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:p=body;This is cool\x1b\\'

Tip

kitty also comes with its own statically compiled command line tool to easily display notifications, with all their advanced features. For example:

kitten notify "Hello world" A good day to you

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 is an Identifier. 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 non-zero value 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). The size of the payload must be no longer than 2048 bytes, before being encoded or 4096 encoded bytes.

Both the title and body payloads must be either Escape code safe UTF-8 text 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. No HTML or other markup in the plain text is allowed. It is strictly plain text, to be interpreted as such.

Allowing users to filter notifications

Added in version 0.36.0: Specifying application name and notification type

Well behaved applications should identify themselves to the terminal by means of two keys f which is the application name and t which is the notification type. These are free form keys, they can contain any values, their purpose is to allow users to easily filter out notifications they do not want. Both keys must have Base64 encoded UTF-8 text as their values. The t key can be specified multiple times, as notifications can have more than one type. See the freedesktop.org spec for examples of notification types.

Note

The application name should generally be set to the filename of the applications desktop file (without the .desktop part) or the bundle identifier for a macOS application. While not strictly necessary, this allows the terminal emulator to deduce an icon for the notification when one is not specified.

Tip

kitty has sophisticated notification filtering and management capabilities via filter_notification.

Being informed when user activates the notification

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. (Ideally i should have been left out from the response, but for backwards compatibility i=0 is used). 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.

Being informed when a notification is closed

Added in version 0.36.0: Notifications of close events

If you wish to be informed when a notification is closed, you can specify c=1 when sending the notification. For example:

<OSC> 99 ; i=mynotification : c=1 ; hello world <terminator>

Then, the terminal will send the following escape code to inform when the notification is closed:

<OSC> 99 ; i=mynotification : p=close ; <terminator>

If no notification id was specified i=0 will be used in the response

If a=report is specified and the notification is activated/clicked on then both the activation report and close notification are sent. If the notification is updated then the close event is not sent unless the updated notification also requests a close notification.

Note that on some platforms, such as macOS, the OS does not inform applications when notifications are closed, on such platforms, terminals reply with:

<OSC> 99 ; i=mynotification : p=close ; untracked <terminator>

This means that the terminal has no way of knowing when the notification is closed. Instead, applications can poll the terminal to determine which notifications are still alive (not closed), with:

<OSC> 99 ; i=myid : p=alive ; <terminator>

The terminal will reply with:

<OSC> 99 ; i=myid : p=alive ; id1,id2,id3 <terminator>

Here, myid is present for multiplexer support. The response from the terminal contains a comma separated list of ids that are still alive.

Updating or closing an existing notification

Added in version 0.36.0: The ability to update and close a previous notification

To update a previous notification simply send a new notification with the same notification id (i key) as the one you want to update. If the original notification is still displayed it will be replaced, otherwise a new notification is displayed. This can be used, for example, to show progress of an operation. How smoothly the existing notification is replaced depends on the underlying OS, for example, on Linux the replacement is usually flicker free, on macOS it isn’t, because of Apple’s design choices. Note that if no i key is specified, no updating must take place, even if there is a previous notification without an identifier. The terminal must treat these as being two unique unidentified notifications.

To close a previous notification, send:

<OSC> i=<notification id> : p=close ; <terminator>

This will close a previous notification with the specified id. If no such notification exists (perhaps because it was already closed or it was activated) then the request is ignored. If no i key is specified, this must be a no-op.

Automatically expiring notifications

A notification can be marked as expiring (being closed) automatically after a specified number of milliseconds using the w key. The default if unspecified is -1 which means to use whatever expiry policy the OS has for notifications. A value of 0 means the notification should never expire. Values greater than zero specify the number of milliseconds after which the notification should be auto-closed. Note that the value of 0 is best effort, some platforms honor it and some do not. Positive values are robust, since they can be implemented by the terminal emulator itself, by manually closing the notification after the expiry time. The notification could still be closed before the expiry time by user interaction or OS policy, but it is guaranteed to be closed once the expiry time has passed.

Adding icons to notifications

Added in version 0.36.0: Custom icons in notifications

Applications can specify a custom icon to be displayed with a notification. This can be the application’s logo or a symbol such as error or warning symbols. The simplest way to specify an icon is by name, using the n key. The value of this key is Base64 encoded UTF-8 text. Names can be either application names, or symbol names. The terminal emulator will try to resolve the name based on icons and applications available on the computer it is running on. The following list of well defined names must be supported by any terminal emulator implementing this spec. The n key can be specified multiple times, the terminal will go through the list in order and use the first icon that it finds available on the system.

