Reasoning

AG-UI provides first-class support for LLM reasoning, enabling chain-of-thought visibility while maintaining privacy and state continuity across conversation turns.

Overview

Modern LLMs increasingly use chain-of-thought reasoning to improve response quality. AG-UI’s reasoning support addresses three key challenges:

Reasoning visibility: Surface reasoning signals (e.g., summaries) to users without exposing raw chain-of-thought
State continuity: Maintain reasoning context across turns using encrypted reasoning items, even under store:false or zero data retention (ZDR) policies
Privacy compliance: Support enterprise privacy requirements while preserving reasoning capabilities

Unlike Activity messages, Reasoning messages are intended to represent the agent’s internal thought process and may be encrypted for privacy and are meant to be sent back to the agent for further processing on subsequent turns.

ReasoningMessage

The ReasoningMessage type represents reasoning content in the message history:

interface ReasoningMessage {
  id: string
  role: "reasoning"
  content: string // Reasoning content (visible to client)
  encryptedValue?: string // Optional encrypted reasoning for state continuity
}

Property	Type	Description
`id`	`string`	Unique identifier for the reasoning message
`role`	`"reasoning"`	Message role discriminator
`content`	`string`	Reasoning content visible to the client
`encryptedValue`	`string?`	Encrypted chain-of-thought blob for state continuity

Key characteristics:

Separate from assistant messages: Reasoning is kept distinct from final responses to avoid polluting conversation history
Streamable: Content arrives via streaming events
Optional encryption: When encryptedValue is present, it represents encrypted chain-of-thought that the client stores and forwards opaquely

Reasoning Events

Reasoning events manage the lifecycle of reasoning messages. See Events for the complete event reference.

Event Flow

A typical reasoning flow follows this pattern:

Event Types

Event	Purpose
`ReasoningStart`	Marks beginning of reasoning phase
`ReasoningMessageStart`	Begins a streaming reasoning message
`ReasoningMessageContent`	Delivers reasoning content chunks
`ReasoningMessageEnd`	Completes a reasoning message
`ReasoningMessageChunk`	Convenience event that auto-manages message lifecycle
`ReasoningEnd`	Marks completion of reasoning
`ReasoningEncryptedValue`	Attaches encrypted chain-of-thought to a message or tool call

Privacy and Compliance

AG-UI reasoning is designed with privacy-first principles:

Zero Data Retention (ZDR)

For deployments requiring zero data retention:

Encrypted reasoning values can carry state across turns without storing decryptable content on the client
The client receives and forwards encryptedValue blobs opaquely via ReasoningEncryptedValue events
Only the agent (or authorized backend) can decrypt the reasoning content

Visibility Control

Agents control what reasoning is visible to users:

Full visibility: Stream the complete chain-of-thought via ReasoningMessageContent events
Summary only: Emit a condensed summary while attaching detailed reasoning as encrypted values
Hidden: Use only ReasoningEncryptedValue events with no visible streaming

Compliance Considerations

Requirement	Solution
GDPR right to erasure	Encrypted content can be discarded without losing reasoning capability
SOC 2 data handling	Reasoning content never stored in plaintext on client
HIPAA minimum necessary	Only summaries exposed; detailed reasoning stays encrypted
Audit logging	`ReasoningStart`/`ReasoningEnd` events provide audit trail without content exposure

Example Implementations

Basic Reasoning Flow

A simple implementation showing visible reasoning:

// Agent emits reasoning start
yield {
  type: "REASONING_START",
  messageId: "reasoning-001",
}

// Stream visible reasoning content
yield {
  type: "REASONING_MESSAGE_START",
  messageId: "msg-123",
  role: "reasoning",
}

yield {
  type: "REASONING_MESSAGE_CONTENT",
  messageId: "msg-123",
  delta: "Let me ",
}

yield {
  type: "REASONING_MESSAGE_CONTENT",
  messageId: "msg-123",
  delta: "think through ",
}

yield {
  type: "REASONING_MESSAGE_CONTENT",
  messageId: "msg-123",
  delta: "this step ",
}

yield {
  type: "REASONING_MESSAGE_CONTENT",
  messageId: "msg-123",
  delta: "by step...",
}

yield {
  type: "REASONING_MESSAGE_END",
  messageId: "msg-123",
}

// End reasoning
yield {
  type: "REASONING_END",
  messageId: "reasoning-001",
}

Encrypted Content for State Continuity

When maintaining reasoning state across turns without exposing content, use the ReasoningEncryptedValue event to attach encrypted chain-of-thought to messages or tool calls:

// Agent emits reasoning start
yield {
  type: "REASONING_START",
  messageId: "reasoning-002",
}

