Godot multiplayer (network programmer)

Target: Godot 4.7.2, SceneMultiplayer over ENet and WebSocket.

Expert level here means the networked game is proven with several processes, under injected latency, before anyone plays it: authority is a written decision per object, every client message is a request the server checks, and replication cost is measured in KiB/s. This skill builds the replication in code, runs a server and 2 to 3 clients headless on localhost (plus a browser client), and passes only on numbers from those runs. If a sibling skill named here is missing from your available skills, ask the user to install it (npx skills add scenario-labs/skills --skill <name>); unattended, proceed from tool schemas and flag the gap.

REQUIRED BACKGROUND: scenario-godot-expert (channels, gd_run, review loop, 4.7 traps).

Status (2026-10-02): 16 live tests (up to 3 clients, ENet, WebSocket, exported server, headless Chrome) and 18 offline tests pass. Evidence: tests/live_evidence/godot-multiplayer/.

Stance (the expert delta)

1. Decide authority per object before writing code. Someone must own each object's data (AndrooDev, jff9oxO8v1s [00:04:01]). Client authority over the body is fine for casual co-op; competitive or cheat-prone games keep the body on the server and give clients their input only (Tamas Galffy, iP_xdJ0peFo [00:03:46, 00:06:47]; BatteryAcidDev, GqHTNmRspjU [00:00:33]). Deciding condition: cost of cheating against cost of input lag. Measured at 250 ms RTT: server authority shows your own move after 294 to 310 ms, client authority after 11 to 22 ms, and a client-authority cheater's 50 m jump stuck.
2. Set authority from the node name in _enter_tree. Name each player node with its peer id; _enter_tree calls set_multiplayer_authority(name.to_int()) so it holds before _ready (Travis Hunter, tK2ACXUGcrY [00:46:16]). Do not read a spawn-synced property there: on every client copy measured, peer_id was still 0 in _enter_tree.
3. Every any_peer RPC is a request. Read multiplayer.get_remote_sender_id() first (it is 0 after an await, measured), check that the sender owns the target and that the values are possible, then act. A forged teleport for another player and a 30 m jump were both rejected in the live test.
4. The RPC checksum compares method names only, whatever the docs say. The high-level docs say it covers every @rpc declaration and skips arguments; two live runs in 4.7.2 (M2, round 1 and round 2) disagree. A name missing on one side logs "The rpc node checksum failed" on both sides and RPCs are matched by sorted index: the client called b and the server ran a. A different mode only fails per call ("not allowed"), a different argument count fails per call ("expected 2 argument(s)"), different call_local or transfer mode passes silently. Keep server and client on one script per node path (one codebase: Davide Di Staso, nyKBuM9Y_-Q).
5. Test with latency and three players. Localhost hides every latency bug; two players hide what a bystander sees (Galffy, iP_xdJ0peFo [00:09:58, 00:42:31]). gd_net.UdpLagProxy adds delay, jitter and loss on localhost without root.
6. Replication cost is a design input. ALWAYS sends every network frame whether or not the value changed; ON_CHANGE is reliable and costs more than ALWAYS while values move (measured: 72.6 against 100 KiB/s for 50 moving props). A headless server is uncapped: 145 fps here, 2.2 to 2.4 times the bandwidth of 60 fps. Cap it (--max-fps, Engine.max_fps) or set replication_interval. Cut by visibility (Di Staso [00:14:29]).
7. Know which connection event fires where. On a joining client, peer_connected fires once per existing peer, server id 1 included; connected_to_server fires on clients only, so send client info there, not from peer_connected (docs delta 7; FinePointCGI e0JLO_5UgQo [00:22:50]). After the server leaves, get_unique_id() reads 0: skip id 0 in registries and spawn loops (pA9RGn87Uag [00:47:31]). All measured (M14).
8. The server stops speed hacks, not wall hacks. Server authority blocks impossible moves and infinite health; it does not stop wall hacks or aimbots, because every client still receives everyone's position. Interest management (synchronizer visibility, server line-of-sight checks) limits the leak (MfjEB1cowsE [00:18:55]; iP_xdJ0peFo [00:53:28]).
9. The dedicated server is the same project exported in "dedicated server" mode. That mode adds the dedicated_server feature, which forces headless display and dummy audio even without --headless (measured on the exported macOS binary), and Strip Visuals removed the 3.1 MB texture from the server pack (BatteryAcidDev, jgJuX04cq7k [00:04:47]).

Establish first

Genre and player count; who may cheat (competitive, co-op, friends only); platforms (browser means WebSocket or WebRTC, no ENet); topology (dedicated server, listen server, relay or P2P); a backend already chosen (Steam, Nakama, Colyseus, own server); hosting budget (cloud costs money: ask before any deployment); tick and target RTT (default 60 Hz server, play tested at 100 and 250 ms). Defaults: one project, server authority with input-only clients for anything competitive, ENet for desktop and mobile, WebSocket behind TLS (wss://) for the web.

