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Mann vs. Machine is a co-op game mode released in the update of the same name. In Mann vs. Machine, a team up to six players attempts to prevent a horde of AI-controlled robots from carrying a bomb to their base. Players can purchase upgrades for their weapons using in-game currency, called credits. All purchased upgrades will last for the duration of the game. If a player leaves during the game for any reason, providing they reconnect, their upgrades will be retained as well as their credits spent and remaining.
The only achievements obtainable in Mann vs. Machine are the Mann vs. Machievements.
Mann vs. Machine uses a separate set of "Best Moments" and thus keeps track of a separate performance report. These will appear after the usual set of "Best Moments" as a client connects to a server.
The game mode currently has five difficulties: Normal, Intermediate, Advanced, Expert, and Nightmare. Contrary to popular belief, Nightmare difficulty is actually easier than Expert difficulty and, oftentimes, Advanced difficulty.

Machines (Day 1)
Mercs (Day 2)
Bounty (Day 3)