The AI has definitely improved compared to the original Metal Gear Solid. Now, when you are seen by the guards, like in Metal Gear Solid 2: Sons of Liberty, the guards can call for backup using its radio, which is a disadvantage for us. But if you don't want any backup to come, you can either paralyze/kill/tranquils the soldiers before they even call for backup.
One of the added things to the AI is a different way of how the AI thinks. In this game, the soldiers will never come at you alone. When you were seen, they will search for you and come at you with groups, much to your disadvantage. Even in the Normal difficulty, the AI will immediately shoot at you; and unlike the first game, when you do approach them, they will try to disable you by attacking using hand-to-hand combat.