Setup multiplayer

/setup-multiplayer — Add Multiplayer Without Repainting the Project

Overview

Multiplayer is a one-way door. Pick the wrong authority model and refactoring later is brutal. Start with Summer Engine's high-level MultiplayerAPI + MultiplayerSpawner + MultiplayerSynchronizer + ENet. Do not roll custom networking without a measured reason.

Core principle: the architecture decision (peer authority vs server authority) is harder than the code. Lock it before touching a single RPC.

Steps

1. Ask the four blocking questions

Before any code:

1. What kind of multiplayer? co-op LAN / co-op online / competitive PvP / lobbies + matchmaking
2. How many players? 2 / 2–4 / 8+
3. Who runs the simulation? host (one of the players) / dedicated server / authoritative peer-to-peer
4. What's the network you're targeting? same Wi-Fi / public internet with ~80ms RTT / mobile

Wait. Each answer flips an irreversible knob.

2. Pick the authority model — irreversible decision

Lock it now. Refactoring authority is a project-killer.

Model	Best for	Pros	Cons
Host authority (one player runs the sim)	Co-op (2–4 players), casual PvP, friends-only	Cheapest. No server bill. Host migration possible.	Host advantage (no lag for them). Cheating possible.
Dedicated server authority	Competitive PvP, matchmaking, > 4 players	Fair. Anti-cheat possible. Stable.	Server bill. Need a deploy story. Higher complexity.
Peer-to-peer with shared authority	Lock-step strategy / RTS, fighting games (rollback)	Symmetric latency. No "host advantage".	Determinism is a project on its own. Hard to debug.

Default recommendation: host authority for co-op (2–4 players), dedicated server for everything else. If user says "competitive PvP" + "lobbies": dedicated.

State the choice explicitly:

Going with host authority for 2–4 player co-op. The first player to start a game is the host. Their scene tree is the source of truth; clients receive replicated state and send input RPCs. Host migration is hard with this model — if the host quits, the game ends. Confirm?

3. Pick the transport

Almost always: ENetMultiplayerPeer. It is UDP with reliability layers and ships with Summer Engine. Validate the actual game topology and player count; this is local netcode, not managed hosting or matchmaking.

When to switch:

• WebSocketMultiplayerPeer: if you need the game to run in the browser (HTML5 export). ENet doesn't work in browsers.
• WebRTCMultiplayerPeer: if you need browser AND P2P (no central server). Higher complexity.
• Custom transport: never, unless you've shipped multiplayer before.

4. Pick what to replicate — the replication graph

Replicate the minimum. Every replicated field is bandwidth + bug surface.

For a typical co-op shooter:

Object	Replicated state	Replication frequency	Authority
Player position / rotation	x, y, z, yaw	20 Hz	Owner peer
Player health	int	On change	Server/host
Player input intent	move_dir, jump, fire	30 Hz	Owner peer (sends to host)
Enemy state	position, hp, FSM state	10 Hz	Host only
World items (pickups)	spawn / despawn events	On change	Host only
UI / menus	nothing	nothing	Local only
Particles, decorative VFX	nothing	nothing	Local only (each peer plays its own)

Don't replicate:

• Local UI state.
• Cosmetic-only VFX (each peer simulates locally).
• Camera / view position (each peer has their own camera).
• Audio (use 3D audio with replicated positions, not replicate the AudioStreamPlayer).

5. Read the scene to find what changes

summer_get_scene_tree
summer_inspect_node "./World/Player"

Note: the existing single-player Player needs to become a per-peer Player. The path is to spawn one Player per connected peer.

6. Propose the change set

I'm about to:

• Add a Network autoload (scripts/network.gd) that owns peer setup + spawn lifecycle.
• Convert ./World/Player from a static scene node into a MultiplayerSpawner-managed instance, spawned per peer on join.
• Add a MultiplayerSynchronizer to the player scene replicating position, rotation.y, health.
• Convert player input from local → input intent RPC sent to the authority.
• Add a basic host/join UI on the main menu.

Authority model: host. Transport: ENet. Tick: 30 Hz input, 20 Hz position. Confirm before I touch anything?

