Try Portal: Cooperative, which recently received a major update.

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Portal: Cooperative is a mod that adds multiplayer functionality to the original Portal game, supporting 2-3 players. Released in 2023, it includes multiple map sets and recently migrated to a new Source Engine branch, making server creation easier. The update also brought quality-of-life improvements like a new server creation menu, pause system fixes, and improved collisions.

You Can Finally Play Portal 1 With Friends In Multiplayer