Extensions to the xterm protocol

kitty has a few extensions to the xterm protocol, to enable advanced features. These are typically in the form of new or re-purposed escape codes. While these extensions are currently kitty specific, it would be nice to get some of them adopted more broadly, to push the state of terminal emulators forward.

The goal of these extensions is to be as small and unobtrusive as possible, while filling in some gaps in the existing xterm protocol. In particular, one of the goals of this specification is explicitly not to "re-imagine" the tty. The tty should remain what it is -- a device for efficiently processing text received as a simple byte stream. Another objective is to only move the minimum possible amount of extra functionality into the terminal program itself. This is to make it as easy to implement these protocol extensions as possible, thereby hopefully encouraging their widespread adoption.

If you wish to discuss these extensions, propose additions/changes to them please do so by opening issues in the github bug tracker.

Colored and styled underlines

kitty supports colored and styled (wavy) underlines. This is of particular use in terminal editors such as vim and emacs to display red, wavy underlines under mis-spelled words and/or syntax errors. This is done by re-purposing some SGR escape codes that are not used in modern terminals (CSI codes)

To set the underline style:

<ESC>[4:0m  # this is no underline
<ESC>[4:1m  # this is a straight underline
<ESC>[4:2m  # this is a double underline
<ESC>[4:3m  # this is a curly underline
<ESC>[4:4m  # this is a dotted underline (not implemented in kitty)
<ESC>[4:5m  # this is a dashed underline (not implemented in kitty)
<ESC>[4m    # this is a straight underline (for backwards compat)
<ESC>[24m   # this is no underline (for backwards compat)

To set the underline color (this is reserved and as far as I can tell not actually used for anything):

<ESC>[58...m

This works exactly like the codes 38, 48 that are used to set foreground and background color respectively.

To reset the underline color (also previously reserved and unused):

<ESC>[59m

The underline color must remain the same under reverse video, if it has a color, if not, it should follow the foreground color.

To detect support for this feature in a terminal emulator, query the terminfo database for the Su boolean capability.

Graphics rendering

See The terminal graphics protocol for a description of this protocol to enable drawing of arbitrary raster images in the terminal.

Keyboard handling

There are various problems with the current state of keyboard handling. They include:

  • No way to use modifiers other than Ctrl and Alt

  • No way to reliably use multiple modifier keys, other than, Shift+Alt.

  • No way to handle different types of keyboard events, such as press, release or repeat

  • No reliable way to distinguish single Esc keypresses from the start of a escape sequence. Currently, client programs use fragile timing related hacks for this, leading to bugs, for example: neovim #2035.

There are already two distinct keyboard handling modes, normal mode and application mode. These modes generate different escape sequences for the various special keys (arrow keys, function keys, home/end etc.) Most terminals start out in normal mode, however, most shell programs like bash switch them to application mode. We propose adding a third mode, named full mode that addresses the shortcomings listed above.

Switching to the new full mode is accomplished using the standard private mode DECSET escape sequence:

<ESC>[?2017h

and to leave full mode, use DECRST:

<ESC>[?2017l

The number 2017 above is not used for any existing modes, as far as I know. Client programs can query if the terminal emulator is in full mode by using the standard DECRQM escape sequence.

The new mode works as follows:

  • All printable key presses without modifier keys are sent just as in the normal mode. This means all printable ASCII characters and in addition, Enter, Space and Backspace. Also any unicode characters generated by platform specific extended input modes, such as using the AltGr key. This is done so that client programs that are not aware of this mode can still handle basic text entry, so if a full mode using program crashes and does not reset, the user can still issue a reset command in the shell to restore normal key handling. Note that this includes pressing the Shift modifier and printable keys. Note that this means there are no repeat and release events for these keys and also for the left and right shift keys.

  • For non printable keys and key combinations including one or more modifiers, an escape sequence encoding the key event is sent. For details on the escape sequence, see below.

The escape sequence encodes the following properties:

  • Type of event: press,repeat,release

  • Modifiers pressed at the time of the event

  • The actual key being pressed

Schematically:

<ESC>_K<type><modifiers><key><ESC>\

Where <type> is one of p -- press, r -- release and t -- repeat. Modifiers is a bitmask represented as a single base64 digit. Shift -- 0x1, Alt -- 0x2, Control -- 0x4 and Super -- 0x8. <key> is a number (encoded in base85) corresponding to the key pressed. The key name to number mapping is defined in this table.

Client programs must ignore events for keys they do not know. The mapping in the above table is stable and will never change, however, new codes might be added to it in the future, for new keys.

For example:

<ESC>_KpGp<ESC>\  is  <Ctrl>+<Alt>+x (press)
<ESC>_KrP8<ESC>\  is  <Ctrl>+<Alt>+<Shift>+<Super>+PageUp (release)

This encoding means each key event is represented by 8 or 9 printable ascii only bytes, for maximum robustness.

To see the full mode in action, run:

kitty +kitten key_demo

Support for this mode is indicated by the fullkbd boolean capability in the terminfo database, in case querying for it via DECQRM is inconvenient.

Setting text styles/colors in arbitrary regions of the screen

There already exists an escape code to set some text attributes in arbitrary regions of the screen, DECCARA. However, it is limited to only a few attributes. kitty extends this to work with all SGR attributes. So, for example, this can be used to set the background color in an arbitrary region of the screen.

The motivation for this extension is the various problems with the existing solution for erasing to background color, namely the background color erase (bce) capability. See this discussion and this FAQ for a summary of problems with bce.