7. Build it

a) Network autoload

scripts/network.gd:

class_name Network
extends Node

const PORT := 7000
const MAX_PLAYERS := 4

signal player_joined(peer_id: int)
signal player_left(peer_id: int)

@export var player_scene: PackedScene

var _peer: MultiplayerPeer

func host_game() -> Error:
    var enet := ENetMultiplayerPeer.new()
    var err := enet.create_server(PORT, MAX_PLAYERS - 1)
    if err != OK:
        return err
    _peer = enet
    multiplayer.multiplayer_peer = _peer
    multiplayer.peer_connected.connect(_on_peer_connected)
    multiplayer.peer_disconnected.connect(_on_peer_disconnected)
    # Host spawns its own player at id=1 (server id)
    _spawn_player(1)
    return OK

func join_game(address: String) -> Error:
    var enet := ENetMultiplayerPeer.new()
    var err := enet.create_client(address, PORT)
    if err != OK:
        return err
    _peer = enet
    multiplayer.multiplayer_peer = _peer
    return OK

func _on_peer_connected(peer_id: int) -> void:
    # Only the host spawns players. Clients receive via MultiplayerSpawner.
    if multiplayer.is_server():
        _spawn_player(peer_id)
    player_joined.emit(peer_id)

func _on_peer_disconnected(peer_id: int) -> void:
    var node_name := str(peer_id)
    var players := get_tree().get_first_node_in_group("players_root")
    if players != null and players.has_node(node_name):
        players.get_node(node_name).queue_free()
    player_left.emit(peer_id)

func _spawn_player(peer_id: int) -> void:
    var player: Node = player_scene.instantiate()
    player.name = str(peer_id)        # name = peer id; sync by name
    player.set_multiplayer_authority(peer_id)
    var players_root := get_tree().get_first_node_in_group("players_root")
    players_root.add_child(player, true)

Register as autoload in project.godot:

summer_project_setting(key="autoload/Network", value="*res://scripts/network.gd")

b) Player scene with MultiplayerSynchronizer

In the player scene, add:

Player (CharacterBody3D)
├── CollisionShape3D
├── Camera3D                         # local-only, attach in script if peer is local
├── MeshInstance3D
└── MultiplayerSynchronizer

summer_add_node(scenePath="res://scenes/player.tscn", parent="./World/Players/Player", type="MultiplayerSynchronizer", name="Sync")

scenePath is required on summer_add_node, summer_set_prop, summer_connect_signal and summer_save_scene; it names the exact res:// scene to mutate and does not default to the open tab.

Configure the synchronizer to replicate position, rotation.y, health. A standalone SceneReplicationConfig.tres is worth it here because the same config is shared by every player instance — not because embedded sub_resources fail (they don't):

# scripts/player_net.gd
extends CharacterBody3D

@export var move_speed: float = 5.0
@export var max_health: int = 100

@onready var camera: Camera3D = $Camera3D
@onready var sync: MultiplayerSynchronizer = $Sync

var health: int = 100

func _ready() -> void:
    health = max_health
    var auth := get_multiplayer_authority()
    var is_local := auth == multiplayer.get_unique_id()
    camera.current = is_local
    set_physics_process(is_local)        # only the owner runs movement
    set_process_input(is_local)

func _physics_process(delta: float) -> void:
    var input_dir := Input.get_vector("move_left", "move_right", "move_forward", "move_back")
    var direction := (transform.basis * Vector3(input_dir.x, 0, input_dir.y)).normalized()
    if direction:
        velocity.x = direction.x * move_speed
        velocity.z = direction.z * move_speed
    else:
        velocity.x = move_toward(velocity.x, 0, move_speed)
        velocity.z = move_toward(velocity.z, 0, move_speed)
    if not is_on_floor():
        velocity.y -= 9.8 * delta
    move_and_slide()
    # position is replicated automatically via MultiplayerSynchronizer

@rpc("any_peer", "call_local", "reliable")
func take_damage(amount: int) -> void:
    if not multiplayer.is_server():
        return                                          # only host applies damage
    health = max(0, health - amount)
    if health == 0:
        rpc("on_died")

@rpc("authority", "call_local", "reliable")
func on_died() -> void:
    queue_free()

c) RPC patterns — the four flavors

Lock the conventions:

RPC config	Use case
@rpc("authority", "call_local", "reliable")	Host tells everyone something happened (health change result, death)
@rpc("any_peer", "call_remote", "unreliable")	Frequent input intent (movement, look) — drop is OK
@rpc("any_peer", "call_remote", "reliable")	Rare input event (fire weapon, pick up item) — drop NOT OK
@rpc("any_peer", "call_local", "reliable")	Player asks host to do something (request damage)

Authority validation is mandatory. Inside any any_peer RPC, check multiplayer.get_remote_sender_id() and validate the sender owns the action.

