Like the original Portal (2007), players solve puzzles by placing portals and teleporting between them. Portal 2 adds features including tractor beams, lasers, light bridges, and paint-like gels that alter player movement or allow portals to be placed on any surface. In the single-player campaign, players control Chell, who navigates the dilapidated Aperture Science Enrichment Center during its reconstruction by the supercomputer GLaDOS (Ellen McLain); new characters include robot Wheatley (Stephen Merchant) and Aperture founder Cave Johnson (J. K. Simmons). In the new cooperative mode, players solve puzzles together as robots Atlas and P-Body (both voiced by Dee Bradley Baker). Jonathan Coulton and the National produced songs for the game.
Valve announced Portal 2 in March 2010, and promoted it with alternate reality games including the Potato Sack, a collaboration with several independent game developers. After release, Valve released downloadable content and a simplified map editor to allow players to create and share levels.
The initial tutorials guide the player through movement controls and interactions with their environment, and in the case of the cooperative campaign, interactions with the other player. Gameplay revolves around the use of the Aperture Science Handheld Portal Device, which can create a pair of two portals on suitable surfaces through which the player or objects can pass through. Characters can use these portals to move between rooms or to "fling" objects or themselves across a distance.
Additional game elements not featured in the original Portal include Thermal Discouragement Beams (lasers), Excursion Funnels (tractor beams), and Hard Light Bridges, all of which can be transmitted through portals. Aerial Faith Plates catapult the player and objects through the air. The player must disable sentient, lethal turrets or avoid their line of sight. The Weighted Storage Cube has been redesigned, and there are new types: Redirection Cubes, which have prismatic lenses that redirect laser beams, spherical Edgeless Safety Cubes, an antique version of the Weighted Storage Cube used in the underground levels, and a cube-turret hybrid created by Wheatley after taking control of Aperture. The heart-decorated Weighted Companion Cube reappears briefly. Early demonstrations included Pneumatic Diversity Vents, shown to transport objects and transfer suction power through portals, but these do not appear in the final game. The typical objective of a test chamber or level is to use the portal gun and provided gameplay elements to open a locked exit door and progress to the next chamber.
Paint-like gels (which are dispensed from pipes and can be transported through portals) impart certain properties to surfaces or objects coated with them. Players can use orange Propulsion Gel to cross surfaces more quickly, blue Repulsion Gel to bounce from a surface, and white Conversion Gel to allow surfaces to accept portals. Only one type of gel can affect a certain surface at a time. Some surfaces, such as grilles, cannot be coated with a gel. Water can block or wash away gels, returning the surface or object to its normal state.
In the cooperative campaign, two players can use the same console with a split screen, or can use a separate computer or console; Windows, Mac OS X, and PlayStation 3 users can play with each other regardless of platform. Both player-characters are robots equipped with independent portal guns, a portal pair placed by either player is usable by both. Most chambers lack strict structure, and require players to use both sets of portals for laser or funnel redirection, launches, and other maneuvers. The game provides voice communication between players, and online players can temporarily enter a split-screen view to help coordinate actions. Players can "ping" to draw the other player's attention to walls or objects, start countdown timers for synchronized actions, and perform joint gestures such as waving or hugging. The game tracks which chambers each player has completed and allows players to replay chambers they have completed with new partners.
In the game's boss fight and ending, Chell confronts Wheatley and attaches corrupted personality cores (Nolan North) to force another core exchange and restore GLaDOS to authority. However, Wheatley demolishes the button necessary to initiate the transfer. As the facility crumbles, Chell places a portal on the Moon; she and Wheatley begin to be pulled into the vacuum of space. GLaDOS, having reasserted control over the facility, rescues Chell and abandons Wheatley to outer space. When Chell awakens, GLaDOS claims to have learned of humanity from the remnants of Caroline, but deletes Caroline's personality. Deciding that Chell is not worth the trouble of killing, GLaDOS frees her from the facility.