For example, to set the background color to blue in a rectangular region of the screen from (3, 4) to (10, 11), you use:

<ESC>[2*x<ESC>[4;3;11;10;44$r<ESC>[*x

Saving and restoring the default foreground/background/selection/cursor colors

It is often useful for a full screen application with its own color themes to set the default foreground, background, selection and cursor colors. This allows for various performance optimizations when drawing the screen. The problem is that if the user previously used the escape codes to change these colors herself, then running the full screen application will lose her changes even after it exits. To avoid this, kitty introduces a new pair of OSC escape codes to push and pop the current color values from a stack:

<ESC>]30001<ESC>\  # push onto stack
<ESC>]30101<ESC>\  # pop from stack

These escape codes save/restore the so called dynamic colors, default background, default foreground, selection background, selection foreground and cursor color.

Pasting to clipboard

kitty implements the OSC 52 escape code protocol to get/set the clipboard contents (controlled via the clipboard_control setting). There is one difference in kitty's implementation compared to some other terminal emulators. kitty allows sending arbitrary amounts of text to the clipboard. It does so by modifying the protocol slightly. Successive OSC 52 escape codes to set the clipboard will concatenate, so:

<ESC>]52;c;<payload1><ESC>\
<ESC>]52;c;<payload2><ESC>\

will result in the clipboard having the contents payload1 + payload2. To send a new string to the clipboard send an OSC 52 sequence with an invalid payload first, for example:

<ESC>]52;c;!<ESC>\

Here ! is not valid base64 encoded text, so it clears the clipboard. Further, since it is invalid, it should be ignored by terminal emulators that do not support this extension, thereby making it safe to use, simply always send it before starting a new OSC 52 paste, even if you aren't chunking up large pastes, that way kitty won't concatenate your paste, and it will have no ill-effects in other terminal emulators.

In case you're using software that can't be easily adapted to this protocol extension, it can be disabled by specifying no-append to the clipboard_control setting.

Desktop notifications

kitty implements an extensible escape code (OSC 99) to show desktop notifications. It is easy to use from shell scripts and fully extensible to show title and body. Clicking on the notification can optionally focus the window it came from, and/or send an escape code back to the application running in that window.

The design of the escape code is partially based on the discussion in the defunct terminal-wg

The escape code has the form:

<OSC> 99 ; metadata ; payload <terminator>

Here <OSC> is <ESC>] and <terminator> is <ESC><backslash>. The metadata is a section of colon separated key=value pairs. Every key must be a single character from the set a-zA-Z and every value must be a word consisting of characters from the set a-zA-Z0-9-_/\+.,(){}[]*&^%$#@!`~. The payload must be interpreted based on the metadata section. The two semi-colons must always be present even when no metadata is present.

Before going into details, lets see how one can display a simple, single line notification from a shell script:

printf '\x1b]99;;Hello world\x1b\\'

To show a message with a title and a body:

printf '\x1b]99;i=1:d=0;Hello world\x1b\\'
printf '\x1b]99;i=1:d=1:p=body;This is cool\x1b\\'

The most important key in the metadata is the p key, it controls how the payload is interpreted. A value of title means the payload is setting the title for the notification. A value of body means it is setting the body, and so on, see the table below for full details.

The design of the escape code is fundamentally chunked, this is because different terminal emulators have different limits on how large a single escape code can be. Chunking is accomplished by the i and d keys. The i key is the notification id which can be any string containing the characters [a-zA-Z0-9_-+.]. The d key stands for done and can only take the values 0 and 1. A value of 0 means the notification is not yet done and the terminal emulator should hold off displaying it. A value of 1 means the notification is done, and should be displayed. You can specify the title or body multiple times and the terminal emulator will concatenate them, thereby allowing arbitrarily long text (terminal emulators are free to impose a sensible limit to avoid Denial-of-Service attacks).

Both the title and body payloads must be either UTF-8 encoded plain text with no embedded escape codes, or UTF-8 text that is base64 encoded, in which case there must be an e=1 key in the metadata to indicate the payload is base64 encoded.

When the user clicks the notification, a couple of things can happen, the terminal emulator can focus the window from which the notification came, and/or it can send back an escape code to the application indicating the notification was activated. This is controlled by the a key which takes a comma separated set of values, report and focus. The value focus means focus the window from which the notification was issued and is the default. report means send an escape code back to the application. The format of the returned escape code is:

<OSC> 99 ; i=identifier ; <terminator>

The value of identifier comes from the i key in the escape code sent by the application. If the application sends no identifier, then the terminal must use i=0. Actions can be preceded by a negative sign to turn them off, so for example if you do not want any action, turn off the default focus action with:

a=-focus

Complete specification of all the metadata keys is in the table below. If a terminal emulator encounters a key in the metadata it does not understand, the key must be ignored, to allow for future extensibility of this escape code. Similarly if values for known keys are unknown, the terminal emulator should either ignore the entire escape code or perform a best guess effort to display it based on what it does understand.

Note

It is possible to extend this escape code to allow specifying an icon for the notification, however, given that some platforms, such as macOS, dont allow displaying custom icons on a notification, at all, it was decided to leave it out of the spec for the time being.

Similarly, features such as scheduled notifications could be added in future revisions.

Key

Value

Default

Description

a

Comma separated list of report, focus, with optional leading -

focus

What action to perform when the notification is clicked

d

0 or 1

1

Indicates if the notification is complete or not.

e

0 or 1

0

If set to 1 means the payload is base64 encoded UTF-8, otherwise it is plain UTF-8 text with no C0 control codes in it

i

[a-zA-Z0-9-_+.]

0

Identifier for the notification

p

One of title or body.

title

Whether the payload is the notification title or body. If a notification has no title, the body will be used as title.

Note

kitty also supports the legacy OSC 9 protocol developed by iTerm2 for desktop notifications.