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OSC 66 text sizing
OSC 66 is the Kitty text sizing protocol, allowing applications to render text at different sizes (scale 1-7x) and explicitly specify character cell widths. The sequence
ESC ] 66 ; params BEL controls text sizing with s=N for scale factor and w=N for explicit cell width.
This protocol addresses the fundamental character width ambiguity problem — applications can tell the terminal exactly how wide a character should be, rather than relying on potentially inconsistent wcwidth() calculations. Supported by kitty and foot, with Ghostty parsing (but not yet rendering) the sequences.How this is tested: Query text sizing state with
OSC 66;? BEL and check for response. Fallback: send OSC 66;s=2 BEL and verify the sequence is consumed.Analysis2026-03-26
Supported by 1 of 18 backends (6%)
Terminal Applications
| Terminal | Version | Support | Notes |
|---|---|---|---|
| cmux | ✓ yes | Consumed (no query) | |
| iTerm2 | 3.6.9 | ? unknown | |
| Ghostty | 1.3.1 | ? unknown | |
| VS Code | 1.112.0 | ? unknown | |
| Cursor | 2.6.21 | ? unknown | |
| Warp | 0.2026.03.18.08.24.03 | ? unknown | |
| Terminal.app | 2.15 | ? unknown | |
| Kitty | 0.46.2 | ? unknown | OSC 66 not exposed via Python batch bridge — kitty supports it natively |
Headless Backends
Parser correctness only — a ✓ means the parser accepts the sequence.
| Backend | Version | Support | Notes |
|---|---|---|---|
| Alacritty | 0.26.0 | ✗ no | OSC 66 text sizing not implemented in alacritty |
| vt100.js | 0.2.1 | ✗ no | Not implemented — pure TypeScript emulator |
| vterm | 0.2.0 | ✗ no | OSC 66 text sizing not implemented |
| WezTerm | 0.1.0-fork.5 | ✗ no | OSC 66 text sizing not implemented in wezterm |
| xterm.js | 5.5.0 | ? unknown | OSC 66 text sizing not implemented in xterm.js |
| xterm.js | 6.0.0 | ? unknown | OSC 66 text sizing not implemented in xterm.js |
| xterm.js | 5.4.0 | ? unknown | OSC 66 text sizing not implemented in xterm.js |
| xterm.js | 6.1.0-beta.195 | ? unknown | OSC 66 text sizing not implemented in xterm.js |