The code segment — code blocks in an AI chat, highlighted

Code segment - syntax highlighted code block

Highlighting is a plugin you opt into; with none installed the block renders plain, which is what core’s zero-dependency promise costs here.

A segment is data, not an element — it has no tag and dispatches nothing. This one is AparteCodeSegment, 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

{
  id: 's1',
  type: 'code',
  language: 'ts',
  filename: 'transport.ts',
  content: "const transport = new AparteBackendTransport({ endpoint: '/api/chat' });",
}

Shape

Discriminated by type: 'code'.

Field	Type	Required	Description
content	string	yes
language	string	—
filename	string	—
showLineNumbers	boolean	—

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
fallback	string	—	One sentence standing in for the segment when no renderer claims its type — DRAWN for every type, not only for custom. More
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

fallback

One sentence standing in for the segment when no renderer claims its type — DRAWN for every type, not only for custom.

It is the answer to a real moment: a conversation replayed in another app, a client that loads its views lazily, an export. Core draws it as text (no markup) under .aparte-segment-fallback. Supplying one also silences the “no renderer” developer warning: an author who wrote a fallback has already said this can happen.

In the HISTORY sent back to the model it stands in for the segment only where core does not serialise the type itself — custom, and any type core does not know (a registered block grammar’s, a plugin’s). A text or a code segment contributes its content and nothing else; thinking and tool_call are left out of the history on purpose (_segmentsToText in AparteClient states why), and so is error.

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

A built-in renderer draws this one, from packages/core/src/renderers/segments/code.ts. It installs itself the first time a segment of this type needs it, so a chat renders it with no setup.

Replace it with your own for this config only:

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

registerSegmentRenderer({
    type: 'code',
    render: (segment: AparteCodeSegment) => {
        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.