godot-multiplayer

# 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

1. **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.
2. **Handle connection signals** on `multiplayer`: `peer_connected(id)`,
   `peer_disconnected(id)`, `connected_to_server`, `connection_failed`,
   `server_disconnected`.
3. **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.
4. **Keep RPC signatures identical** on every peer that runs the script — Godot checksums
   all `@rpc` methods in a script; mismatches break silently.
5. **Assign authority** per node with `set_multiplayer_authority(id)`; gate input/RPCs by
   `is_multiplayer_authority()`.
6. **Replicate state** with `MultiplayerSpawner` (auto-instances scenes on clients) and
   `MultiplayerSynchronizer` (auto-syncs selected properties).
7. **Validate on the server.** Don't trust client-reported positions/results.

## Patterns

### 1. Host or join (ENet)

```gdscript
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)

```gdscript
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)

```gdscript
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)

```gdscript
# 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.