Features›Extensions›Kitty color protocol (OSC 21)

Kitty color protocol (OSC 21)

Category: extensions · Baseline: rich · Tags: Operating System Commands (OSC), Kitty Extensions · Specification ↗

ESC ] 21 ; key=value BEL

OSC 21 is Kitty's key-value color protocol, designed as a structured replacement for the scattered OSC 10–19 color sequences. Instead of issuing separate sequences for foreground, background, cursor, selection, and palette colors, applications send a single ESC ] 21 ; key=value;... BEL sequence that updates any combination of named color slots. Both setting and querying are supported: key=? requests the current value, and the terminal responds with the same key-value framing. The protocol is well-defined and avoids the parsing ambiguities of the legacy xterm color sequences. Currently supported by Kitty, Ghostty, and foot.

How this is testedpartial
Send OSC 21 ; foreground=? BEL and verify the terminal responds with the foreground color in key-value form.

The same probe runs against headless backends (via Termless) and real terminal apps (via a daemon launched in each terminal). This lets us distinguish parser correctness from rendering correctness.

Analysis2026-04-06

Supported by 4 of 9 terminals (44%). Not supported by: vterm.js, vt100.js, Alacritty, WezTerm, xterm.js. Part of the Rich TUI baseline.

Supported by 6 of 14 backends (43%)

Terminal Applications

Terminal	Version	Support	Notes
iTerm2	3.6.9	✓ yes	Consumed (no query response)
Ghostty	1.3.1	✓ yes
Terminal.app	2.15	✓ yes	Consumed (no query response)
Kitty	0.46.2	✓ yes	OSC 21 query responses not exposed via Python batch bridge
VS Code	1.113.0	? unknown
Warp	0.2026.03.18.08.24.03	? unknown
Cursor	2.6.21	? unknown

Headless Backends

Parser correctness only — a ✓ means the parser accepts the sequence.

Backend	Version	Support	Notes
Alacritty	0.26.0	✗ no	Kitty color protocol (OSC 21) not implemented by alacritty
vt100.js	0.2.1	✗ no	Kitty color protocol (OSC 21) not implemented — post-dates VT220 baseline
vterm	0.2.0	✗ no	Kitty color protocol (OSC 21) not implemented in vterm.js
WezTerm	0.1.0-fork.5	✗ no	Kitty color protocol (OSC 21) not implemented by wezterm
xterm.js	5.5.0	✗ no	Kitty color protocol (OSC 21) not implemented in xterm.js

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