Universally available icon names

Name	Description
error	An error symbol
warn, warning	A warning symbol
info	A symbol denoting an informational message
question	A symbol denoting asking the user a question
help	A symbol denoting a help message
file-manager	A symbol denoting a generic file manager application
system-monitor	A symbol denoting a generic system monitoring/information application
text-editor	A symbol denoting a generic text editor application

If an icon name is an application name it should be an application identifier, such as the filename of the application’s .desktop file on Linux or its bundle identifier on macOS. For example if the cross-platform application FooBar has a desktop file named: foo-bar.desktop and a bundle identifier of net.foo-bar-website.foobar then it should use the icon names net.foo-bar-website.foobar and foo-bar so that terminals running on both platforms can find the application icon.

If no icon is specified, but the f key (application name) is specified, the terminal emulator should use the value of the f key to try to find a suitable icon.

Adding icons by transmitting icon data

This can be done by using the p=icon key. Then, the payload is the icon image in any of the PNG, JPEG or GIF image formats. It is recommended to use an image size of 256x256 for icons. Since icons are binary data, they must be transmitted encoded, with e=1.

When both an icon name and an image are specified, the terminal emulator must first try to find a locally available icon matching the name and only if one is not found, fallback to the provided image. This is so that users are presented with icons from their current icon theme, where possible.

Transmitted icon data can be cached using the g key. The value of the g key must be a random globally unique UUID like Identifier. Then, the terminal emulator will cache the transmitted data using that key. The cache should exist for as long as the terminal emulator remains running. Thus, in future notifications, the application can simply send the g key to display a previously cached icon image with needing to re-transmit the actual data with p=icon. The g key refers only to the icon data, multiple different notifications with different icon or application names can use the same g key to refer to the same icon. Terminal multiplexers must cache icon data themselves and refresh it in the underlying terminal implementation when detaching and then re-attaching. This means that applications once started need to transmit icon data only once until they are quit.

Note

To avoid DoS attacks terminal implementations can impose a reasonable max size on the icon cache and evict icons in order of last used. Thus theoretically, a previously cached icon may become unavailable, but given that icons are small images, practically this is not an issue in all but the most resource constrained environments, and the failure mode is simply that the icon is not displayed.

Note

How the icon is displayed depends on the underlying OS notifications implementation. For example, on Linux, typically a single icon is displayed. On macOS, both the terminal emulator’s icon and the specified custom icon are displayed.

Adding buttons to the notification

Buttons can be added to the notification using the buttons payload, with p=buttons. Buttons are a list of UTF-8 text separated by the Unicode Line Separator character (U+2028) which is the UTF-8 bytes 0xe2 0x80 0xa8. They can be sent either as Escape code safe UTF-8 or Base64. When the user clicks on one of the buttons, and reporting is enabled with a=report, the terminal will send an escape code of the form:

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

Here, button_number is a number from 1 onwards, where 1 corresponds to the first button, two to the second and so on. If the user activates the notification as a whole, and not a specific button, the response, as described above is:

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

If no identifier was specified when creating the notification, i=0 is used. The terminal must not send a response unless report is requested with a=report.

Note

The appearance of the buttons depends on the underlying OS implementation. On most Linux systems, the buttons appear as individual buttons on the notification. On macOS they appear as a drop down menu that is accessible when hovering the notification. Generally, using more than two or three buttons is not a good idea.

Playing a sound with notifications

Added in version 0.36.0: The ability to control the sound played with notifications

By default, notifications may or may not have a sound associated with them depending on the policies of the OS notifications service. Sometimes it might be useful to ensure a notification is not accompanied by a sound. This can be done by using the s key which accepts Base64 encoded UTF-8 text as its value. The set of known sounds names is in the table below, any other names are implementation dependent, for instance, on Linux, terminal emulators will probably support the standard sound names

Standard sound names

Name	Description
system	The default system sound for a notification, which may be some kind of beep or just silence
silent	No sound must accompany the notification
error	A sound associated with error messages
warn, warning	A sound associated with warning messages
info	A sound associated with information messages
question	A sound associated with questions

Support for sound names can be queried as described below.

Querying for support

Added in version 0.36.0: The ability to query for support

An application can query the terminal emulator for support of this protocol, by sending the following escape code:

<OSC> 99 ; i=<some identifier> : p=? ; <terminator>

A conforming terminal must respond with an escape code of the form:

<OSC> 99 ; i=<some identifier> : p=? ; key=value : key=value <terminator>

The identifier is present to support terminal multiplexers, so that they know which window to redirect the query response too.

