steam-multiplayer-networking

name	steam-multiplayer-networking
description	Integrate Steam multiplayer networking into your game. Covers lobby creation, matchmaking, Steam Networking Sockets (relay), dedicated game servers, and auth tickets. Use when building multiplayer features, setting up lobbies, or implementing peer-to-peer or server-based networking with Steamworks.
standards-version	1.10.0

Steam Multiplayer Networking

Trigger

Use this skill when the user:

• Is adding multiplayer support to a Steam game
• Needs help with lobby creation, joining, or matchmaking
• Wants to use Steam Networking Sockets (relay network)
• Is setting up dedicated game servers with Steam
• Asks about auth tickets, P2P networking, or skill-based matchmaking

Required Inputs

• App ID - the game's Steam App ID
• Multiplayer type - lobbies, P2P, dedicated servers, or a combination

Workflow

Lobby-Based Matchmaking (ISteamMatchmaking)

1. Create a lobby:

C++ (Steamworks SDK):

SteamMatchmaking()->CreateLobby(k_ELobbyTypePublic, maxPlayers);
// Handle result in OnLobbyCreated callback

C# (Steamworks.NET):

SteamMatchmaking.CreateLobby(ELobbyType.k_ELobbyTypePublic, maxPlayers);

Lobby types: Private, FriendsOnly, Public, Invisible.

2. Set lobby metadata (game mode, map, version - used for filtering):

SteamMatchmaking()->SetLobbyData(lobbyId, "gamemode", "deathmatch");
SteamMatchmaking()->SetLobbyData(lobbyId, "map", "arena_01");

3. Search for lobbies:

SteamMatchmaking()->AddRequestLobbyListStringFilter("gamemode", "deathmatch", k_ELobbyComparisonEqual);
SteamMatchmaking()->AddRequestLobbyListDistanceFilter(k_ELobbyDistanceFilterDefault);
SteamMatchmaking()->RequestLobbyList();

4. Join a lobby:

SteamMatchmaking()->JoinLobby(lobbyId);

5. Lobby chat and data exchange - use SetLobbyMemberData and SendLobbyChatMsg for pre-game coordination.

Steam Networking Sockets (Relay Network)

The modern networking API that relays traffic through Valve's network, protecting player IPs.

1. Initialize and create a listen socket (host):

SteamNetworkingSockets()->CreateListenSocketP2P(0, 0, nullptr);

2. Connect to a host (client):

SteamNetworkingIdentity identity;
identity.SetSteamID(hostSteamId);
SteamNetworkingSockets()->ConnectP2P(identity, 0, 0, nullptr);

3. Send messages:

SteamNetworkingSockets()->SendMessageToConnection(conn, data, dataSize, k_nSteamNetworkingSend_Reliable, nullptr);

4. Receive messages:

SteamNetworkingMessage_t* msgs[16];
int numMsgs = SteamNetworkingSockets()->ReceiveMessagesOnConnection(conn, msgs, 16);
for (int i = 0; i < numMsgs; i++) {
    // process msgs[i]->m_pData (size: msgs[i]->m_cbSize)
    msgs[i]->Release();
}

Dedicated Game Servers (ISteamGameServer)

1. Initialize the game server:

SteamGameServer_Init(0, gamePort, queryPort, eServerModeAuthenticationAndSecure, "1.0.0");
SteamGameServer()->SetProduct("mygame");
SteamGameServer()->SetGameDescription("My Game Server");
SteamGameServer()->LogOnAnonymous();

2. Auth tickets - validate connecting players:

SteamGameServer()->BeginAuthSession(ticket, ticketLen, clientSteamId);
// Check EAuthSessionResponse in callback

3. Server browser registration - servers appear in Steam's server browser automatically after LogOnAnonymous().

Skill-Based Matchmaking

Steam does not provide built-in Elo/MMR. Recommended pattern:

• Store skill ratings in Steam Stats (ISteamUserStats)
• Use lobby metadata filters to match similar skill ranges
• Or run a backend matchmaking service that creates lobbies for matched players

Key References

Resource	URL
Multiplayer overview	https://partner.steamgames.com/doc/features/multiplayer
Matchmaking & Lobbies	https://partner.steamgames.com/doc/features/multiplayer/matchmaking
ISteamMatchmaking API	https://partner.steamgames.com/doc/api/ISteamMatchmaking
ISteamNetworkingSockets	https://partner.steamgames.com/doc/api/ISteamNetworkingSockets
ISteamGameServer	https://partner.steamgames.com/doc/api/ISteamGameServer

Example Interaction

User: "I'm building a 4-player co-op game (App ID 567890). How do I set up lobbies where the host can pick a map and others can join?"

Agent:

1. Shows CreateLobby with k_ELobbyTypePublic and maxPlayers=4
2. Sets lobby metadata: map name, game version
3. Implements lobby search with map filter
4. Shows how to transition from lobby to gameplay using Steam Networking Sockets
5. Notes: "Use SetLobbyData for host-controlled settings and SetLobbyMemberData for per-player info like character selection."

MCP Usage

Multiplayer networking uses in-process Steamworks SDK APIs (ISteamMatchmaking, ISteamNetworkingSockets, ISteamGameServer) with no Web API equivalents for P2P connections or dedicated server management.

Step	MCP Tool	Auth	Purpose
Lobby code examples	steam_createLobby({ type, max_members, metadata })	SDK guide	Get C++/C#/GDScript code for lobby creation

The steam_createLobby tool returns SDK code examples and documentation, not an HTTP API call. The actual lobby creation happens in-process via the Steamworks SDK.

If the MCP server is not available, use the SDK code examples in the Workflow section above.

Common Pitfalls

1. Not calling SteamAPI_RunCallbacks() frequently enough — networking callbacks (connection state changes, messages) are only dispatched during RunCallbacks(). Missing or infrequent calls cause lag and missed events.
2. Trusting client-reported game state — never trust client-sent positions, damage values, scores, or inventory. Validate everything server-side or with an authoritative host.
3. Forgetting relay fallback — Steam Networking Sockets uses Valve's relay network for NAT traversal. If you implement direct P2P only, players behind strict NATs can't connect.
4. Not handling k_ESteamNetworkingConnectionState_ClosedByPeer — players disconnect without warning. Always handle this state to clean up sessions and notify other players.
5. Sending too much data per tick — Steam Networking has per-connection bandwidth limits. Sending full game state every frame instead of deltas causes packet loss and disconnects.

See Also

• Steam Friends & Social - game invites, rich presence, and overlay for multiplayer games
• Steam Leaderboards - competitive ranking systems for multiplayer games