Castella Agent UI Components

High-level components for building conversational interfaces and agent management UIs.

When to use: "create a chat UI", "AgentChat", "chat with agent", "display tool calls", "multi-agent chat", "AgentHub", "message history", "MultiAgentChat"

Quick Start (3 Lines)

Create a chat UI connected to an A2A agent:

from castella.agent import AgentChat

chat = AgentChat.from_a2a("http://localhost:8080")
chat.run()

Installation

uv sync --extra agent   # Agent UI + A2A + A2UI support

AgentChat

High-level chat component with minimal setup:

from castella.agent import AgentChat

# Connect to A2A agent
chat = AgentChat.from_a2a("http://localhost:8080")
chat.run()

# Or use custom handler function
chat = AgentChat(
    handler=lambda msg: f"Echo: {msg}",
    title="Echo Bot",
    system_message="Welcome! How can I help?",
)
chat.run()

Parameters

AgentChat(
    a2a_client=None,           # A2AClient instance
    handler=None,              # Custom handler: (str) -> str
    title="Agent Chat",        # Window title
    placeholder="Type...",     # Input placeholder
    system_message=None,       # Initial system message
    show_agent_card=True,      # Show agent card for A2A
    width=700,                 # Window width
    height=550,                # Window height
)

Factory Methods

# From A2A agent URL
chat = AgentChat.from_a2a("http://localhost:8080")

# From A2A client
from castella.a2a import A2AClient
client = A2AClient("http://localhost:8080")
chat = AgentChat.from_a2a(client)

Chat Components

Build custom chat UIs with lower-level components.

ChatContainer

Complete chat UI (messages + input):

from castella.agent import ChatContainer, ChatMessageData
from castella.core import ListState

messages = ListState([])

def on_send(text: str):
    messages.append(ChatMessageData(role="user", content=text))
    # Get response from agent...
    response = get_response(text)
    messages.append(ChatMessageData(role="assistant", content=response))

container = ChatContainer(
    messages,
    on_send=on_send,
    title="My Chat",
    placeholder="Type a message...",
)

ChatMessage

Display a single message:

from castella.agent import ChatMessage, ChatMessageData

msg = ChatMessageData(
    role="assistant",  # "user", "assistant", or "system"
    content="Hello! How can I help you today?",
)
widget = ChatMessage(msg)

ChatInput

Text input with send button:

from castella.agent import ChatInput

input_widget = ChatInput(
    placeholder="Type a message...",
    on_send=lambda text: print(f"Sent: {text}"),
)

ChatView

Scrollable message list:

from castella.agent import ChatView
from castella.core import ScrollState

scroll_state = ScrollState()
view = ChatView(messages, scroll_state=scroll_state)

ChatMessageData

Message data structure:

from castella.agent import ChatMessageData, ToolCallData

msg = ChatMessageData(
    role="assistant",
    content="Let me check the weather for you.",
    tool_calls=[
        ToolCallData(
            id="call_1",
            name="get_weather",
            arguments={"location": "Tokyo"},
            result="Sunny, 22°C",
        )
    ],
)

Tool Call Visualization

ToolCallView

Display a single tool call:

from castella.agent import ToolCallView

tool = ToolCallView(
    name="get_weather",
    arguments={"location": "Tokyo"},
    result="Sunny, 22°C",
)

ToolHistoryPanel

Display history of tool calls:

from castella.agent import ToolHistoryPanel
from castella.core import ListState

tool_calls = ListState([...])
panel = ToolHistoryPanel(tool_calls)

Agent Card Display

AgentCardView

Show agent information:

from castella.agent import AgentCardView
from castella.a2a import A2AClient

client = A2AClient("http://agent.example.com")
card_view = AgentCardView(
    client.agent_card,
    show_skills=True,
    compact=False,
)

AgentListView

Display multiple agents:

from castella.agent import AgentListView

agent_list = AgentListView(
    agents=[client1.agent_card, client2.agent_card],
    on_select=lambda card: print(f"Selected: {card.name}"),
)

MultiAgentChat

Tabbed interface for multiple agents:

from castella.agent import MultiAgentChat
from castella.a2a import A2AClient

chat = MultiAgentChat({
    "weather": A2AClient("http://localhost:8081"),
    "travel": A2AClient("http://localhost:8082"),
    "restaurant": A2AClient("http://localhost:8083"),
})
chat.run()

Each agent gets its own chat tab with independent message history.

AgentHub

Agent discovery and management dashboard:

from castella.agent import AgentHub
from castella.a2a import A2AClient

# Create hub
hub = AgentHub(title="Agent Hub")

# Add agents
hub.add_agent("http://localhost:8081")
hub.add_agent(A2AClient("http://localhost:8082"))

hub.run()

Or initialize with agents:

hub = AgentHub(agents=[
    A2AClient("http://agent1.example.com"),
    A2AClient("http://agent2.example.com"),
])

Features:

• Left panel: List of agents with add/remove
• Right panel: Chat with selected agent
• URL input to add new agents at runtime

Scroll Position Pattern

Important pattern for chat UIs - set scroll before adding message:

class ChatComponent(Component):
    def __init__(self):
        super().__init__()
        self._messages = ListState([])
        self._messages.attach(self)
        self._scroll_state = ScrollState()
        # DON'T attach scroll state

    def _send_message(self, text: str):
        # Add user message
        self._messages.append(ChatMessageData(role="user", content=text))

        # Get response...
        response = get_response(text)

        # Set scroll BEFORE adding response (so re-render picks it up)
        self._scroll_state.y = 999999
        self._messages.append(ChatMessageData(role="assistant", content=response))

Lazy State Attachment

For components created before App exists (like AgentHub):

class MyComponent(Component):
    def __init__(self):
        super().__init__()
        self._state = State(0)
        self._states_attached = False
        # DON'T attach here - App may not exist yet

    def view(self):
        # Attach lazily when view() is called
        if not self._states_attached:
            self._state.attach(self)
            self._states_attached = True
        return Text(str(self._state()))

Message Markdown Support

Messages support Markdown formatting:

msg = ChatMessageData(
    role="assistant",
    content="""
# Weather Report

**Tokyo**: Sunny, 22°C

| Day | High | Low |
|-----|------|-----|
| Mon | 24°C | 18°C |
| Tue | 22°C | 17°C |
""",
)

Best Practices

1. Use ScrollState without attaching for chat scroll
2. Set scroll position before adding new messages
3. Use Markdown for rich message content
4. Handle loading states for async responses
5. Use ListState.append() for new messages
6. Use lazy state attachment for hub-style components

Reference

• references/components.md

  - Component API reference

• references/data_classes.md

  - ChatMessageData, ToolCallData types

• scripts/

  - Executable examples (simple_chat.py, multi_agent.py, agent_hub.py)