Protocols

Rune advertises only what it implements. Agreeing to an option and then ignoring its subnegotiations breaks real servers, so the compatibility table and the feature list are the same list.

Protocol	Option	What rune does
ECHO	1	Server-controlled local echo (passwords hidden)
SGA / EOR	3 / 25	Prompt detection modes
TTYPE / MTTS	24	Reports RUNE, terminal type, and an honest MTTS bitvector (ANSI, VT100, UTF-8, 256 colors, truecolor, MNES; bit 2048 on TLS connections)
NAWS	31	Window size, re-sent on every resize
NEW-ENVIRON / MNES	39	CLIENT_NAME, CLIENT_VERSION, CHARSET, MTTS, TERMINAL_TYPE
CHARSET	42	Accepts UTF-8, rejects everything else
MCCP2	86	zlib decompression; a clean stream end resumes plain telnet
GMCP	201	Framing and JSON in Go; handlers and Core.Supports policy in Lua (rune.gmcp)

TLS is address-level rather than a telnet option: tls://host:port verifies certificates, and tls+insecure://host:port accepts self-signed ones (common on MUDs). Plain telnet is the default; telnet:// is accepted explicitly.

Refused (not implemented): MCCP3, MSSP, ZMP, MXP, MSP, LINEMODE.

Related: GMCP