100 GMCP resources for MUD players

Send IAC WILL GMCP (255 251 201) to the client during the initial login sequence. If the client responds with IAC DO GMCP (255 253 201), the out-of-band channel is established.

Wrap all GMCP payloads in Subnegotiation markers. Format: IAC SB GMCP <Package.Message> <JSON data> IAC SE. Ensure no raw text or ANSI codes leak into this stream.

Use a robust C/C++ library like cJSON or RapidJSON. Avoid manual string concatenation for JSON to prevent malformed payloads when player names contain quotes or special characters.

Parse the client's initial Core.Hello message to identify the client name and version (e.g., 'Mudlet 4.17'). Use this to toggle client-specific workarounds or features.

Track the packages the client explicitly requests. Do not send Char.Vitals or Room.Info until the client confirms support via the Core.Supports.Set or Add messages.

Implement a throttle for high-frequency updates like Char.Vitals. Limit updates to 100ms intervals to prevent client lag during rapid combat rounds or heavy regeneration.

Some legacy clients require a null byte (\0) at the end of the JSON payload before the IAC SE. Test compatibility across Mudlet, Mushclient, and BeipMU.

If a player disables GMCP or switches to a non-compliant proxy, the server must receive IAC DONT GMCP and immediately stop all subnegotiation to prevent protocol desync.

Use standard package names (Core, Char, Room, Comm) for compatibility. Use a unique game-specific prefix (e.g., 'MyMUD.Quest') for custom game mechanics.

Hook your GMCP broadcast functions into your game engine's event system (e.g., on_damage, on_move) rather than polling player state in the main loop.

Send current and max values for hp, mana, and moves. Example: { "hp": 150, "maxhp": 200 }. Essential for client-side health bars.

Provide 'num' (unique ID), 'name', 'zone', and 'exits' (a map of directions to room IDs). This allows Mudlet's mapper to build an accurate layout.

Send chat messages with 'chan' and 'msg' keys. This allows clients to capture 'gossip' or 'clan' chat into separate scrollable windows.

Send a list of items in a specific location (inv, room, or container). Include 'id', 'name', and optional 'icon' for graphical inventory systems.

Transmit character-level metadata like level, race, class, and current gold. Update this only on state change to save bandwidth.

Send arrays of entities present in the current room. Useful for creating clickable 'target' lists in the client UI.

List skill categories available to the player. Used to populate client-side spellbooks or talent trees without querying the server manually.

Respond to a client Core.Ping with a server-side timestamp. Helps players diagnose network issues versus server processing lag.

Send active debuffs (poison, blind) and buffs (shield, haste). Essential for high-level automated combat scripting in modern clients.

Notify the client when a movement fails (e.g., 'The door is locked'). Prevents the client-side mapper from desyncing when a move command is sent.

Register functions using registerAnonymousEventHandler('gmcp.Char.Vitals', 'myCallback'). This is more efficient than checking the gmcp table in a loop.

Use the Mudlet command 'lua display(gmcp)' to see the current state of all received data packages in the debug console.

Directly link GMCP vital values to Geyser.Gauge objects. Update the gauge's value inside the gmcp.Char.Vitals event handler.

If the client debug console shows 'JSON error', check for unescaped newlines or backslashes in the server-side string serialization.

Use sendGMCP('Core.Supports.Add ["Comm.Channel 1"]') to tell the server your script is ready to process specific data streams.

Use the OnPluginTelnetSubnegotiation callback to capture option 201. Use a Lua JSON module to decode the buffer into a table.

Trigger centerview(gmcp.Room.Info.num) whenever the Room.Info package arrives to keep the visual map locked to the player's position.

Show or hide UI elements based on GMCP data (e.g., hide the 'Mount' gauge if gmcp.Char.Status.mounted is false).

Use client-side scripts to bridge GMCP Comm.Channel data to Discord webhooks for out-of-game monitoring of guild chats.

Use Wireshark with the Telnet filter to inspect the raw hex bytes of GMCP subnegotiations if the client is failing to parse the JSON.