With a game as popular as Agario, it is no surprise that bots have been made. The bots designed for Agar.io have threat detection algorithms while, surprisingly, not considered cheating. They are very easy to detect, you can detect a bot by looking at the way it moves. Most bots go for the small cells. You can usually see them moving jaggedly and abnormal, making these bots an easy target. They don't break any game mechanics, nor do they claim any superior advantage via code. It's entirely via the use of AI that they stand a chance at all. Popular examples of bots are "AposBot", "LioBot", and "NotReallyABot". "TigerBot" is another recent development. "TigerBot" has a very advanced AI, and it can split-eat you! Be careful, especially if you see "TigerBot" split into two cells. If it's done it once, it'll do it again. Bots can also cause the game to lag.
Feeder Bots usually consist of 7 to 10 bots in one server. They work by having one person install a program in their browser which will track which Agar.io gamemode and server they go into and brings the bots with them, these bots will then follow the mouse cursor of whomever controls them and are used to feed them, hence the name "Feeder Bots". Every time a bot is eaten they will immediately respawn and go back to the mouse cursor. This type of bot has recently become generalized and seen throughout the game. It's an unfair advantage as they can cause lag and track players to give them extra mass. Popular examples of feeder bots are "AgarVIPBots.com", "AgarBot.ovh", and "OP-Bots.com". "Morebots.ovh" is another recent source. Once a player leaves the server, all of the bots will go to the center of the map endlessly, being eaten and respawning until the server closes. Be careful! Some bots may be within a virus. So maybe you should check twice. Make sure to keep a heads up for these bots!