These next tests require cooperation. Consequently, they have never been solved by a human. That's where you come in. You don't know pride; you don't know fear. You don't know anything. You'll be perfect.
— GLaDOS
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The Cooperative Testing Initiative, or simply co-op, is a multi-player gamemode and campaign in Portal 2 whereby two players can join up and take part in test courses built specifically for cooperative testing.
Players take the form of two robot test subjects built by GLaDOS, Atlas; the short and round blue bot, P-body; the tall and pea-shaped orange bot that looks like a turret with arms and legs. Each bot comes equipped with their own Handheld Portal Device with separate portal colors. The portals are capable of maintaining the flow of things, even if it enters a partner's portal, which is essential during some test chambers.
There are five official testing courses, each with a number of test chambers within, which will test both the robots' ability to solve the different puzzles as they progress. Each chamber becomes much harder than the last, with the final test taking place outside the official Aperture testing facilities. In these tests, the bots are then put to the test, so to speak, and must use every bit of knowledge earned about the previous tests in the final one.
For the last chambers of each courses, excluding the fifth, the bots are instructed by GLaDOS to go out and locate a certain disk to be inserted into a DVD player found at the end of the course. Once found and inserted within the device, the players are given a few lines from GLaDOS before she triggers a self-destruct sequence on the duo. This is the only means of retrieving them back to the Hub.
During the testing, GLaDOS will periodically chime in and say a few choice words, either to bemoan the bots' progress or point out their tiny flaws. She even goes beyond to try and break their partnership by pointing fingers at one or the other. If either both or one of the bots' stands within range of GLaDOS' monitoring cameras and perform a gesture, it will elicit her response, depending on the gesture performed.
In this campaign, the bots will be notified that their partner has been destroyed and exactly where it did. The destroyed bot will be easily rebuilt seconds later in a chute from the Reassembly Machine. The gamemode gives an infinite amount of lives, and GLaDOS will comment on every death that occurs with dry humor or irritability. As the bots progress, they will cross checkpoints that lead into the next part of the test chamber. Checkpoints allow the bot that died to respawn at the checkpoint for shorter travel time to the next testing chamber.