@rpc("any_peer", "call_local", "reliable")
func request_fire() -> void:
    var sender := multiplayer.get_remote_sender_id()
    if sender != get_multiplayer_authority():
        return                       # someone else is trying to control this player; drop
    # ... actually fire

d) Lag compensation outline

Defer to a specialist skill, but flag the basics:

• For projectiles: spawn locally on owner's machine at fire time, replicate the spawn event with a timestamp. Other peers fast-forward the projectile by their RTT.
• For hit detection: server-authoritative + simple latency tolerance (host accepts hits within ~100ms past server time).
• For movement: client-side prediction plus server reconciliation is the standard model. Summer Engine does not provide this game-specific layer; you write it. Competitive PvP requires specialist lag-compensation work.

8. Save and verify

summer_save_scene(scenePath="res://scenes/player.tscn")
summer_get_script_errors(path="res://scripts/player_net.gd")   # one file per call; path required
summer_play
# user tests by running two instances locally (Debug → Customize Run Instances → 2)
summer_stop

Test checklist:

• Two instances connect (one hosts, one joins).
• Each instance sees its own player as local (camera attached, can move).
• Each instance sees the other player move smoothly (sync at 20 Hz).
• Damage RPC routes through host (not double-applied on each peer).
• Disconnect cleans up the disconnected player's node.

Common mistakes

Don't	Do	Why
Decide authority model halfway through	Lock it in step 2 before touching code	Refactoring authority is a project-killer
Replicate every property	Replicate position, rotation, health, that's it	Bandwidth + bug surface
Run physics on every peer for every player	Physics on owner only, others receive transform	Otherwise jitter and divergence
Trust the client	Validate get_remote_sender_id() in every any_peer RPC	Cheaters route through the wrong RPC
Copy-pasting a SceneReplicationConfig per player scene	One standalone .tres, referenced everywhere	Replication config must match on every peer; duplicated copies drift
Roll custom transport	ENet (or WebSocket for browser)	You will hit edge cases ENet already solved
Skip authority check in damage RPC	if not multiplayer.is_server(): return at the top of damage handlers	Otherwise damage applied N times
Spawn players on every peer	MultiplayerSpawner — host spawns, clients receive	Otherwise N copies of each player
@rpc("any_peer") everything	Use authority for server→clients, any_peer only for client→server	Mismatched flow = exploits
Camera replicated	Camera is local-only, set current = is_local on spawn	Each peer needs their own camera

Anti-patterns

• "Let's also have rollback netcode for our co-op shooter." Rollback is a 6-month project on its own. Use it for fighting games and lock-step RTS, not co-op.
• NAT punchthrough on day one. Use Steam Networking or a relay; don't write punchthrough yourself for v1.
• Dedicated servers without a deploy story. If you can't redeploy in < 5 min, the dedicated path isn't ready.
• Custom serialization. Summer Engine's variant serialization is fine. Do not write your own packing until you measure bandwidth.
• Host migration in v1. It's a feature, not a primitive. Ship without it; add later if churn data demands.

Collaborative protocol

This skill rewires the project at the architectural level. Always ask before each block of changes. Group writes — "I'm about to add the autoload + 2 scenes + 3 scripts + InputMap entries. OK?".

Want a working starter?

For a known-good multiplayer scaffold (host + join UI, ENet transport, MultiplayerSpawner, replicated player), point users at:

→ template-id: none ships today. Run summer list templates for the live list rather than naming one from memory.

For now: this skill produces the scaffold inline. When the template lands, link from here.

See also

• multiplayer-and-networking/multiplayerapi-basics/SKILL.md — deeper MultiplayerAPI patterns
• multiplayer-and-networking/client-server-pattern/SKILL.md — dedicated server architecture
• multiplayer-and-networking/peer-replication/SKILL.md — synchronizer config patterns
• multiplayer-and-networking/enet-vs-websocket/SKILL.md — transport choice
• multiplayer-and-networking/lag-compensation/SKILL.md — deeper netcode for competitive (when shipped)
• multiplayer-and-networking/state-sync/SKILL.md — selective replication patterns
• multiplayer-and-networking/lobby-and-matchmaking/SKILL.md — lobby UI + Steam matchmaking