The Cooperative Testing Initiative, or Co-op for short, is the separate game mode where two players can join up and take part in test chambers built specifically for cooperative play. The players take the form of either Atlas, the short and blue bot, or P-body, the tall and orange bot. 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.
Atlas and P-body, the Co-op bots, in
Portal 2
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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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There are five test courses, each with several testing chambers within, which will test both players' ability to solve the different puzzles as they progress. Each chamber becomes much harder than the last, with the final test taking place outside of the official Aperture testing facilities. In these tests, the players 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. The last chambers of each course, excluding the fifth, the players are told to go out and locate a certain disk for the 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 exploding violently. This is the only means of getting out of the test chamber. Upon completion, players are then returned to the Hub.
During the testing, GLaDOS will periodically chime in and say a few choice words, either to bemoan the players' 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 the player stands within range of one of GLaDOS' monitoring cameras and perform a gesture, it will elicit a response from GLaDOS, depending on the gesture performed. Players do not have to fear death while testing, as it's commonplace in the machine world. If a player dies, the other player will be given an indication that the other has died and where they died. The dead player will then respawn moments later from the chute the Reassembly Machine sends them through, ready for a second try. The players have an infinite amount of lives, and GLaDOS will comment on every death that occurs with dry humor or irritability. As players progress, they will cross checkpoints that lead into the next part of the test chamber. The checkpoints will allow players who die to respawn at the checkpoint for shorter travel time to the next testing chamber.
Course One: Team Building
This is one of the first courses available to the player upon entering the Hub, the others remaining locked until the other courses are completed. In this course, the players are given basic tests that emphasize the use of teamwork through portals and pinging, which is a function that allows players to communicate even without text or even through voice. Pings come in a variety of forms, from a simple "Look" ping, to a "Timer" ping that can be used to coordinate timed events effectively.
Course Two: Mass and Velocity
After completing Course One, this course will unlock for players to continue testing. This chamber utilizes the effects portals have when an object has both mass and velocity as it enters the portal, meaning the players must utilize both in order to pass this course.
Course Three: Hard-Light Surfaces
This course opens up upon completion of Course Two. In this course, the players must make use of the Light Bridges to both navigate chambers and act as a wall for other chambers as a defense mechanism or a creative way of stopping something from hurtling into a water hazard.
Course Four: Excursion Funnels
This course unlocks upon completion of Course Three. During this course, the player will have to make use of both Light Bridges and Excursion Funnels in order to complete these chambers. It will employ elements from the previous courses as well, making it much more difficult as the players progress.
Course Five: Mobility Gels
This is the final course that unlocks after the completion of Course Four. Upon entering, GLaDOS will chime in the explain the purpose of the testing and what their true purpose is. The players are then dropped into the defunct Aperture testing spheres encountered during single player, and must make use of all elements from the previous chambers along with the Gels that are found in the course. The final course is very difficult, requiring both players to act as a true team. The last chamber is halted by a large vault door, which holds the frozen humans inside. Both players must perform any gesture to ultimately unlock the door.
Additional Course: Art Therapy
Set after the conclusion of the Mobility Gels course, GLaDOS has reassembled the bots for testing again and claims that it has been millions of years since they were last reassembled. As the testing progresses, the bots are able to observe the defunct surroundings of the Enrichment Center, which GLaDOS assures them as part of an "art exhibit". Further on in the course, players will be sent to a Robot Assembly Annex station to repair their own reassembly machines as GLaDOS finally makes it clear that it has only been a week since the Mobility Gels course and that she needs their help in restoring her control over the facility as something is overriding her controls. At the end of the course, both players are sent out of the testing area to resolve the problem in a control room housing a prototype chassis of the central core.
Interestingly, the sixth course was initially going to consist of advanced test chambers during Portal 2's development, months before the game was released. However, this was scrapped and was removed from the final game. The Peer Review DLC to Portal 2 instead adds only challenge modes to single-player and co-op, with no sign of advanced test chambers. The Art Therapy course is referred to as an additional course instead of the "sixth course".
