Appearance
Underline color (SGR 58)
ESC [ 58;2;R;G;B mSGR 58 sets the underline color independently of the text foreground color. The syntax mirrors SGR 38 (foreground):
ESC [ 58:2::R:G:B m for truecolor, ESC [ 58:5:N m for 256-color. Reset the underline color to default with SGR 59.
This is particularly useful with curly underlines (SGR 4:3) for diagnostic indicators: red underline for errors, yellow for warnings, blue for hints — all while keeping the text color unchanged. Introduced by Kitty and adopted by other modern terminals.How this is tested: Send
\x1b[4m\x1b[58;2;255;0;128mX, verify the cell has underline active and underlineColor matches RGB(255, 0, 128).Analysis2026-03-26
Supported by all 8 tested terminals — universal adoption. Part of the Modern TUI baseline.
Supported by 10 of 13 backends (77%)
Terminal Applications
| Terminal | Version | Support | Notes |
|---|---|---|---|
| iTerm2 | 3.6.9 | ✓ yes | |
| Ghostty | 1.3.1 | ✓ yes | |
| VS Code | 1.112.0 | ✓ yes | |
| Warp | 0.2026.03.18.08.24.03 | ✓ yes | |
| Cursor | 2.6.21 | ✓ yes | |
| tmux | 3.6a | ✓ yes | |
| Kitty | 0.46.2 | ✓ yes | Not exposed via Python batch bridge |
| Terminal.app | 2.15 | ✓ yes | |
| GNU Screen | 5.0.1 | ✓ yes |
Headless Backends
Parser correctness only — a ✓ means the parser accepts the sequence.
| Backend | Version | Support | Notes |
|---|---|---|---|
| vterm | 0.2.0 | ✓ yes | |
| Alacritty | 0.26.0 | ~ partial | Not exposed via alacritty_terminal API |
| WezTerm | 0.1.0-fork.5 | ~ partial | Not exposed via wezterm-term API |
| vt100 | 0.2.1 | ✗ no | Not implemented — pure TypeScript emulator |