// Stream a visible summary for the user
yield {
  type: "REASONING_MESSAGE_START",
  messageId: "msg-456",
  role: "reasoning",
}

yield {
  type: "REASONING_MESSAGE_CONTENT",
  messageId: "msg-456",
  delta: "Analyzing your request...",
}

yield {
  type: "REASONING_MESSAGE_END",
  messageId: "msg-456",
}

// Attach encrypted chain-of-thought to the reasoning message
yield {
  type: "REASONING_ENCRYPTED_VALUE",
  subtype: "message",
  entityId: "msg-456",
  encryptedValue: "eyJhbGciOiJBMjU2R0NNIiwiZW5jIjoiQTI1NkdDTSJ9...",
}

yield {
  type: "REASONING_END",
  messageId: "reasoning-002",
}

// On subsequent turns, client sends back the message with encryptedValue
// which the agent can decrypt to restore reasoning context

Attaching Encrypted Reasoning to Tool Calls

You can also attach encrypted reasoning to tool calls to capture why the agent chose specific arguments or how it interpreted results:

// Tool call with encrypted reasoning
yield {
  type: "TOOL_CALL_START",
  toolCallId: "tool-123",
  toolCallName: "search_database",
  parentMessageId: "msg-789",
}

yield {
  type: "TOOL_CALL_ARGS",
  toolCallId: "tool-123",
  delta: '{"query": "user preferences"}',
}

yield {
  type: "TOOL_CALL_END",
  toolCallId: "tool-123",
}

// Attach encrypted reasoning explaining why this tool was called
yield {
  type: "REASONING_ENCRYPTED_VALUE",
  subtype: "tool-call",
  entityId: "tool-123",
  encryptedValue: "encrypted-reasoning-about-tool-selection...",
}

ZDR-Compliant Implementation

For zero data retention scenarios:

// Server-side: encrypt reasoning before sending
const encryptedReasoning = await encrypt(detailedChainOfThought, secretKey)

yield {
  type: "REASONING_START",
  messageId: "reasoning-003",
}

// Only emit a high-level summary to the client
yield {
  type: "REASONING_MESSAGE_CHUNK",
  messageId: "summary-001",
  delta: "Processing your request securely...",
}

yield {
  type: "REASONING_MESSAGE_CHUNK",
  messageId: "summary-001",
  delta: "", // Empty delta closes the message
}

// Attach the encrypted chain-of-thought
yield {
  type: "REASONING_ENCRYPTED_VALUE",
  subtype: "message",
  entityId: "summary-001",
  encryptedValue: encryptedReasoning,
}

yield {
  type: "REASONING_END",
  messageId: "reasoning-003",
}

// Client stores only:
// - The encrypted blob (cannot decrypt)
// - The summary text (no sensitive details)
// Full reasoning is never persisted in plaintext

Using the Convenience Chunk Event

The ReasoningMessageChunk event simplifies implementation by auto-managing message lifecycle:

// First chunk with messageId starts the message automatically
yield {
  type: "REASONING_MESSAGE_CHUNK",
  messageId: "msg-789",
  delta: "Analyzing the problem space...",
}

// Subsequent chunks continue the stream
yield {
  type: "REASONING_MESSAGE_CHUNK",
  messageId: "msg-789",
  delta: " Considering multiple approaches...",
}

// Empty delta (or next non-reasoning event) closes automatically
yield {
  type: "REASONING_MESSAGE_CHUNK",
  messageId: "msg-789",
  delta: "",
}

Client Integration

Handling Reasoning Events

import { EventType, type BaseEvent } from "@ag-ui/core"

function handleEvent(event: BaseEvent) {
  switch (event.type) {
    case EventType.REASONING_START:
      // Initialize reasoning UI (e.g., "thinking" indicator)
      console.log("Agent is reasoning...")
      break

    case EventType.REASONING_MESSAGE_CONTENT:
      // Append visible reasoning to UI
      appendReasoningText(event.messageId, event.delta)
      break

    case EventType.REASONING_ENCRYPTED_VALUE:
      // Store encrypted value for the referenced entity
      if (event.subtype === "message") {
        storeMessageEncryptedValue(event.entityId, event.encryptedValue)
      } else if (event.subtype === "tool-call") {
        storeToolCallEncryptedValue(event.entityId, event.encryptedValue)
      }
      break

    case EventType.REASONING_END:
      // Finalize reasoning UI
      console.log("Reasoning complete")
      break
  }
}

Passing Encrypted Reasoning Back

When making subsequent requests, include stored encrypted values:

const response = await agent.run({
  threadId: "thread-123",
  messages: [
    ...previousMessages,
    {
      id: "reasoning-002",
      role: "reasoning",
      content: "Analyzing your request...", // Visible summary
      encryptedValue: storedEncryptedBlob, // Opaque to client
    },
    {
      id: "user-msg-001",
      role: "user",
      content: "Follow up question...",
    },
  ],
})

Migration from Thinking Events

The THINKING_* events are deprecated and will be removed in version 1.0.0. New implementations should use REASONING_* events.