Workflow

1. Network design note. Per object: owner, what replicates (synchronizer property and mode), what is an event (RPC with mode, transfer and channel), what a client may request. Streams of unreliable_ordered updates get their own transfer channel, channel last in @rpc (docs deltas 4, 5); idle players send nothing below a velocity threshold (pA9RGn87Uag [00:42:16]). GATE: no object without an owner; no client-owned health, score or inventory unless the brief accepts cheating.
2. Build the replication in code. gd_net.install_netkit(P), then net_build.gd (make_sync, make_spawner) writes scenes with SceneReplicationConfig set in code; the Replication panel is for humans (references/gui-paths.md). Players: name = peer id, add_child(node, true), authority in _enter_tree. GATE: net_audit.gd:scene has no flags (root paths resolve, no Object or RID property, no @Node@2 names, intervals match modes).
3. Session test on localhost. gd_net.session(P, roles) runs a server and 2 to 3 clients, each in its own APFS clone, all machine slots held first. Cover the lobby gate, a late joiner, a leaver, cheat RPCs, and a password through auth_callback plus complete_auth on both sides (P5). GATE: every client sees every player, the late joiner sees all, the leaver's node is freed everywhere, authority report correct, forged RPCs rejected, logs clean.
4. Latency pass. Rerun through UdpLagProxy at 100 and 250 ms, then 250 ms with 30 ms jitter and 5 % loss; replication_delay() measures how far behind the server truth a remote client shows a player. GATE: the numbers fit the genre, or prediction is planned (netfox or own).
5. Bandwidth pass. Server KiB/s per client from ENet counters (bandwidth in the result), per mode, interval and visibility. GATE: under the budget at max players, fps capped.
6. Web and transports. Same session with transport="websocket"; for a browser build, export Web and run the client in headless Chrome (procedure P9). WebSocket is TCP: "unreliable" RPCs arrive reliable and ordered, with head-of-line blocking under loss; WebRTC gives unreliable browser channels but needs signalling, STUN/TURN and webrtc-native on desktop (WebRTC doc; wgIqB6JNcro [00:19:35]). An HTTPS page needs wss://. A background tab stops _process and the socket drops: build reconnect (web doc). GATE: the browser client joins, sees all players, RTT measured.
7. Dedicated server. gd_net.server_preset(), export, compare client and server packs (pack_report), run the exported binary without --headless and connect clients. Set application/run/flush_stdout_on_print=true for systemd or journald logs (docs delta 12; default false, measured). GATE: dedicated_server feature true, display headless, no client textures in the pack, exit 0.
8. Look. A windowed observer client captures the replicated world (capture_at); open it. GATE: players where the server says, colors per peer, nothing missing.

Harness in one call

import sys; sys.path[:0] = ["<skills>/scenario-godot-expert/scripts", "<skills>/scenario-godot-multiplayer/scripts"]
import gd_net
gd_net.install_netkit(P); gd_net.build_scenes(P)          # res://net/world.tscn, player.tscn, props
roles = [gd_net.role("server", role="server", port=24010, lobby_size=2),
         gd_net.role("c1", role="client", port=24011, index=0, delay_s=0.3, cheat_at=2.0),
         gd_net.role("c2", role="client", port=24011, index=1, delay_s=0.3)]
with gd_net.UdpLagProxy(24011, 24010, rtt_ms=100, jitter_ms=0, loss=0.0):
    s = gd_net.session(P, roles)                            # one APFS clone and one Godot per role
s["roles"]["c1"]["result"]["rtt_median_ms"]                 # every NetLab number is in AGENT_RESULT

Your own game uses the same pattern: a job script per role (role(name, script=...)), user args after --, AGENT_RESULT printed by each process. Keep ports unique per project; two roles never share a project folder.

Prediction, rollback and backends (not run here)

netfox gives server authority with client prediction and rollback: input gathered on the network tick, logic in _rollback_tick, NetworkTime.physics_factor around move_and_slide, TickInterpolator for display (GqHTNmRspjU [00:06:58, 00:18:55, 00:21:24]; iP_xdJ0peFo [00:28:00]). Prove it on a small prototype first: it is opinionated (GqHTNmRspjU [00:34:53]). Deterministic input-only rollback fits fighting and RTS-like games with 2 to 8 players (David Snopek, zvqQPbT8rAE). Steam (GodotSteam, SteamMultiplayerPeer), Nakama, Colyseus and WebRTC P2P swap the peer and lobby calls; authority, spawner and synchronizer code stays the same (Gwizz, fUBdnocrc3Y; AndrooDev, wgIqB6JNcro [00:07:18]). None of these ran here: they need a third-party addon, an account or a signalling server. Verify against the installed release before quoting an API.

