git:20260716.53f9451 to git:20260911.7afa6ea

---

name: online-multiplayer-and-game-backend

description: Use when designing or implementing authoritative multiplayer topology, replication, RPCs, prediction, reconciliation, matchmaking, sessions, reconnect, game backend persistence, regional hosting, protocol evolution, or network tests.

metadata: {portable: true, compatible_with: [claude-code, codex]}

---

# Online Multiplayer and Game Backend

Make authority, replicated state, session transitions, persistence, and degraded network behaviour explicit and testable.

## Use When

- A game has remote players, lobbies, matchmaking, dedicated/listen servers, replicated actors/state, or backend-owned progression.

## Do Not Use When

- The request is local multiplayer only.

- Anti-cheat, moderation, or infrastructure deployment is the main task; compose with their specialist skills.

## Required Inputs

Engine/version, player/region/load envelope, topology, trust boundaries, tick/update budgets, gameplay state model, matchmaking rules, persistence ownership, privacy/security constraints, service objectives, cost limits, and version policy.

## Workflow

1. Choose topology from threat, fairness, latency, cost, and operational evidence; name the authority for every mutable field.

2. Specify session states from discovery through allocation, join, play, disconnect, reconnect, migration/termination, and cleanup.

3. Define replicated state, relevance/frequency, RPC direction/reliability/idempotency, prediction/reconciliation, and late-join snapshots.

4. Separate ephemeral match state from durable profile/economy state; make retries and ownership transfers safe.

5. Version protocol and content contracts; test compatible skew and reject incompatible clients with a recoverable message.

6. Exercise latency, loss, jitter, reorder, duplication, disconnect, reconnect, server crash, region loss, overload, and malicious input.

## Quality Standards

- The server validates consequential actions; a client request is never proof that an action was legal.

- Reliable delivery is used only for state that must arrive; queues must be bounded.

- Tests retain seed, build/protocol version, topology, network profile, server logs, and outcome.

## Outputs

Authority matrix; protocol/session contracts; replication budget; backend data ownership map; network test pack; capacity/region plan; compatibility and rollback runbook.

## References

- [Multiplayer authority and session gates](references/multiplayer-authority-session-gates.md)