The custom segment — your own message content (generative UI)

Custom segment - for framework-specific views (Onboarding, Tools, etc.)

A segment is data, not an element — it has no tag and dispatches nothing. This one is AparteCustomSegment, and it is the object you push into a bubble; something else draws it.

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Core's own renderer, drawing the example printed below inside a real viewport. A segment is data — this is what the library makes of it, and it looks the same in every framework.

Example

// With no renderer registered for `subType`, core draws `fallback` — which is why the
// field exists: an unknown view degrades to a sentence instead of to nothing.
{
  id: 's1',
  type: 'custom',
  subType: 'weather-widget',
  data: { city: 'Lille', celsius: 11 },
  fallback: 'Lille — 11°C, overcast.',
}

Shape

Discriminated by type: 'custom'.

Field	Type	Required	Description
subType	string	yes	Unique string structure to identify the view (e.g. ‘onboarding’, ‘webcontainer’, ‘weather-widget’)
data	unknown	—	Arbitrary data payload for the component
fallback	string	—	Optional fallback text representation

Shared by every segment

From AparteSegmentBase, which is also what a segment type of your own extends.

Field	Type	Required	Description
id	string	yes	Unique segment identifier
type	string	yes	Segment type discriminator
isStreaming	boolean	—	Whether segment is currently being streamed
messageId	string	—	Id of the message this segment belongs to. Stamped on insertion.
index	number	—	Position in the owning message’s segments[], maintained across insertions and removals.
meta	Record<string, unknown> & { aparte?: AparteSegmentTiming }	—	Extras the producer of the segment knows — token counts, cost, compute device — plus aparte, the one sub-object core writes. More

Notes on individual fields

meta

Extras the producer of the segment knows — token counts, cost, compute device — plus aparte, the one sub-object core writes.

Everything except meta.aparte is yours: fill it with updateSegment(segmentId, { meta }), which MERGES rather than replaces. Mirrors AparteMessage.metadata.

Who emits it

An app pushes one with addSegment() on the viewport, or a parser produces it while a reply streams. Core stamps its identity and timing on insert, so an emitted segment does not have to carry them.

Who draws it

Nothing in core draws this one — it is the escape hatch, and the renderer is yours. Without one registered, core falls back: it draws the segment’s fallback string when there is one, and otherwise a [Unknown segment type: custom] placeholder with a console warning naming the type.

import { registerSegmentRenderer } from '@aparte/core';
import type { AparteCustomSegment } from '@aparte/core';

registerSegmentRenderer({
    type: 'custom',
    render: (segment: AparteCustomSegment) => {
        const el = document.createElement('div');
        el.textContent = JSON.stringify(segment);
        return el;
    },
});

See Customization for the whole renderer seam, and bring your own loop for emitting segments yourself.