Each platform will have cross-play enabled for co-op, with up to four players able to jump in and play in one world together at any time.
343 Industries has made it clear that progress will not carry over once campaign co-op fully releases, but save progress for every player will count towards the save they’re playing from. This means players will not need a specific save for co-op or have to repeat missions even when playing as number two.
On top of co-op, the same update will also include a mission replay feature. On that note, lead world designer John Mulkey shared:
“In both solo and co-op play, you can open your Tac-Map, highlight a completed mission, and then choose replay. You will then be prompted to select the difficulty and be offered to activate any Skulls you (or your teammates) have collected.
“Setting a mission to replay resets it and teleports you (and your fireteam) to the location to tackle the challenge. You are free to wander off to do something else, fast travel away, or resume your furthest main mission.”
Players will also be able to unlock new achievements, which will target new opportunities opened up by campaign co-op and mission replay in Halo Infinite.