Alright, people. Don't mess this up.
There are rare situations known as softlocks, where a mission certainly cannot be completed successfully, but the mission failure condition is not actually met. In this case, there may be no in-game action the player can take to progress the mission, or the mission might be continued indefinitely but fruitlessly.
Most unwinnable situations are not softlocks; for example, when the last remaining agent is overwatched by a guard with no way to escape, the player can take an in-game action to continue (i.e. the agent can say their “final words” and get shot), but cannot prolong the mission indefinitely.
If a softlock does occur, the mission can only end in failure, either by losing every agent, or by selecting “Retire Agency” from the game menu.
There is a possible softlock in the Tutorial mission which can occur when Decker drags the first Security Guard to the door before the second Security Guard, and opens the door to distract the second guard while pinning the first guard.
If this happens, the Tutorial mission script requires the player to prepare an ambush for the second guard, but this is impossible while Decker is pinning another guard. The player therefore has no in-game action to progress the Tutorial, and must retire the agency.
The OMNI Mainframe mission can become unwinnable if Central or Monst3r become lethally-wounded before completing their respective objectives, and there are no remaining Med Gels in the facility to revive them.
In the Research Facility, OMNI Foundry Lab and OMNI Mainframe missions, a softlock can occur if the agency finds themselves with no way to hack a compulsory mainframe device such as the Access Switches or Security Hub. Missions in the extended campaign after the OMNI Foundry Lab can also become unwinnable if the agency has no way to hack or reboot the Redundant Power Relay.
This can happen if the agency runs out of PWR with no way to generate more, such as when using Fusion, or Seed with a breaker which costs at least 5 PWR to use. Alternatively, it can happen if the agency sells its breaker program(s). In either case, to be a true softlock the agency must have no alternative such as hacking using Buster Chips or generating PWR from augments.
In the OMNI Mainframe mission, Central's Antiviral Proxy augment can usually be used to generate PWR by triggering a daemon, either by walking through the Infrared Emitter or firing the Overclocked Neural DART. However, the softlock can still occur if Central gets KO'd (preventing her augment from granting power), the Infrared Emitter's Power Supply is controlled by the agency and the Overclocked Neural DART becomes irretrievably lost.
The Research Facility mission can become unwinnable if all but one agent is lethally-wounded, and there are no remaining Med Gels in the facility to revive another agent. The two Access Switches are typically too far apart from each other for one agent to activate both switches on the same turn; so in this case, there is no way to unlock the exit teleporter in order to complete the mission.
The OMNI Foundry Lab mission can become unwinnable if the Quantum Reservoir is lost, missions in the extended campaign can become unwinnable if the Universal Power Cell is lost, and missions in Endless mode can become unwinnable if the Exit Access Card is lost before being used to open the Exit Door.
One of these compulsory items can become irretrievably lost if:
In the OMNI Mainframe mission, the Incognita Memory Drive should appear in Central's inventory at the start of the mission, and the OMNI Security Chip augment should be installed on Monst3r or placed in his inventory after hacking the Security Hub. If Central has no free inventory slot then the Incognita Memory Drive appears on the floor, and if Monst3r has four augments and no free inventory slot then the OMNI Security Chip appears on the floor.
Due to a bug in the game, in either case the item cannot be picked up, leaving the game in an unwinnable state. This bug is fixed in the Sim Constructor mod.
Due to a bug in the game, the OMNI Foundry Lab and OMNI Mainframe missions can become unwinnable if the Research Security Terminal, Security Hub or Mainframe Lock have Rubiks installed. If either device is hacked using a breaker which breaks exactly the number of firewalls the device has, then Rubiks leaves the device in an inconsistent state where it still has 1 firewall (so it cannot be accessed), but the device is also “controlled” by the agency (so its firewall cannot be hacked by Incognita). In this case the device cannot be used to progress the mission.
This can also occur if the device has Castle installed, if Castle targets the device itself.
When this bug occurs, the mission can still be recovered by hacking the device with a Buster Chip or the Prototype Chip. However, if the agency does not have one of these, and has no way to acquire one, then the mission becomes unwinnable. The bug is fixed by the Function Library mod.
The OMNI Foundry Lab starting room has a unique layout of cover objects which allows up to four agents to hide in cover, such that even if a guard notices an agent there, he cannot step past the cover objects to see and overwatch the agent, because the agent on that tile is blocking his only path there.
This can cause a softlock when all active agents are hidden on these four tiles, if the agency finds themselves with no way to attack or distract the guard who notices them. When this happens, the guard will continue to peek every turn without moving, also blocking the agents' only path out.
The augment Gel Injectors initially gives an agent +3 base AP, but then reduces their base AP by 1 at each alarm level; the penalty continues to apply beyond alarm level 6, down to a minimum of 0 AP. It is therefore possible for an agent with Gel Injectors to become completely immobile, if there is no other way to gain AP such as by using Stims, and the agent cannot become knocked out or lethally-wounded in order for another agent to drag them. This can happen if every surviving guard in the facility is knocked out on few enough tiles to be pinned by one or more agents with 0 base AP due to Gel Injectors, causing a softlock.