Here, the key=value parts specify details about what the terminal implementation supports. Currently, the following keys are defined:

Key	Value
a	Comma separated list of actions from the a key that the terminal implements. If no actions are supported, the a key must be absent from the query response.
c	c=1 if the terminal supports close events, otherwise the c must be omitted.
o	Comma separated list of occassions from the o key that the terminal implements. If no occasions are supported, the value o=always must be sent in the query response.
p	Comma spearated list of supported payload types (i.e. values of the p key that the terminal implements). These must contain at least title.
s	Comma separated list of sound names from the table of standard sound names above. Terminals will report the list of standard sound names they support. Terminals should support atleast system and silent.
u	Comma separated list of urgency values that the terminal implements. If urgency is not supported, the u key must be absent from the query response.
w	w=1 if the terminal supports auto expiring of notifications.

In the future, if this protocol expands, more keys might be added. Clients must ignore keys they do not understand in the query response.

To check if a terminal emulator supports this notifications protocol the best way is to send the above query action followed by a request for the primary device attributes. If you get back an answer for the device attributes without getting back an answer for the query action the terminal emulator does not support this notifications protocol.

Specification of all keys used in the protocol

Key	Value	Default	Description
a	Comma separated list of report, focus, with optional leading -	focus	What action to perform when the notification is clicked
c	0 or 1	0	When non-zero an escape code is sent to the application when the notification is closed.
d	0 or 1	1	Indicates if the notification is complete or not. A non-zero value means it is complete.
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
f	Base64 encoded UTF-8 application name	unset	The name of the application sending the notification. Can be used to filter out notifications.
g	Identifier	unset	Identifier for icon data. Make these globally unqiue, like an UUID.
i	Identifier	unset	Identifier for the notification. Make these globally unqiue, like an UUID, so that terminal multiplexers can direct responses to the correct window. Note that for backwards compatibility reasons i=0 is special and should not be used.
n	Base64 encoded UTF-8 application name	unset	Icon name. Can be specified multiple times.
o	One of always, unfocused or invisible	always	When to honor the notification request. unfocused means when the window the notification is sent on does not have keyboard focus. invisible means the window both is unfocused and not visible to the user, for example, because it is in an inactive tab or its OS window is not currently active. always is the default and always honors the request.
p	One of title, body, close, icon, ?, alive, buttons	title	Type of the payload. If a notification has no title, the body will be used as title. A notification with not title and no body is ignored. Terminal emulators should ignore payloads of unknown type to allow for future expansion of this protocol.
s	Base64 encoded sound name	system	The sound name to play with the notification. silent means no sound. system means to play the default sound, if any, of the platform notification service. Other names are implementation dependent.
t	Base64 encoded UTF-8 notification type	unset	The type of the notification. Used to filter out notifications. Can be specified multiple times.
u	0, 1 or 2	unset	The urgency of the notification. 0 is low, 1 is normal and 2 is critical. If not specified normal is used.
w	>=-1	-1	The number of milliseconds to auto-close the notification after.

Added in version 0.35.0: Support for the u key to specify urgency

Added in version 0.31.0: Support for the o key to prevent notifications from focused windows

Note

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

Base64

The base64 encoding used in the this specification is the one defined in RFC 4648. When a base64 payload is chunked, either the chunking should be done before encoding or after. When the chunking is done before encoding, no more than 2048 bytes of data should be encoded per chunk and the encoded data must include the base64 padding bytes, if any. When the chunking is done after encoding, each encoded chunk must be no more than 4096 bytes in size. There may or may not be padding bytes at the end of the last chunk, terminals must handle either case.

Escape code safe UTF-8

This must be valid UTF-8 as per the spec in RFC 3629. In addition, in order to make it safe for transmission embedded inside an escape code, it must contain none of the C0 and C1 control characters, that is, the unicode characters: U+0000 (NUL) - U+1F (Unit separator), U+7F (DEL) and U+80 (PAD) - U+9F (APC). Note that in particular, this means that no newlines, carriage returns, tabs, etc. are allowed.

Identifier

Any string consisting solely of characters from the set [a-zA-Z0-9_-+.], that is, the letters a-z, A-Z, the underscore, the hyphen, the plus sign and the period. Applications should make these globally unique, like a UUID for maximum robustness.

Important

Terminals must sanitize ids received from client programs before sending them back in responses, to mitigate input injection based attacks. That is, they must either reject ids containing characters not from the above set, or remove bad characters when reading ids sent to them.