Which Assassin's Creed game was the first to feature playable co-op missions?

Assassin's Creed Unity was the first Assassin's Creed game to feature playable cooperative missions.

Released in 2014, Unity lets up to four players join selected missions and heists as different versions of Arno Dorian. Cooperative activities use the same Paris open world as the main campaign, while allowing participants to coordinate movement, stealth, combat, and objectives.

The feature was distinct from earlier multiplayer modes. Brotherhood and later games included competitive or social multiplayer experiences, but those modes did not provide the same four-player cooperative story missions. Unity's co-op missions were designed around teamwork and could be played online.

Assassin's Creed Syndicate followed Unity and included a separate two-player local mode for certain activities, but it did not replace Unity's role as the series' first game with this form of cooperative mission play. Unity's single-player story remains centered on Arno and the French Revolution.