kovidgoyal/kitty · info

The application is trying to use xterm's modifyOtherKeys. Th

Error message

The application is trying to use xterm's modifyOtherKeys. This is superseded by the kitty keyboard protocol https://sw.kovidgoyal.net/kitty/keyboard-protocol. The application should be updated to use that.

What it means

The application enabled xterm's modifyOtherKeys (CSI > 4 n with a non-zero level) while the kitty keyboard protocol is not active; kitty logs an advisory that this legacy mechanism is superseded. modifyOtherKeys still works - this is only a warning.

Source

Thrown at kitty/screen.c:2076

}

void
screen_decsace(Screen *self, unsigned int val) {
    self->modes.mDECSACE = val == 2 ? true : false;
}

void
screen_reset_mode(Screen *self, unsigned int mode) {
    set_mode_from_const(self, mode, false);
}

void
screen_modify_other_keys(Screen *self, unsigned val, unsigned val2) {
    // Only report an error about modifyOtherKeys if the kitty keyboard
    // protocol is not in effect and the application is trying to turn it on.
    // There are some applications that try to enable both.
    if (self->test_child == Py_None && !screen_current_key_encoding_flags(self) && val == 4 && val2 != INT_MAX && val2 != 0) {
        log_error(
            "The application is trying to use xterm's modifyOtherKeys. This is superseded by the kitty keyboard protocol "
            "https://sw.kovidgoyal.net/kitty/keyboard-protocol. The application should be updated to use that.");
    }
}

uint8_t
screen_current_key_encoding_flags(Screen *self) {
    for (unsigned i = arraysz(self->main_key_encoding_flags); i-- > 0;) {
        if (self->key_encoding_flags[i] & 0x80) return self->key_encoding_flags[i] & 0x7f;
    }
    return 0;
}

void
screen_report_key_encoding_flags(Screen *self) {
    char buf[16] = {0};
    debug_input("\x1b[35mReporting key encoding flags: %u\x1b[39m\n", screen_current_key_encoding_flags(self));
    snprintf(buf, sizeof(buf), "?%uu", screen_current_key_encoding_flags(self));

View on GitHub (pinned to 6d5d0c4406)

Solutions

1. Ignore - modifyOtherKeys still functions
2. Enable the kitty keyboard protocol in your editor/app instead of legacy modifyOtherKeys
3. Filter the message from kitty logs if noisy

Example fix

# vim, before
let &t_TI = "\e[>4;2m"
# after (use kitty protocol-aware terminal settings)
set term=kitty

# detect the kitty keyboard protocol before enabling legacy modes
# send CSI ? u, parse the reply; only fall back to modifyOtherKeys when unsupported

When it happens

Trigger: Program sends CSI > 4 ; 1 h or CSI > 4 ; 2 h without having negotiated the kitty keyboard protocol (and not running under kitty's debug child).

Common situations: ssh sessions, tmux, or vim/emacs/tui configs that enable modifyOtherKeys by default.

Related errors

• Invalid shortcut
• This terminal does not support the kitty keyboard protocol,
• Shortcut: {sc} has unknown modifier, ignoring
• Ignoring invalid remap_modifiers "{val}": {msg}
• Application escape mode is not supported, the extended keybo

AI-assisted analysis of kovidgoyal/kitty@6d5d0c4406 (2026-08-27). Data as JSON: /api/errors/02100a9285a4ae29. Report an issue: GitHub.