CopilotKit

Reasoning

Surface the agent's thinking chain in the chat — default or fully custom.

"use client";// Reasoning — Custom//// Pairs with `reasoning-default` so users can compare default vs custom// reasoning rendering side by side. Both demos share the same backend// (`reasoning_agent` graph) and runtime URL (/api/copilotkit). This cell// overrides the `reasoningMessage` slot on the `messageView` slot with// `ReasoningBlock` — a tagged amber banner that emphasizes the agent's// thinking chain.//// Reasoning is a first-class message type in v2: see// packages/react-core/src/v2/components/chat/CopilotChatMessageView.tsx,// which discriminates messages by `message.role === "reasoning"` and// renders them via the `reasoningMessage` slot (default component:// `CopilotChatReasoningMessage`). The slot override below is the public,// stable way to customize that output.import type { CopilotChatReasoningMessage } from "@copilotkit/react-core/v2";import { CopilotKit, CopilotChat } from "@copilotkit/react-core/v2";import { ReasoningBlock } from "./reasoning-block";import { useReasoningCustomSuggestions } from "./suggestions";const AGENT_ID = "reasoning-custom";export default function ReasoningCustomDemo() {  return (    <CopilotKit runtimeUrl="/api/copilotkit" agent={AGENT_ID}>      <div className="flex justify-center items-center h-screen w-full">        <div className="h-full w-full max-w-4xl">          <Chat />        </div>      </div>    </CopilotKit>  );}function Chat() {  // Hooks that consume CopilotKit context (suggestions, agent, etc.) must  // run INSIDE the <CopilotKit> provider — otherwise the context is null  // and `.subscribe(...)` blows up at mount.  useReasoningCustomSuggestions();  return (    <CopilotChat      agentId={AGENT_ID}      className="h-full rounded-2xl"      messageView={{        reasoningMessage:          ReasoningBlock as unknown as typeof CopilotChatReasoningMessage,      }}    />  );}

What is this?#

Some models (OpenAI's o1, o3, and o4-mini, Anthropic's thinking variants) emit reasoning tokens, internal chain-of-thought traces that explain how the model is working toward its answer. CopilotKit surfaces these as first-class messages: when a REASONING_MESSAGE_* event arrives from the agent, the chat renders it inline so the user can follow the agent's thinking.

Reasoning isn't a custom-renderer plumb-in; it's a dedicated message type on the chat view. You can either accept the built-in rendering or override the reasoningMessage slot with your own component.

When should I use this?#

Expose reasoning in the UI when you want to:

  • Give users real-time insight into the agent's thought process
  • Show progress on long or multi-step problems
  • Debug prompt behavior during development
  • Brand the reasoning card to match the rest of your product

Default reasoning rendering (zero-config)#

Out of the box, reasoning events render inside CopilotKit's built-in CopilotChatReasoningMessage card:

  • A "Thinking…" label with a pulsing indicator while the model reasons.
  • Auto-expanded content so users can follow the chain of thought live.
  • Collapses to "Thought for X seconds" once reasoning finishes, with a chevron to re-expand.
  • Reasoning text rendered as Markdown.

No configuration is needed; if your model emits reasoning tokens, the card appears automatically:

Here's what the built-in card looks like while the model thinks through a multi-step problem:

"use client";// Reasoning — Default//// Pairs with `reasoning-custom` (the Custom variant) so users can// compare default vs custom reasoning rendering side by side. This cell// renders <CopilotChat> with NO slot override — reasoning messages are// rendered by the built-in `CopilotChatReasoningMessage` component// (Thinking… / Thought for X header with an expandable content region).//// Both demos share the same backend (`reasoning_agent` graph) and the// same runtime URL (/api/copilotkit). The only difference is whether the// `messageView.reasoningMessage` slot is overridden.import { CopilotKit, CopilotChat } from "@copilotkit/react-core/v2";import { useReasoningDefaultSuggestions } from "./suggestions";const AGENT_ID = "reasoning-default";export default function ReasoningDefaultDemo() {  return (    <CopilotKit runtimeUrl="/api/copilotkit" agent={AGENT_ID}>      <div className="flex justify-center items-center h-screen w-full">        <div className="h-full w-full max-w-4xl">          <Chat />        </div>      </div>    </CopilotKit>  );}function Chat() {  useReasoningDefaultSuggestions();  return <CopilotChat agentId={AGENT_ID} className="h-full rounded-2xl" />;}

Custom reasoning rendering#

For full control over the reasoning card, pass a component to the reasoningMessage slot on messageView. Your component receives the ReasoningMessage object (.content holds the streaming text), the full messages list, and isRunning, enough to decide whether this block is still streaming and whether it's the active trailing message:

"use client";// Custom `reasoningMessage` slot renderer.//// Receives the `ReasoningMessage` plus (optionally) the full message list and// the running state from the slot system. Renders the content inline with a// visibly tagged amber banner so the user can always see the agent's thinking// chain — this is the focal UI of the demo.import React from "react";import type { ReasoningMessage, Message } from "@ag-ui/core";export function ReasoningBlock({  message,  messages,  isRunning,}: {  message: ReasoningMessage;  messages?: Message[];  isRunning?: boolean;}) {  const isLatest = messages?.[messages.length - 1]?.id === message.id;  const isStreaming = !!(isRunning && isLatest);  const hasContent = !!(message.content && message.content.length > 0);  return (    <div      data-testid="reasoning-block"      className="my-2 rounded-xl border border-[#DBDBE5] bg-[#BEC2FF1A] px-3.5 py-2.5 text-sm"    >      <div className="flex items-center gap-2 font-medium text-[#010507]">        <span className="inline-block rounded-full border border-[#BEC2FF] bg-white px-2 py-0.5 text-[10px] uppercase tracking-[0.14em] text-[#57575B]">          Reasoning        </span>        <span className="text-[#57575B]">          {isStreaming ? "Thinking…" : hasContent ? "Agent reasoning" : "…"}        </span>      </div>      {hasContent && (        <div className="mt-1.5 whitespace-pre-wrap italic text-[#57575B]">          {message.content}        </div>      )}    </div>  );}

The ReasoningBlock (imported above) renders the reasoning as an amber-tagged inline banner, intentionally louder than the default card so the thinking chain is the focal UI of the demo. Swap in your own component to match your product's tone.

The messageView.reasoningMessage slot accepts either a full component (as shown) or a sub-slot object like { header, contentView, toggle } if you just want to tweak parts of the default card. See the reference docs for sub-slot props.