Initially, Valve planned to exclude portals from Portal 2. For five months, they focused on a gameplay mechanic called "F-Stop"; Valve did not discuss the specifics of the idea as they may use it in a future game. In 2020, developer LunchHouse Software revealed they were using Valve's F-Stop code in their upcoming game Exposure. The mechanic was based on an "Aperture Camera", with which users could take photos of objects, store the object in a camera, and then replace it while rotating or scaling it. Valve's F-Stop game was set in the 1980s, and would not have featured Chell or GLaDOS; instead, it followed a new test subject involved in a conflict within Aperture after Johnson, in an attempt to reach immortality, uploaded himself into an artificial intelligence and took control of a robot army. Though the playtesters liked F-Stop, they expressed disappointment at the omission of portals. Based on the playtesting feedback, Newell directed the team to reconsider direction around October 2008.
Valve aimed to teach new players the portal mechanics while still entertaining experienced players. To this end, they streamlined some elements; for example, the moving energy balls of Portal were replaced with lasers, which provide immediate feedback. To evoke a sense of nostalgia and time having passed between the games, Valve included test chambers from the original Portal; they used higher-resolution textures supported by the improved game engine, and applied decay, collapse and overgrowth effects.
The middle section of the single-player campaign takes place in large spaces where few portals can be placed, forcing players to find creative ways to cross. The architecture in these sections was inspired by photographs of industrial complexes such as CERN, NASA, and the abandoned Soviet space program. When Wheatley controls the Aperture facility, the designers "had a blast" creating deranged chambers reflecting Wheatley's stupidity. As solving constant puzzles would tire players, the designers inserted occasional "experiences" to provide respite and advance the plot.
The Repulsion (jumping) and Propulsion (running) gels in Portal 2 originated in Tag: The Power of Paint. Valve hired the Tag creators to develop the idea further and later decided to include it in Portal 2. Journalists have likened Tag to Narbacular Drop, the DigiPen student project whose mechanics became Portal. As the third Tag gel, which allows the character to walk on any surface regardless of gravity, gave playtesters motion sickness, it was replaced by Conversion gel, which integrates with the portal mechanic. The gels give the player more control over the environment, which increased the challenge for the puzzle designers. The gels are rendered using fluid dynamics routines specially developed at Valve by the former Tag Team. Rendering techniques developed for Left 4 Dead 2 were used to render pools of liquid; Portal 2 combines "flowing" surface maps to mimic the motion of water with "debris flow" maps and random noise to create realistic, real-time rendering of water effects.
Each puzzle chamber in the cooperative mode requires actions from both players. As soon as a playtester discovered a way to complete a puzzle with one set of portals, the level was sent back to the designers for further work. With few exceptions, Valve designed the chambers so that both players would remain in sight of each other to promote communication and cooperation. Some of the puzzle chambers were designed asymmetrically; one player would manipulate portals and controls to allow the other player to cross the room, emphasizing that the two characters, while working together, are separate entities. The designers soon realized that the ability to tag surfaces with instructional icons for one's partner was a necessary element, since they found this to be more effective for cooperation than simple, verbal instructions.
Valve considered a competitive mode. According to Wolpaw, the mode was similar to the video game Speedball; one team would try to transport a ball from one side of the playing field to the other using portals, while the other team would attempt to stop them with their own use of portals. Matches would commence with this objective in mind, but quickly descended into chaos. Valve realized that people enjoyed solving puzzles with portals more and therefore they focused on the cooperative mode.
The writers originally conceived several premature joke game endings if the player performed certain actions, but these required too much development effort for little payback and were scrapped. One of these joke endings was triggered by shooting a portal onto the moon's surface, after which the player's character would die from asphyxiation over a closing song, but the idea of creating a portal on the moon was incorporated into the game's final ending. The writers planned that Chell would say a single word during the ending, but this was not considered funny enough. In an early version of the script, Chell finds a lost "tribe" of turrets looking for their leader, a huge "Animal King" turret which can be seen in in-game videos of the retail product. As a reward, the Animal King would have married Chell to a turret, which would have followed Chell around the game without visible movement. The cooperative campaign was planned to feature a more detailed storyline, in which GLaDOS would send two robots to discover human artifacts, such as a comic based on a pastiche of Garfield. The writers hoped to use this idea to make the robots human-like for testing purposes, but recognized that unlike the captive audience of the single-player campaign, the two players in cooperative mode may simply talk over the story, and thus the story was condensed into very basic elements. 2b1af7f3a8