Changelog

Version 0.15.0

Oct 9, 2026

Lit Inspector is on the Chrome Web Store, and the Timeline now explains itself: every update records what caused it, a column of rails draws the chain from a click or a parent render through @lit/task runs to the re-render, and a time range can be selected, summarised and shared as a link. The Components tab shows a component’s slots, parts, context, Lit’s warnings and tags that were never defined, and its details pane can be filtered, folded, expanded and copied from. Nothing needs action to upgrade.

Added

Lit Inspector is on the Chrome Web Store. Install it with Add to Chrome instead of loading an unpacked zip, and Chrome keeps it up to date. Firefox still loads the zip from the GitHub release.
Updates know what caused them. The cause is recorded at the requestUpdate call that scheduled an update, so the Updates tab reads “after click (412, 88)” or “after my-parent update” instead of guessing from timing, and the Timeline details pane names the cause with a show link to it.
The Timeline draws why each update ran. A column beside the list connects an update to the row that caused it: the parent whose render set a property on it, or the click, key or custom event whose handler requested it. Rows stay in time order, each chain has its own colour, and hovering a row highlights its chain.
@lit/task runs show in the Timeline’s cause chain. A task run is a row of its own, from the update or handler that started it to when it settled, and the re-render it asks for hangs from it, so a click, the fetch it started and the render that showed the result read as one chain.
Select a time range in the Timeline. Shift+drag on the tracks, or drag the time ruler, to see which layers fired and which components updated in that stretch, then zoom to it or filter the list to it. Drag either edge, or Tab to it and use the arrow keys, to adjust it a tick step at a time; Shift moves five.
Share a Timeline range as a link. Copy link in the range summary gives an address with #tab=timeline&range=<start>-<end> that opens the Tracks on that stretch, including in an exported snapshot.
Coding agents can summarise a window of the Timeline. The new lit_range-summary MCP tool takes a start and an end from lit_recent-events times and returns which layers fired and which components updated between them.
See a component’s slots and parts. The Components tab lists each slot with what is assigned to it, flags empty, fallback, forwarded and duplicate slots, and shows in red the children no slot takes. Turn on Anatomy to draw the slots and ::part exports on the page; hovering a row in the Slots or Parts table pulses that region and fades the others.
See parts forwarded with exportparts. The Parts list and the Anatomy overlay now include parts a nested component re-exports, marked forwarded, and show the outer name when exportparts renames one.
See which provider a Lit context consumer reads from. The Components details pane now shows each @lit/context provider and consumer with its key and value, links a consumer to its provider and a provider to its consumers, and selects the element when you click a link.
Record the custom events your components dispatch. The new Custom events layer on the Timeline, off by default, shows each event’s type, flags and detail next to the component that dispatched it.
Lit dev-mode warnings show up. Mistakes Lit warns about, like scheduling an update from inside updated(), record a warning event in the Timeline naming the component, including ones issued before recording started. A component Lit has warned about gets a warning chip and a Warnings section in the Components tab, with Lit’s message and a link to its explanation; agents get the same list from the component details tool.
Updates that shouldUpdate vetoes leave a trace. A refused update records an update skipped event in the Timeline with the property keys that changed, and the Updates tab counts each component’s refused updates and marks them in its list.
Components that were never defined show up in the tree. A custom tag on the page with no definition, from a missing import or a typo, is listed with a “not defined” chip instead of being left out; selecting it explains what is missing and links to the template that renders it. Agents see it in the list-components tool too.
Property options show on the Properties rows. A custom hasChanged or converter, noAccessor, useDefault and attribute: false each get a badge, so you can see how a property is declared without opening the source. lit_component-details lists the same options for agents.
Filter the Components tree. A box in the toolbar narrows the tree to elements whose tag or class name matches, keeps their parents for context, and restores your expanded branches when cleared.
Filter the details pane. A box under the tag name narrows properties, state, attributes, instance fields, slots and parts to rows whose name or value matches, and stays set as you select other components.
Details sections fold, and remember it. Properties, State, Attributes, Instance, Slots and Parts each fold on their heading, show how many rows they hold, and stay folded as you select other components.
Expand values in the details pane. Objects, arrays, Maps and Sets open level by level from a caret, so values nested past the preview limits are no longer cut off, and open levels follow the value as it changes. Maps and Sets also preview their first entries, such as Map(2) {"a" => 1, "b" => 2}.
Values in the details pane are syntax-coloured and pretty-printed. Strings, numbers, keywords and keys are coloured like code, and objects or arrays too wide for a line show one entry per line.
Changed values light up in the details pane. When the selected component updates, each row whose value changed briefly highlights, so you can see what an interaction touched.
Copy a value from the details pane. Hover a row for a copy button that puts the value, as shown, on the clipboard.
Shift-hover a tree row to outline every instance of that component. The page shows each element with the same tag in a dashed box, and the tree marks their rows.

Changed

Timeline list rows group by update. Each component update is one collapsed row showing its duration and changed properties; expand it to see its phases, skips, warnings and events from the same update.
The Timeline’s detail pane is resizable and easier to scan. Drag its top edge to give a selected event more room; the height sticks. A header shows the event’s name, layer, time and duration, and the facts below line up in a compact grid with pretty-printed data and any recorded error.
The Components details header is compact. The class and template locations, and the render root, now sit in a three-line block under the tag name, and a status shows next to the tag only while an update is pending or the first render has not happened.
Long values in the details pane get the full width. Objects and long strings move to their own line instead of wrapping in a narrow column, badges follow the value, and arrays, dates and DOM nodes are tagged with their type.
Task status shows as a coloured dot in the details pane. Instance rows name their kind in a quiet tag after the name instead of outlined chips, a task’s status is green, amber or red, and attribute values show as quoted strings.
Details sections are easier to tell apart. A thin rule and more space now separate Properties, State, Attributes, Instance, Slots and Parts.
The Anatomy toggle is remembered. Leaving it on keeps it on after the panel reloads.

Fixed

The Slots table follows light-child changes. Changing a child’s slot attribute, or adding or removing children, now updates the selected element’s slots without a re-render or a re-select.
Timeline ticks line up with the marks. With many lanes and a scrollbar, the time ruler was offset from the marks by the scrollbar’s width, and the range edges could block clicks on marks beneath them.
The Timeline range no longer overshoots the tracks. With many lanes and a vertical scrollbar, the shaded range’s right edge stopped a few pixels past the last mark; it now ends where the marks do.
Long strings in the details pane wrap under their own text. A string that wraps inside a pretty-printed object now continues indented under its line instead of at the left edge of the value.