Chain of Thought

Group reasoning and tool calls into a collapsible accordion UI.

LLMs often produce reasoning steps and tool calls in succession. Chain of Thought lets you visually group these consecutive parts into a single collapsible accordion, giving users a clean "thinking" UI.

Overview

When a model like OpenAI's o4-mini responds, it may emit a sequence of reasoning tokens and tool calls before producing its final text answer. By default, these parts render individually. ChainOfThoughtPrimitive groups consecutive reasoning + tool-call parts together and renders them through a single component.

Key benefits:

Cleaner UI — Collapse intermediate steps behind a "Thinking" toggle
Better context — Users see that reasoning and tool calls are related
Built-in accordion — Expand/collapse with a single click; collapsed by default

Quick Start

Pass a ChainOfThought component to MessagePrimitive.Parts

MessagePrimitive.Parts accepts a ChainOfThought component. When provided, consecutive reasoning and tool-call parts are automatically grouped and rendered through it.

import {
  AuiIf,
  ChainOfThoughtPrimitive,
  MessagePrimitive,
} from "@assistant-ui/react";
import type { FC } from "react";

const Reasoning: FC<{ text: string }> = ({ text }) => {
  return (
    <p className="whitespace-pre-wrap px-4 py-2 text-muted-foreground text-sm italic">
      {text}
    </p>
  );
};

const ChainOfThought: FC = () => {
  return (
    <ChainOfThoughtPrimitive.Root className="my-2 rounded-lg border">
      <ChainOfThoughtPrimitive.AccordionTrigger className="flex w-full cursor-pointer items-center gap-2 px-4 py-2 font-medium text-sm hover:bg-muted/50">
        Thinking
      </ChainOfThoughtPrimitive.AccordionTrigger>
      <AuiIf condition={({ chainOfThought }) => !chainOfThought.collapsed}>
        <ChainOfThoughtPrimitive.Parts
          components={{ Reasoning, tools: { Fallback: ToolFallback } }}
        />
      </AuiIf>
    </ChainOfThoughtPrimitive.Root>
  );
};

const AssistantMessage: FC = () => {
  return (
    <MessagePrimitive.Root>
      <MessagePrimitive.Parts
        components={{
          Text: MarkdownText,
          ChainOfThought, // groups reasoning + tool parts
        }}
      />
    </MessagePrimitive.Root>
  );
};

Use a reasoning model

Chain of Thought is most useful with models that produce reasoning tokens (e.g. OpenAI o4-mini). Here's an example backend route using the Vercel AI SDK:

app/api/chat/route.ts

import { openai } from "@ai-sdk/openai";
import { streamText, convertToModelMessages } from "ai";

export async function POST(req: Request) {
  const { messages } = await req.json();

  const result = streamText({
    model: openai("o4-mini"),
    messages: await convertToModelMessages(messages),
  });

  return result.toUIMessageStreamResponse();
}

<AuiIf condition={({ chainOfThought }) => !chainOfThought.collapsed}>
  <ChainOfThoughtPrimitive.Parts
    components={{
      Reasoning,
      tools: { Fallback: ToolFallback },
      Layout: ({ children }) => (
        <div className="border-l-2 border-muted pl-4">{children}</div>
      ),
    }}
  />
</AuiIf>

Prop	Type	Description
`components.Reasoning`	`FC<{ text: string }>`	Component to render reasoning parts
`components.tools.Fallback`	`ToolCallMessagePartComponent`	Fallback component for tool-call parts
`components.Layout`	`ComponentType<PropsWithChildren>`	Wrapper component around each rendered part when expanded

Reading Collapsed State

Use AuiIf to conditionally render based on the accordion state:

import { AuiIf, ChainOfThoughtPrimitive } from "@assistant-ui/react";
import { ChevronDownIcon, ChevronRightIcon } from "lucide-react";

const ChainOfThoughtAccordionTrigger = () => {
  return (
    <ChainOfThoughtPrimitive.AccordionTrigger className="flex w-full cursor-pointer items-center gap-2 px-4 py-2 text-sm">
      <AuiIf condition={({ chainOfThought }) => chainOfThought.collapsed}>
        <ChevronRightIcon className="size-4" />
      </AuiIf>
      <AuiIf condition={({ chainOfThought }) => !chainOfThought.collapsed}>
        <ChevronDownIcon className="size-4" />
      </AuiIf>
      Thinking
    </ChainOfThoughtPrimitive.AccordionTrigger>
  );
};

Full Example

See the complete with-chain-of-thought example for a working implementation with tool calls and reasoning.

Generative UI — Custom UI for tool calls
Tools — Defining and using tools