godot-multiplayer

SKILL.md

Godot Multiplayer (4.x high-level)

Connect peers, call functions remotely with @rpc, assign authority, and replicate state
with MultiplayerSpawner/MultiplayerSynchronizer. Targets Godot 4.7 (ENet). Treat
all client input as untrusted; keep the server authoritative.

When to use

Use when adding networked multiplayer: hosting/joining over ENet, calling RPCs,
assigning per-node authority, or auto-spawning/syncing nodes across peers.

When not to use: local split-screen (no networking); raw TCP/UDP/WebSocket protocol
work (low-level PacketPeer); HTTP requests. For save/persistence → save-systems.

Core workflow

Create a peer (ENetMultiplayerPeer), call create_server(port, max) or
create_client(ip, port), and assign it to multiplayer.multiplayer_peer. The
server's unique ID is always 1; clients get random positive IDs.

Handle connection signals on multiplayer: peer_connected(id),
peer_disconnected(id), connected_to_server, connection_failed,
server_disconnected.

Define RPCs with @rpc(...). Call them on a Callable via rpc() (all peers) or
rpc_id(peer_id) (one peer). Inside, multiplayer.get_remote_sender_id() tells you who
sent it.

Keep RPC signatures identical on every peer that runs the script — Godot checksums
all @rpc methods in a script; mismatches break silently.

Assign authority per node with set_multiplayer_authority(id); gate input/RPCs by
is_multiplayer_authority().

Replicate state with MultiplayerSpawner (auto-instances scenes on clients) and
MultiplayerSynchronizer (auto-syncs selected properties).

Validate on the server. Don't trust client-reported positions/results.

Patterns

1. Host or join (ENet)

const PORT := 7000
const MAX_PLAYERS := 8

func host() -> void:
    var peer := ENetMultiplayerPeer.new()
    var err := peer.create_server(PORT, MAX_PLAYERS)
    if err != OK:
        push_error("Cannot host: %s" % err); return
    multiplayer.multiplayer_peer = peer
    multiplayer.peer_connected.connect(_on_peer_connected)

func join(ip := "127.0.0.1") -> void:
    var peer := ENetMultiplayerPeer.new()
    peer.create_client(ip, PORT)
    multiplayer.multiplayer_peer = peer
    multiplayer.connected_to_server.connect(func(): print("connected"))

func leave() -> void:
    multiplayer.multiplayer_peer = OfflineMultiplayerPeer.new()

2. RPCs: client sends input to the server (any_peer, call_local)

func _unhandled_input(event: InputEvent) -> void:
    if event.is_action_pressed("fire") and is_multiplayer_authority():
        request_fire.rpc_id(1)          # send only to the server (id 1)

# Clients may call this; it runs on the server (and locally if server is a player).
@rpc("any_peer", "call_local", "reliable")
func request_fire() -> void:
    var sender := multiplayer.get_remote_sender_id()
    if not _can_fire(sender):           # server-side validation
        return
    spawn_projectile.rpc(sender)        # tell everyone to spawn it

@rpc("authority", "call_local", "reliable")
func spawn_projectile(owner_id: int) -> void:
    _do_spawn(owner_id)

3. Per-node authority (each player controls their own avatar)

extends CharacterBody2D

func _ready() -> void:
    # The node name is the owning peer's id; that peer is the authority.
    set_multiplayer_authority(name.to_int())

func _physics_process(delta: float) -> void:
    if not is_multiplayer_authority():
        return                          # only the owner reads input & moves
    velocity = Input.get_vector("left", "right", "up", "down") * 200.0
    move_and_slide()

4. MultiplayerSynchronizer config (editor + replication)

# Add a MultiplayerSynchronizer child; in its Replication editor add the properties to
# sync (e.g. position, velocity). Set "Sync"/"Spawn" flags per property. From code you
# can scope visibility:
@onready var sync: MultiplayerSynchronizer = $MultiplayerSynchronizer

func _ready() -> void:
    # Only replicate this node to a specific peer (e.g. private info).
    sync.set_visibility_for(target_peer_id, true)

Pitfalls

RPC signature checksum. Every @rpc method in a script must exist with the same
declaration on both client and server builds — even unused ones. A mismatch causes
errors that may point at the wrong function. Argument names/count are not checked, but
the set of RPCs and their annotations are.

Default @rpc is "authority". Clients calling it are ignored unless you set
"any_peer". Use "call_local" so the host (also a player) runs it too.

NodePaths must match across peers. RPC routing uses the node's path/name; spawn nodes
with identical names on all peers (use MultiplayerSpawner or add_child(node, true) for
readable, deterministic names).

Trusting the client. Never let clients set authoritative state (health, position,
hits) directly. Send intent, validate on the server, then broadcast results.

RPC on non-Node classes fails. @rpc methods must be on Node-derived classes, not
plain Resource/RefCounted.

RPCs don't serialize Objects/Callables. Pass plain data (ints, strings, arrays,
dictionaries, PackedArrays).

Forgetting to reset the peer. To disconnect cleanly, set
multiplayer.multiplayer_peer = OfflineMultiplayerPeer.new().

Android needs INTERNET permission in the export preset or all networking is blocked.

References

For MultiplayerSpawner setup, transfer modes/channels, SceneMultiplayer
authentication (auth_callback/complete_auth), a lobby skeleton, and dedicated-server
export notes, read references/replication-and-rpc.md.

Related skills

godot-nodes-scenes — instancing the scenes that get spawned/synced.

godot-signals-groups — connection signals and event flow.

godot-export — exporting a headless dedicated server build.