Appearance
Insert/replace mode (IRM)
ESC [ 4 hIRM (Insert/Replace Mode) controls whether new characters insert (shifting existing content right) or replace (overwriting) at the cursor. Enable insert mode with
ESC [ 4 h, disable (replace mode) with ESC [ 4 l. Replace mode is the default. Most modern TUI applications use explicit ICH/DCH sequences rather than IRM.How this is testedautomated
Write "ABC", move to col 1, send
Write "ABC", move to col 1, send
\x1b[4h (IRM), write "X", verify X is at col 0 and A shifted to col 1.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 11 of 12 terminals (92%). Not supported by: vt100.js. Part of the Modern TUI baseline. Notes: vt100.js: IRM (insert/replace mode) not implemented in vt100.js baseline emulator.
Supported by 12 of 14 backends (86%)
Terminal Applications
| Terminal | Version | Support | Notes |
|---|---|---|---|
| iTerm2 | 3.6.9 | ✓ yes | |
| Ghostty | 1.3.1 | ✓ yes | |
| VS Code | ✓ yes | ||
| Warp | ✓ yes | ||
| Cursor | ✓ yes | ||
| Terminal.app | ✓ yes |
Headless Backends
Parser correctness only — a ✓ means the parser accepts the sequence.
| Backend | Version | Support | Notes |
|---|---|---|---|
| Alacritty | 0.26.0 | ✓ yes | |
| vterm | 0.2.0 | ✓ yes | |
| WezTerm | 0.1.0-fork.5 | ✓ yes | |
| xterm.js | 5.5.0 | ✓ yes | |
| Kitty | 0.40.0 | ~ partial | Not exposed via Python batch bridge |
| vt100.js | 0.2.1 | ✗ no | IRM (insert/replace mode) not implemented in vt100.js baseline emulator |