Chat - Message parts

Render reasoning, sources, files, step markers, and data parts with your own plain React markup.

Each message part type produced by the streaming protocol gets its own rendering branch. Every assistant message has a parts array where each entry is a typed object. Core does not render these parts for you—you branch on part.type and render whatever you want.

Key concepts

The part type union

Each part in message.parts has a type discriminant:

message.parts.map((part, index) => {
  switch (part.type) {
    case 'text':
      return <p key={index}>{part.text}</p>;
    case 'reasoning':
      return (
        <details key={index}>
          <summary>Thinking</summary>
          {part.text}
        </details>
      );
    case 'file':
      return (
        <a key={index} href={part.url}>
          {part.filename ?? 'File'}
        </a>
      );
    case 'source-url':
      return (
        <a key={index} href={part.url}>
          {part.title ?? part.url}
        </a>
      );
    case 'source-document':
      return <cite key={index}>{part.title}</cite>;
    case 'tool':
      return <pre key={index}>{JSON.stringify(part.toolInvocation, null, 2)}</pre>;
    case 'step-start':
      return <hr key={index} />;
    default:
      // data-* parts and custom parts
      return <pre key={index}>{JSON.stringify(part, null, 2)}</pre>;
  }
});

Built-in part types

Part type	Key fields	Description
`text`	`text`, `state`	Plain text content
`reasoning`	`text`, `state`	Chain-of-thought trace
`file`	`mediaType`, `url`, `filename`	Inline file
`source-url`	`sourceId`, `url`, `title`	URL citation
`source-document`	`sourceId`, `title`, `text`	Document citation
`tool`	`toolInvocation`	Tool call with state lifecycle
`dynamic-tool`	`toolInvocation`	Unregistered tool call
`step-start`	(none)	Step boundary marker
`data-*`	`type`, `data`, `transient`	Custom data payload

Streaming state on parts

Text and reasoning parts have an optional state field:

'streaming' — the part is still receiving deltas
'done' — the part is complete

Use part.state to show a typing indicator or pulsing cursor while content is arriving.

Key takeaways

Message parts are typed data—you own the rendering completely.
Branch on part.type in a switch statement to render each variant.
Use part.state on text and reasoning parts to show streaming indicators
For app-specific part types, register renderers via partRenderers or use useChatPartRenderer()

API

ChatRoot