Since: 20210502-130208-bff6815d

The format-tab-title event is emitted when the text for a tab title needs to be recomputed.

This event is a bit special in that it is synchronous and must return as quickly as possible in order to avoid blocking the GUI thread.

The most notable consequence of this is that some functions that are asynchronous (such as wezterm.run_child_process) are not possible to call from inside the event handler and will generate a format-tab-title: runtime error: attempt to yield from outside a coroutine error.

This example overrides the default tab title so that the background color is blue for the active tab. This is partially redundant because there is already configuration for this in tab_bar_style, but it demonstrates that it is possible to format more than just the text shown in the tab.

wezterm.on("format-tab-title", function(tab, tabs, panes, config, hover, max_width)
  if tab.is_active then
    return {
      {Text=" " .. tab.active_pane.title .. " "},
  return tab.active_pane.title

The parameters to the event are:

  • tab - the TabInformation for the active tab
  • tabs - an array containing TabInformation for each of the tabs in the window
  • panes - an array containing PaneInformation for each of the panes in the active tab
  • config - the effective configuration for the window
  • hover - true if the current tab is in the hover state
  • max_width - the maximum number of cells available to draw this tab when using the retro tab bar style.

The return value of the event can be:

  • a string, holding the text to use for the tab title
  • a table holding FormatItems as used in the wezterm.format function. This allows formatting style and color information for individual elements within the tab.

If the event encounters an error, or returns something that is not one of the types mentioned above, then the default tab title text will be computed and used instead.

When the tab bar is computed, this event is called twice for each tab; on the first pass, hover will be false and max_width will be set to tab_max_width. WezTerm will then compute the tab widths that will fit in the tab bar, and then call the event again for the set of tabs, this time with appropriate hover and max_width values.

Only the first format-tab-title event will be executed; it doesn't make sense to define multiple instances of the event with multiple wezterm.on("format-tab-title", ...) calls.

A more elaborate example:

local wezterm = require 'wezterm';

-- The filled in variant of the < symbol
local SOLID_LEFT_ARROW = utf8.char(0xe0b2)

-- The filled in variant of the > symbol
local SOLID_RIGHT_ARROW = utf8.char(0xe0b0)

wezterm.on("format-tab-title", function(tab, tabs, panes, config, hover, max_width)
  local edge_background = "#0b0022"
  local background = "#1b1032"
  local foreground = "#808080"

  if tab.is_active then
    background = "#2b2042"
    foreground = "#c0c0c0"
  elseif hover then
    background = "#3b3052"
    foreground = "#909090"

  local edge_foreground = background

  -- ensure that the titles fit in the available space,
  -- and that we have room for the edges.
  local title = wezterm.truncate_to_width(tab.active_pane.title, max_width-2)

  return {

return {}