AG-UI Protocol

The Agent-User Interaction (AG-UI) Protocol is an open, lightweight, event-based protocol that standardizes how AI agents connect to user-facing applications.

When to Use This Skill

Use this skill when:

Implementing AG-UI protocol events in your code

Building agents that communicate with frontends via AG-UI

Understanding the event types and their structure

Implementing state management, tool calls, or message streaming

Debugging AG-UI event flows

Documentation

See the

docs/2025-11-27/

directory for complete AG-UI protocol documentation:

introduction.md

- Protocol overview and integrations

concepts/architecture.md

- Core architecture and design

concepts/events.md

- Event types and patterns

concepts/messages.md

- Message structure and types

concepts/state.md

- State management and synchronization

concepts/tools.md

- Tool definitions and lifecycle

concepts/agents.md

- Agent implementation

concepts/middleware.md

- Middleware patterns

concepts/serialization.md

- Event serialization and compaction

quickstart/introduction.md

- Getting started guide

quickstart/server.md

- Server implementation

quickstart/clients.md

- Client implementation

Quick Reference

Event Types

enum

EventType

{

// Lifecycle

RUN_STARTED

=

"RUN_STARTED"

,

RUN_FINISHED

=

"RUN_FINISHED"

,

RUN_ERROR

=

"RUN_ERROR"

,

STEP_STARTED

=

"STEP_STARTED"

,

STEP_FINISHED

=

"STEP_FINISHED"

,

// Text Messages

TEXT_MESSAGE_START

=

"TEXT_MESSAGE_START"

,

TEXT_MESSAGE_CONTENT

=

"TEXT_MESSAGE_CONTENT"

,

TEXT_MESSAGE_END

=

"TEXT_MESSAGE_END"

,

// Tool Calls

TOOL_CALL_START

=

"TOOL_CALL_START"

,

TOOL_CALL_ARGS

=

"TOOL_CALL_ARGS"

,

TOOL_CALL_END

=

"TOOL_CALL_END"

,

// State

STATE_SNAPSHOT

=

"STATE_SNAPSHOT"

,

STATE_DELTA

=

"STATE_DELTA"

,

MESSAGES_SNAPSHOT

=

"MESSAGES_SNAPSHOT"

,

// Custom

RAW

=

"RAW"

,

CUSTOM

=

"CUSTOM"

,

}

Event Patterns

Start-Content-End

Streams content incrementally (text, tool arguments)

Snapshot-Delta

State synchronization using complete snapshots + JSON Patch updates

Lifecycle

Run monitoring with mandatory start/end events Message Roles user - User messages (text and multimodal) assistant - AI responses (text and tool calls) system - Instructions or context tool - Tool execution results activity - Progress updates developer - Internal debugging Tool Definition Structure interface Tool { name : string ; // Unique identifier description : string ; // Purpose explanation parameters : JSONSchema ; // Accepted arguments } Tool Call Lifecycle TOOL_CALL_START - Initiates with unique ID TOOL_CALL_ARGS - Streams JSON arguments TOOL_CALL_END - Marks completion State Synchronization STATE_SNAPSHOT - Complete state replacement STATE_DELTA - Incremental JSON Patch (RFC 6902) updates Source Documentation downloaded from: https://github.com/ag-ui-protocol/ag-ui/tree/main/docs