Appearance
Curly underline (SGR 4:3)
ESC [ 4:3 mSGR 4:3 draws a wavy/curly underline beneath text, using the ECMA-48 colon sub-parameter syntax:
ESC [ 4:3 m. This underline style was introduced by Kitty and is commonly used by editors and IDEs to indicate spelling errors, lint warnings, or diagnostic information — similar to the red squiggly underline in graphical editors.
Combine with SGR 58 (underline color) to set the underline color independently of the text color, e.g., red curly underline under normally-colored text for errors. Adopted by Ghostty, WezTerm, foot, and other modern terminals. Reset with SGR 24.How this is tested: Send
\x1b[4:3mX (colon sub-parameter syntax), verify underline === "curly".Analysis2026-03-26
Supported by all 8 tested terminals — universal adoption. Part of the Modern TUI baseline.
Supported by 12 of 13 backends (92%)
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 | |
| Terminal.app | 2.15 | ✓ yes | |
| GNU Screen | 5.0.1 | ✓ yes |