Deprecated Events

The following events are deprecated:

Deprecated Event	Replacement
`THINKING_START`	`REASONING_START`
`THINKING_END`	`REASONING_END`
`THINKING_TEXT_MESSAGE_START`	`REASONING_MESSAGE_START`
`THINKING_TEXT_MESSAGE_CONTENT`	`REASONING_MESSAGE_CONTENT`
`THINKING_TEXT_MESSAGE_END`	`REASONING_MESSAGE_END`

Migration Steps

Update event types: Replace all THINKING_* event types with their REASONING_* equivalents
Update message types: Use ReasoningMessage with role: "reasoning" instead of any thinking-specific message types
Add encrypted value support: Consider using ReasoningEncryptedValue events for improved privacy compliance
Test thoroughly: Ensure existing functionality works with the new event types

Example Migration

Before (deprecated):

// ❌ Deprecated - do not use
yield { type: "THINKING_START", messageId: "think-001" }
yield { type: "THINKING_TEXT_MESSAGE_START", messageId: "msg-001" }
yield { type: "THINKING_TEXT_MESSAGE_CONTENT", messageId: "msg-001", delta: "..." }
yield { type: "THINKING_TEXT_MESSAGE_END", messageId: "msg-001" }
yield { type: "THINKING_END", messageId: "think-001" }

After (current):

// ✅ Current implementation
yield { type: "REASONING_START", messageId: "reasoning-001" }
yield { type: "REASONING_MESSAGE_START", messageId: "msg-001", role: "assistant" }
yield { type: "REASONING_MESSAGE_CONTENT", messageId: "msg-001", delta: "..." }
yield { type: "REASONING_MESSAGE_END", messageId: "msg-001" }
yield { type: "REASONING_END", messageId: "reasoning-001" }

Best Practices

Always pair start/end events: Every ReasoningStart should have a corresponding ReasoningEnd
Use encrypted values for sensitive reasoning: When chain-of-thought contains sensitive information, use ReasoningEncryptedValue to attach encrypted content to messages or tool calls
Provide user feedback: Even with encrypted reasoning, emit visible summaries so users know the agent is working
Handle missing events gracefully: Clients should be resilient to incomplete event streams
Consider bandwidth: For very long reasoning chains, consider emitting only summaries to reduce data transfer

Events - Complete event type reference
Messages - Message type documentation
Serialization - State continuity and lineage

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Tutorials

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Reasoning

Reasoning

Overview

ReasoningMessage

Reasoning Events

Event Flow

Event Types

Privacy and Compliance

Zero Data Retention (ZDR)

Visibility Control

Compliance Considerations

Example Implementations

Basic Reasoning Flow

Encrypted Content for State Continuity

Attaching Encrypted Reasoning to Tool Calls

ZDR-Compliant Implementation

Using the Convenience Chunk Event

Client Integration

Handling Reasoning Events

Passing Encrypted Reasoning Back

Migration from Thinking Events

Deprecated Events

Migration Steps

Example Migration

Best Practices

​Reasoning

​Overview

​ReasoningMessage

​Reasoning Events

​Event Flow

​Event Types

​Privacy and Compliance

​Zero Data Retention (ZDR)

​Visibility Control

​Compliance Considerations

​Example Implementations

​Basic Reasoning Flow

​Encrypted Content for State Continuity

​Attaching Encrypted Reasoning to Tool Calls

​ZDR-Compliant Implementation

​Using the Convenience Chunk Event

​Client Integration

​Handling Reasoning Events

​Passing Encrypted Reasoning Back

​Migration from Thinking Events

​Deprecated Events

​Migration Steps

​Example Migration

​Best Practices

​Related Documentation

Reasoning

Overview

ReasoningMessage

Reasoning Events

Event Flow

Event Types

Privacy and Compliance

Zero Data Retention (ZDR)

Visibility Control

Compliance Considerations

Example Implementations

Basic Reasoning Flow

Encrypted Content for State Continuity

Attaching Encrypted Reasoning to Tool Calls

ZDR-Compliant Implementation

Using the Convenience Chunk Event

Client Integration

Handling Reasoning Events

Passing Encrypted Reasoning Back

Migration from Thinking Events

Deprecated Events

Migration Steps

Example Migration

Best Practices

Related Documentation