assistant-ui-primitives

Design

Guide for assistant-ui UI primitives - ThreadPrimitive, ComposerPrimitive, MessagePrimitive. Use when customizing chat UI components.

QUICK START

How to use this skill

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Source SKILL.md: https://github.com/compozy/agh/blob/HEAD/.agents/skills/assistant-ui/assistant-ui-primitives/SKILL.md

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assistant-ui Primitives

Always consult assistant-ui.com/llms.txt for latest API.

Composable, unstyled components following Radix UI patterns.

References

./references/thread.md -- ThreadPrimitive deep dive
./references/composer.md -- ComposerPrimitive deep dive
./references/message.md -- MessagePrimitive deep dive
./references/action-bar.md -- ActionBarPrimitive deep dive

Import

import {
  AuiIf,
  ThreadPrimitive,
  ComposerPrimitive,
  MessagePrimitive,
  ActionBarPrimitive,
  BranchPickerPrimitive,
  AttachmentPrimitive,
  ThreadListPrimitive,
  ThreadListItemPrimitive,
} from "@assistant-ui/react";

Primitive Parts

Primitive	Key Parts
`ThreadPrimitive`	`.Root`, `.Viewport`, `.Messages`, `.Empty`, `.ScrollToBottom`
`ComposerPrimitive`	`.Root`, `.Input`, `.Send`, `.Cancel`, `.Attachments`
`MessagePrimitive`	`.Root`, `.Parts`/`.Content`, `.If`, `.Error`
`ActionBarPrimitive`	`.Copy`, `.Edit`, `.Reload`, `.Speak`, `.FeedbackPositive`, `.FeedbackNegative`, `.ExportMarkdown`
`BranchPickerPrimitive`	`.Previous`, `.Next`, `.Number`, `.Count`

Custom Thread Example

function CustomThread() {
  return (
    <ThreadPrimitive.Root className="flex flex-col h-full">
      <ThreadPrimitive.Empty>
        <div className="flex-1 flex items-center justify-center">
          Start a conversation
        </div>
      </ThreadPrimitive.Empty>

      <ThreadPrimitive.Viewport className="flex-1 overflow-y-auto p-4">
        <ThreadPrimitive.Messages components={{
          UserMessage: CustomUserMessage,
          AssistantMessage: CustomAssistantMessage,
        }} />
      </ThreadPrimitive.Viewport>

      <ComposerPrimitive.Root className="border-t p-4 flex gap-2">
        <ComposerPrimitive.Input className="flex-1 rounded-lg border px-4 py-2" />
        <ComposerPrimitive.Send className="bg-blue-500 text-white px-4 py-2 rounded-lg">
          Send
        </ComposerPrimitive.Send>
      </ComposerPrimitive.Root>
    </ThreadPrimitive.Root>
  );
}

Conditional Rendering

Prefer AuiIf for new code. Primitive .If components still exist but are deprecated.

<AuiIf condition={({ message }) => message.role === "user"}>
  User only
</AuiIf>
<AuiIf condition={({ thread }) => thread.isRunning}>
  Generating...
</AuiIf>
<AuiIf condition={({ message }) => message.branchCount > 1}>
  Has edit history
</AuiIf>

<AuiIf condition={({ thread }) => thread.isRunning}>
  <ComposerPrimitive.Cancel>Stop</ComposerPrimitive.Cancel>
</AuiIf>

<AuiIf condition={({ thread }) => thread.isEmpty}>No messages</AuiIf>

Content Parts

<MessagePrimitive.Content components={{
  Text: ({ part }) => <p>{part.text}</p>,
  Image: ({ part }) => <img src={part.image} alt="" />,
  ToolCall: ({ part }) => <div>Tool: {part.toolName}</div>,
  Reasoning: ({ part }) => <details><summary>Thinking</summary>{part.text}</details>,
}} />

Branch Picker

<MessagePrimitive.If hasBranches>
  <BranchPickerPrimitive.Root className="flex items-center gap-1">
    <BranchPickerPrimitive.Previous>←</BranchPickerPrimitive.Previous>
    <span><BranchPickerPrimitive.Number /> / <BranchPickerPrimitive.Count /></span>
    <BranchPickerPrimitive.Next>→</BranchPickerPrimitive.Next>
  </BranchPickerPrimitive.Root>
</MessagePrimitive.If>

Common Gotchas

Primitives not rendering

Wrap in AssistantRuntimeProvider
Ensure parent primitive provides context

Styles not applying

Primitives are unstyled by default
Add className and style with your app's Tailwind/CSS system