Making alliances is a good strategy for finding allies, hunting in packs and quickly defeating the enemy. You can more easily block other cells in a corner
A very fun tactic that can pay off in Agar.io is to split the opponent in half with the space bar. You will then put yourself between two of your enemy's cells to prevent them from merging.
Cells of the same size cannot be eaten. Don't waste time trying to eat them and focus on cells that are smaller than you. You can find out the mass of the opposite cells by checking the "Show mass" option in the settings.
Viruses, green balls, are very dangerous in Agar.io; You must never touch them or you will lose. But especially when your cell is still small, you can use it as a shield by placing yourself behind it. So large cells can burst on the virus. With a little luck, you can even eat leftovers...
At the beginning of the game, when you're still young, you really have to avoid corners. This is the best way to get blocked by bigger guys. However, if you've already gained weight, it's a good strategy to try to push around corners to thwart your opponents and eventually swallow them!
The W button can help you gain agility as it will detach part of your cell, making you faster. Additionally, in a team game, it can enlarge a cell of your color to strengthen it.
By pressing the spacebar you split your cell in half, one of the halves is pushed forward. This is very helpful when you find yourself in a difficult situation to escape quickly. It's also an effective way to quickly eat smaller cells, knowing that after a few minutes your two cells will combine to become one and the same.