keep this updated.
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