Agar.io is a massively multiplayer online action game created by Matheus Valadares. Players control one or more cells in a map representing a petri dish. The goal is to gain as much mass as possible by eating agar and cells smaller than the player's cell while avoiding larger ones which can eat the player's cells. Each player starts with one cell, but players can split a cell into two once it reaches a sufficient mass, allowing them to control multiple cells. The name comes from the substance agar, used to culture bacteria.
The game was released to positive critical reception; critics particularly praised its simplicity, competition, and mechanics, while criticism targeted its repetitive gameplay. Largely due to word of mouth on social networks, it was a quick success, becoming one of the most popular web and mobile games in its first year. A Steam version was announced on May 3, 2015 (though never released as of 2018), and the mobile version of Agar.io for iOS and Android was released on July 24, 2015 by Miniclip. Agar.io has inspired similar web games called ".io games", including games with a similar objective but different characters, and games that incorporate elements of other genres like shooter games.
The objective of Agar.io is to grow a cell on a petri dish by swallowing both randomly generated pellets (agar), which slightly increases a cell's mass, and smaller cells, without being swallowed by larger cells. The browser version currently holds five game modes: FFA (Free-For-All), Battle Royale, Teams, Experimental and Party. The mobile version of the game includes FFA (Free-For-All) and Rush Mode. The goal of the game is to obtain the largest cell; players must restart when all their cells are eaten. Players can change their cell's appearance with predefined words, phrases, symbols or skins. The more mass a cell has, the slower it will move. Cells gradually lose a small amount of mass over time.
Viruses are green, spiky circles that split large cells. Viruses are normally randomly generated, but can also be generated when receiving enough mass, to the point of splitting into two, hence creating a new virus.
Players can split their cell into two, and one of the two evenly divided cells will be flung in the direction of the cursor. This can be used as a ranged attack to shoot a cell in order to swallow other smaller cells or to escape an attack and move quickly around the map. Split cells eventually merge back into one cell. Aside from feeding viruses, players can release a small fraction of their mass to feed other cells, an action commonly recognized as an intention to team with another player. However, a small portion of the ejected mass is lost. Players can also spawn from ejected mass.