HA Frontend Contexts

Use this skill when a component reads Home Assistant state, registries, localization, services, config, UI data, connection state, or API helpers.

Goal

Move leaf components away from the broad hass: HomeAssistant object. Broad hass access rerenders components for unrelated changes, hides the data a component depends on, and makes tests harder to mock.

Container components may keep hass when they own it and feed providers. Leaf components should consume the narrowest context that covers their reads.

Core Files

• Context definitions: src/data/context/index.ts
• consume decorator, ContextSubscriptionController, and consumeContext / ContextController for reactive controllers: src/common/decorators/consume.ts (use instead of @lit/context's consume and ContextConsumer, enforced by ESLint)
• Entity-scoped consume helpers: src/common/decorators/consume-context-entry.ts
• Transform decorator: src/common/decorators/transform.ts
• Canonical migration example: src/panels/lovelace/cards/hui-button-card.ts
• Providers are wired by contextMixin on HassBaseEl; consumers do not wire providers manually.

Context Selection

Lazy contexts subscribe on first consumer and tear down after the last consumer: labelsContext, fullEntitiesContext, configEntriesContext, manifestsContext, triggerDescriptionsContext, and conditionDescriptionsContext.

The single-field contexts such as localizeContext, themesContext, and userContext are deprecated. Use grouped contexts instead.

Consumption Patterns

Import consume from src/common/decorators/consume, not from @lit/context. The @lit/context version forces a host update on every context change, so every consumed field rerenders the component, even without @state(), and @transform cannot skip anything. Our consume leaves update scheduling to the field:

• @state() plus @consume: rerenders when the context value changes.
• @state() plus @consume plus @transform: rerenders only when the transformed value changes.
• @consume alone: the field stays current but never triggers a render. Use this for values only read in event handlers or callbacks, such as apiContext, or for data read on demand, such as picker item callbacks.

In reactive controllers, extend ContextController and decorate fields with consumeContext({ context, subscribe, transform }) from the same file. transform stores one part of the context value, and contextUpdated() runs only when that part changes. @state() and @transform do not work on controller fields; call host.requestUpdate() when the host needs to rerender. Example: src/data/service-info-controller.ts.

For lazily created subscriptions, use ContextSubscriptionController from the same file instead of @lit/context's ContextConsumer, which also forces a host update. Its callback must store the value in a reactive field or call host.requestUpdate() itself when the host needs to rerender.

Use entity-scoped helpers when the component watches an entity ID held on the host:

@state() @consumeEntityState({ entityIdPath: ["_config", "entity"] })
private _stateObj?: HassEntity;

@state() @consumeEntityRegistryEntry({ entityIdPath: ["_config", "entity"] })
private _entity?: EntityRegistryDisplayEntry;

@state() @consumeLocalize()
private _localize!: LocalizeFunc;

Use consumeEntityStates when the host property contains one or more entity IDs. It filters missing entities and preserves the previous record when none of the selected entities changed.

@state() @consumeEntityStates({ entityIdPath: ["_config", "entities"] })
private _stateObjs?: Record<string, HassEntity>;

For a single field from a grouped context, pair @consume with @transform:

@state()
@consume({ context: uiContext, subscribe: true })
@transform<HomeAssistantUI, Themes>({ transformer: ({ themes }) => themes })
private _themes!: Themes;

Use @transform with watch when the transformer depends on a host property, such as a computed entity ID. consumeEntityState and consumeEntityStates only watch the first path segment.

To consume a whole group untransformed, omit @transform and type the field as ContextType<typeof statesContext> or the matching context type.

Review Checklist

• The component consumes the narrowest context needed for the data it reads.
• A broad hass property is kept only when the component is a container or external API requires it.
• Entity-scoped reads use the consume helpers rather than ad hoc context transforms.
• Context fields read during render or willUpdate are marked @state() so updates trigger rendering. Fields only read outside the render cycle are left without @state().
• Tests and mocks only provide the data the component actually consumes.