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OSC 133;B command start (FTCS_COMMAND_START)
ESC ] 133 ; B BELOSC 133;B (also known as FTCS_COMMAND_START) marks the end of the prompt and the start of the user's command line. Shells emit this immediately after drawing the prompt and before the user begins typing. It tells the terminal: everything from here until Enter is what the user is typing.
This marker lets the terminal distinguish prompt text (which it shouldn't let users edit) from command text (which is the user's input). Editors and shell-aware terminals use this to implement features like command-line history navigation, syntax highlighting of the typed command, and copy-command-only operations that exclude the prompt.
How this is testedpartial
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OSC 133 ; B BEL and verify the terminal consumes the sequence (cursor doesn't advance, terminal remains responsive).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 all 9 tested terminals — universal adoption. Part of the Rich TUI baseline.
Supported by 10 of 14 backends (71%)
Terminal Applications
| Terminal | Version | Support | Notes |
|---|---|---|---|
| iTerm2 | 3.6.9 | ✓ yes | |
| Ghostty | 1.3.1 | ✓ yes | |
| Terminal.app | 2.15 | ✓ yes | |
| Kitty | 0.46.2 | ✓ yes | |
| VS Code | 1.113.0 | ? unknown | |
| Warp | 0.2026.03.18.08.24.03 | ? unknown | |
| Cursor | 2.6.21 | ? unknown |