Net Messages

Everything a CS2 server and its clients say to each other is a protobuf message: the chat line, the screen shake, a sound starting, a voice packet, the player's movement input. Each one is a message type from Valve's .proto files plus a numeric message id. Source2Mod lets you build and send them, and sit on the wire in both directions to read, change or drop them.

Kinds of messages

User messages are what most plugins want: short server-to-client messages that drive the HUD and effects — text in chat or the centre of the screen, screen fades and shakes, hint boxes. The net_ / clc_ / svc_ messages are the engine's own protocol; you mostly hook those rather than send them.

Ids and names

Every message type is registered with the engine's INetworkMessages under both an id and the name of its protobuf class. Source2Mod takes either:

• By id — the value from the .proto enum, e.g. UM_Shake = 120, CS_UM_HintText = 323, clc_VoiceData = 22. The generated headers give them to you as plain enums: include usermessages.pb.h, cstrike15_usermessages.pb.h, netmessages.pb.h, … (generated into your build by the SDK). The SDK also has them as enum class types under source2mod/schema/entity/enums/ (EBaseUserMessages, ECstrike15UserMessages, SVC_Messages, CLC_Messages, …).
• By name — the protobuf class name, e.g. "CUserMessageShake", "CCSUsrMsg_HintText", "CSVCMsg_GetCvarValue". The lookup is a partial match: "Shake" also works, but it matches CUserMessageShake, CUserMessageShakeDir and CCSUsrMsg_Shake alike, and you get whichever the engine finds first. Use the full class name.

The id is what everything else keys on: sending needs it, and hooks report it. Allocating by name is a convenience for when you do not have the enum at hand — you still pass the id when you send.

Fields

A message's fields are exactly those in its .proto definition. Source2Mod reads and writes them by name through protobuf reflection, so you do not need the generated class — only the field names and types from the .proto:

message CUserMessageShake {
	optional uint32 command = 1;
	optional float amplitude = 2;
	optional float frequency = 3;
	optional float duration = 4;
}

→ SetUInt32(msg, "command", 0), SetFloat(msg, "amplitude", 10.0f), … The .proto files for CS2 are published in the SteamTracking/Protobufs repository.

How messages travel

The server sends every message through IGameEventSystem::PostEventAbstract, with a recipient set of player slots. Messages from clients arrive per client and pass through the engine's pre-filter (INetworkMessageProcessingPreFilter::FilterMessage) before the server handles them. The s2m hooks both places once and hands every plugin a callback:

All of it is on ISource2ModNetworkMessages, reached through g_pSource2ModNetworkMessages once SOURCE2MOD_SAVEVARS() has run. Reference: ISource2ModNetworkMessages. Scripts call the same methods on Source2ModNetworkMessages (source2mod/netmessages.inc), with the ids from source2mod/netmessage_ids.inc.