Numbers

Value	Measured here (2026-10-02, M-series Mac, localhost)	Relative to
RPC round trip, no proxy	16 to 19 ms	60 fps both ends; one frame each way dominates
ENet RTT statistic	15 to 23 ms	same
Own input to visible motion, server authority	47 to 52 ms	about 3 frames
Remote view behind server truth	20 to 30 ms, 0.07 m error at 3 m/s	no proxy
Same at 100 / 250 ms RTT	70 to 80 / 150 to 155 ms; 0.22 / 0.44 m	UdpLagProxy, adds about 6 ms per direction
50 props, ALWAYS, 60 fps	72.6 KiB/s, moving or idle	1 client
ON_CHANGE moving / idle	98 to 101 / 2.3 KiB/s	same
ALWAYS, replication_interval 0.05	21 to 22 KiB/s	same
ALWAYS, uncapped headless server (145 fps)	160 to 175 KiB/s	same
auth_timeout default; kick of a silent peer	3.0 s; 1.50 s after setting 1.5	server
Web export, single-thread	40 MB, wasm 39.5 MB (10.1 MB gzip)	handoff to scenario-godot-performance-export
Dedicated server pack / client pack	0.11 / 3.27 MB (3.1 MB texture stripped)	same project
spawn_limit	0 = unlimited (default); limit 2: third spawn() logs "Spawn limit reached!", returns null, clients get 2	M14; raise it to max players (jgJuX04cq7k [00:18:34])

Quality gates

• Measurable: net_audit flags empty; every role ok with no unexpected engine error; players_seen_max equals the player count everywhere; authority body 1, input = peer; teleport counts as expected; RTT and replication_delay at 100 and 250 ms; KiB/s per mode; rpc_audit reviewed, rpc_pair_check ok; exported server boot dict correct.
• Visual: observer capture shows every player; under 250 ms no streak after a teleport; the browser screenshot shows the replicated players.

Common mistakes

Mistake	What it looks like	Fix
Players spawned only on the server without a spawner, or spawner spawn_path empty	client sees nobody	MultiplayerSpawner with spawn_path and the scene in its list; server calls add_child
Authority read from a synced property in _enter_tree	authority 1 everywhere on clients	authority from name.to_int()
Runtime node without add_child(node, true)	@Node@2 names, "node not found" on RPC	readable names
await before get_remote_sender_id()	sender 0, validation passes or fails wrongly	read the sender first
RPC added on the server script only	checksum error, the wrong method runs	one script, or rpc_pair_check
Object or Resource as RPC argument	no error; receiver gets EncodedObjectAsID	send ids, paths or plain data
Client owns its body in a competitive game	speed and teleport hacks stick	input-only authority plus server simulation
Headless server not capped	2.4 times the bandwidth	--max-fps 60 or intervals
server_relay turned off for a P2P-style feature	"Attempt to call RPC with unknown peer ID"	keep relay on, or route through the server
Freeing a networked branch in the wrong order	"isn't currently active", "Node not found"	offline peer, free contents, drop custom API, free branch
Server export in normal mode	full-size pack, window opens	dedicated server export mode, Strip Visuals
Server start inside assert()	works from the editor; the release export never listens	var err := peer.create_server(port), then check err (M15: assert body stripped in release)
Client info sent from peer_connected, or id 0 registered	duplicate nodes on late joiners, a phantom player	connected_to_server; skip id 0
Client and server on different Godot versions	connection or protocol failures	same 4.7.2 build (protocol changed in 4.3)

Handoffs

• Receives: the game scene and player controller from scenario-godot-gameplay and scenario-godot-3d-world or scenario-godot-2d; architecture rules from scenario-godot-architecture.
• Delivers: network design note, kit scenes or scripts, session evidence JSON, server preset; the web build size and headers go to scenario-godot-performance-export, CI jobs to scenario-godot-pipeline-automation, lobby screens to scenario-godot-ui.

Godot 4.7 notes

• get_rpc_config became get_node_rpc_config (4.5); @rpc tables live on Script.get_rpc_config().
• auth_timeout is the property; the docs page spells auth_timout.
• 4.7.2 fixed peers that stopped replicating after deleting a spawner-spawned node.
• Godot 3 names (NetworkedMultiplayerENet, remote, rset, get_rpc_sender_id) do not exist; gd_net.rpc_audit flags them.
• WebRTC on desktop needs the webrtc-native extension: initialize() returns OK and only logs an error without it (measured).
• Open: under heavy machine load, 4 of 20 ALWAYS runs sent about 10 packets/s instead of 68 (ENet throttle at 0). Log NetUtil.enet_throttle() in long sessions (P8).
• Two peers leaving in one server frame log "Unable to send packet on channel 0" (ENet) or ready_state != STATE_OPEN (WebSocket); harmless, whitelisted by exact text.

References

• references/procedures.md: P1 to P14 with code, test ids and results.
• references/expert-notes.md: claims by expert with timestamps and verified corrections.
• references/critique.md, references/gui-paths.md (Replication panel, Run Instances, Network Profiler), references/sources.md.
• scripts/gd_net.py, scripts/agentkit/multiplayer/: the harness and kit.