Co-op & Competition

What it is. The design of other players as a system. Two engines: co-op (players pursue a shared goal) and competition (players pursue opposed goals) — and the richest designs make players interdependent (co-op) or rivalrous (PvP) rather than merely co-located. Overcooked forces communication by making one person’s job depend on another’s; It Takes Two gives each player a different verb so neither can proceed alone.

Player fantasy / why it’s fun. You needed me / I beat you. Co-op’s pull is shared triumph and the panic-laughter of a plan under pressure; competition’s is the sharp joy of a read that beat a real mind. Both put the most reactive, least predictable system in the room — a person — at the center.

When to use / when NOT

Use it when	Skip it when
Two+ players share a screen/session	A solo experience — don’t bolt on multiplayer
Interdependence or rivalry is the point	”Multiplayer” that’s just two solo games side by side
Communication-under-pressure is the fun	Turn-based async where presence adds nothing

Interdependence is the design, not the player count. Two players who could each finish alone are playing near each other, not with each other. Force the dependency — split the verbs, split the resources, split the information — and co-op becomes a conversation (anchor-overcooked, anchor-it-takes-two).

Variants

Variant	Structure	Feels like	Anchor
Symmetric co-op	same abilities, shared goal	”divide the work”	most horde co-op
Asymmetric co-op	different verbs, shared goal	”I can’t do your job”	`anchor-it-takes-two`
Forced-comms co-op	one’s task gates another’s	”TALK to me”	`anchor-overcooked`
Symmetric PvP	mirror match	”pure skill read”	fighting/RTS
Asymmetric PvP	different roles/goals	”hunter vs. hunted”	1-vs-many
Shared-world rivalry	indirect competition	”I beat your score/ghost”	leaderboards, ghosts

Tuning levers

Lever	Effect	Watch for
Interdependence depth	how much players need each other	too much = one weak link stalls all; too little = parallel play
Communication pressure	how hard they must coordinate	time pressure forces talk (Overcooked); own it in `genre-coop-chaos`
Asymmetry balance	different roles, comparable impact	one role feeling like a passenger kills it
Rivalry stakes	what winning takes from the loser	too harsh = griefing; too soft = no bite
Solo-fallback	can one carry a struggling partner?	none = brittle; total = interdependence lost

How it wires to Hayao

Multiple players = multiple input streams into one deterministic sim. Each player’s intent flows as input actions on the shared world.state; because the sim is pure and rolls through world.rng, a co-op or PvP session is one deterministic, replayable artifact — the same seam Workshop records (docs/WORKSHOP.md, FUN.md law 7). (Grep docs/API.md before assuming any specific netcode primitive — Hayao’s guarantee is the deterministic sim, not a bundled transport.)
Rollback-with-carryover is already here. The knob-change / hot-swap “snapshot → restore → attach” contract is the same contract rollback netplay uses (WORKSHOP knob-change semantics) — the determinism you need for co-op is the determinism the engine already enforces.
Asymmetric roles reuse faction design. “Different but fair” is exactly system-faction-asymmetry applied to players — assert each role’s viability the way you’d assert a faction’s.
Shared encounters are still system-encounter-design — pockets and exit lanes now serve two bodies, not one.

Fails when…

Parallel play. Players who never need each other are two solo games sharing a screen — no co-op fun (interdependence missing).
A passenger role. In asymmetric designs, a role with no real impact benches a player — balance the roles like factions.
Griefing headroom. PvP stakes so harsh (or tools so one-sided) that winning means ruining someone’s session.
Coordination with no pressure. Co-op that permits silence rarely creates talk — pressure (time, scarcity, split info) is what forces the conversation.
Non-determinism across clients. Any wall-clock, Math.random, or iteration-order drift and the shared sim desyncs (CLAUDE.md invariant 2).

Verify

Determinism across streams: golden-hash a scripted two-player session; the shared sim replays bit-exactly regardless of which player acted when (FUN.md law 7).
Every role viable (asymmetric): assert each role’s contribution clears a floor — the faction-viability check applied per player (system-faction-asymmetry; FUN.md law 2 skill-delta shape).
Interdependence proof: a scripted “one player idles” run fails the shared objective — if solo-carry always works, the dependency isn’t real (FUN.md law 4 null-strategy shape).
Shared encounter fairness: pockets/exit-lanes hold for the group (system-encounter-design, FUN.md §4/§5).

Composes with

genre-coop-chaos — the genre where forced-comms co-op is the whole game.
system-faction-asymmetry — asymmetric roles are player-side factions.
system-encounter-design — fights now serve multiple bodies.
pattern-emergence — the most emergent system in any game is another person.