Player-session MUD protocol

ATCP

Achaea Telnet Client Protocol

Legacy compatibility Documented current use Legacy

An out-of-band channel for game data and client capabilities that directly preceded GMCP.

Protocol overview

First seen: 2008
Present-day status: Legacy compatibility
In use today?: Yes, with documented current use.
Legacy?: Yes. It may still matter for compatibility or historical systems.
Layer: Player-session MUD protocol
Transport: Telnet option 200
Telnet option: 200
Research validated: 2026-08-29

Current use and fit

Still encountered in Iron Realms-derived games and compatible clients, but new systems should use GMCP.

Implementation essentials

Negotiate the Telnet option before exchanging module and value messages.
Keep ATCP behind the same semantic event layer used by GMCP.

Compatibility and safety notes

Do not create new ATCP namespaces.
Client identity and capability claims are not credentials.

Original specifications and sources

MUD Standards preservation Preservation
IRE Rapture documentation Original

Where an original site has disappeared, a preservation archive is linked and identified as such. MUDVerse does not claim authorship of the original protocol.