Implement GMCP Core (Telnet Option 201)

[mystiker]

Summary

Implement the GMCP Core protocol layer (Telnet option 201) in the client.
Scope is strictly limited to Core functionality, no MUD-specific modules and no UI integration.

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Background

GMCP (Generic MUD Communication Protocol) is a de-facto standard used by many MUDs to exchange structured (JSON) data over Telnet.

This issue implements:

• GMCP transport & parsing
• Core namespace handling
• Capability negotiation

Non-Core modules (Char, Room, Comm, etc.) are explicitly out of scope.

Reference:

• Mudlet GMCP standard
  https://wiki.mudlet.org/w/Standards:MUD_Client_Media_Protocol
• GMCP in Aardwolf
  https://www.aardwolf.com/wiki/index.php/Clients/GMCP#aardmodules

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Scope

✅ In scope

1. Telnet / Transport

• Support TELOPT_GMCP (201)
• Handle WILL / DO negotiation
• Receive and send GMCP sub-negotiation payloads as UTF-8 text

2. GMCP Message Parsing

• Parse messages in the form:

  <Package>.<Command> [JSON payload]
  

• Package and command are case-insensitive
• JSON payload is optional
• Unknown or malformed payloads must not crash the client

3. Core Namespace Support

Implement handling for the following Core messages:

Core.Hello

• Accept both directions (Client ↔ Server)
• Payload is a JSON object
• No strict schema validation (store raw object)

Core.Supports

• Client → Server
• Support:
  • Core.Supports.Set
  • Core.Supports.Add
  • Core.Supports.Remove
• Payload: JSON array of strings (e.g. "Char 1")
• No assumptions about server responses

Core.Ping

• Bidirectional
• Respond with Core.Ping when received (no payload)

Core.Goodbye

• Server → Client
• Graceful connection shutdown handling

4. Dispatch & Logging

• Central GMCP dispatcher
• Unknown packages / commands must be logged (debug level)
• Core handlers must be clearly separated from non-Core logic

❌ Out of scope

• Non-Core GMCP modules (Char, Room, Comm, etc.)
• UI bindings
• MUD-specific profiles
• Hard-coded payload schemas
• Files, sound, editor, browser info, or custom extensions

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Acceptance Criteria

• GMCP negotiation works with a GMCP-capable MUD
• Core messages are parsed and dispatched correctly
• Client does not crash on unknown GMCP messages
• Core implementation is isolated and reusable
• Logging allows inspection of all incoming GMCP traffic

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Notes / Design Constraints

• GMCP is loosely specified by design
• Avoid strict schema enforcement
• Prefer tolerant parsing and forward compatibility
• This implementation is intended as a foundation for later extensions