Custom Visualizer

This example is available on GitHub: examples/01_standalone_sdk/26_custom_visualizer.py

The SDK provides flexible visualization options. You can use the default rich-formatted visualizer, customize it with highlighting patterns, or build completely custom visualizers by subclassing ConversationVisualizerBase.

Basic Example

examples/01_standalone_sdk/26_custom_visualizer.py

"""Custom Visualizer Example

This example demonstrates how to create and use a custom visualizer by subclassing
ConversationVisualizer. This approach provides:
- Clean, testable code with class-based state management
- Direct configuration (just pass the visualizer instance to visualizer parameter)
- Reusable visualizer that can be shared across conversations
- Better separation of concerns compared to callback functions
- Event handler registration to avoid long if/elif chains

This demonstrates how you can pass a ConversationVisualizer instance directly
to the visualizer parameter for clean, reusable visualization logic.
"""

import logging
import os

from pydantic import SecretStr

from openhands.sdk import LLM, Conversation
from openhands.sdk.conversation.visualizer import ConversationVisualizerBase
from openhands.sdk.event import (
    Event,
)
from openhands.tools.preset.default import get_default_agent


class MinimalVisualizer(ConversationVisualizerBase):
    """A minimal visualizer that print the raw events as they occur."""

    def __init__(self, name: str | None = None):
        """Initialize the minimal progress visualizer.

        Args:
            name: Optional name to identify the agent/conversation.
                                  Note: This simple visualizer doesn't use it in output,
                                  but accepts it for compatibility with the base class.
        """
        # Initialize parent - state will be set later via initialize()
        super().__init__(name=name)

    def on_event(self, event: Event) -> None:
        """Handle events for minimal progress visualization."""
        print(f"\n\n[EVENT] {type(event).__name__}: {event.model_dump_json()[:200]}...")


api_key = os.getenv("LLM_API_KEY")
assert api_key is not None, "LLM_API_KEY environment variable is not set."
model = os.getenv("LLM_MODEL", "openhands/claude-sonnet-4-5-20250929")
base_url = os.getenv("LLM_BASE_URL")
llm = LLM(
    model=model,
    api_key=SecretStr(api_key),
    base_url=base_url,
    usage_id="agent",
)
agent = get_default_agent(llm=llm, cli_mode=True)

# ============================================================================
# Configure Visualization
# ============================================================================
# Set logging level to reduce verbosity
logging.getLogger().setLevel(logging.WARNING)

# Start a conversation with custom visualizer
cwd = os.getcwd()
conversation = Conversation(
    agent=agent,
    workspace=cwd,
    visualizer=MinimalVisualizer(),
)

# Send a message and let the agent run
print("Sending task to agent...")
conversation.send_message("Write 3 facts about the current project into FACTS.txt.")
conversation.run()
print("Task completed!")

# Report cost
cost = llm.metrics.accumulated_cost
print(f"EXAMPLE_COST: ${cost:.4f}")

Running the Example

export LLM_API_KEY="your-api-key"
cd agent-sdk
uv run python examples/01_standalone_sdk/26_custom_visualizer.py

Visualizer Configuration Options

The visualizer parameter in Conversation controls how events are displayed:

from openhands.sdk import Conversation
from openhands.sdk.conversation import DefaultConversationVisualizer, ConversationVisualizerBase

# Option 1: Use default visualizer (enabled by default)
conversation = Conversation(agent=agent, workspace=workspace)

# Option 2: Disable visualization
conversation = Conversation(agent=agent, workspace=workspace, visualizer=None)

# Option 3: Pass a visualizer class (will be instantiated automatically)
conversation = Conversation(agent=agent, workspace=workspace, visualizer=DefaultConversationVisualizer)

# Option 4: Pass a configured visualizer instance
custom_viz = DefaultConversationVisualizer(
    name="MyAgent",
    highlight_regex={r"^Reasoning:": "bold cyan"}
)
conversation = Conversation(agent=agent, workspace=workspace, visualizer=custom_viz)

# Option 5: Use custom visualizer class
class MyVisualizer(ConversationVisualizerBase):
    def on_event(self, event):
        print(f"Event: {event}")

conversation = Conversation(agent=agent, workspace=workspace, visualizer=MyVisualizer())

Customizing the Default Visualizer

DefaultConversationVisualizer uses Rich panels and supports customization through configuration:

from openhands.sdk.conversation import DefaultConversationVisualizer

# Configure highlighting patterns using regex
custom_visualizer = DefaultConversationVisualizer(
    name="MyAgent",                       # Prefix panel titles with agent name
    highlight_regex={
        r"^Reasoning:": "bold cyan",      # Lines starting with "Reasoning:"
        r"^Thought:": "bold green",       # Lines starting with "Thought:"
        r"^Action:": "bold yellow",       # Lines starting with "Action:"
        r"\[ERROR\]": "bold red",         # Error markers anywhere
        r"\*\*(.*?)\*\*": "bold",         # Markdown bold **text**
    },
    skip_user_messages=False,             # Show user messages
)

conversation = Conversation(
    agent=agent,
    workspace=workspace,
    visualizer=custom_visualizer
)

When to use: Perfect for customizing colors and highlighting without changing the panel-based layout.

Creating Custom Visualizers

For complete control over visualization, subclass ConversationVisualizerBase:

from openhands.sdk.conversation import ConversationVisualizerBase
from openhands.sdk.event import ActionEvent, ObservationEvent, AgentErrorEvent, Event

class MinimalVisualizer(ConversationVisualizerBase):
    """A minimal visualizer that prints raw event information."""
    
    def __init__(self, name: str | None = None):
        super().__init__(name=name)
        self.step_count = 0
    
    def on_event(self, event: Event) -> None:
        """Handle each event."""
        if isinstance(event, ActionEvent):
            self.step_count += 1
            tool_name = event.tool_name or "unknown"
            print(f"Step {self.step_count}: {tool_name}")
            
        elif isinstance(event, ObservationEvent):
            print(f"  → Result received")
                
        elif isinstance(event, AgentErrorEvent):
            print(f"❌ Error: {event.error}")

# Use your custom visualizer
conversation = Conversation(
    agent=agent,
    workspace=workspace,
    visualizer=MinimalVisualizer(name="Agent")
)

Key Methods

__init__(self, name: str | None = None)

Initialize your visualizer with optional configuration
name parameter is available from the base class for agent identification
Call super().__init__(name=name) to initialize the base class

initialize(self, state: ConversationStateProtocol)

Called automatically by Conversation after state is created
Provides access to conversation state and statistics via self._state
Override if you need custom initialization, but call super().initialize(state)

on_event(self, event: Event) (required)

Called for each conversation event
Implement your visualization logic here
Access conversation stats via self.conversation_stats property

When to use: When you need a completely different output format, custom state tracking, or integration with external systems.

Next Steps

Now that you understand custom visualizers, explore these related topics:

Events - Learn more about different event types
Conversation Metrics - Track LLM usage, costs, and performance data
Send Messages While Running - Interactive conversations with real-time updates
Pause and Resume - Control agent execution flow with custom logic

Guides

Architecture

API Reference

Basic Example

Visualizer Configuration Options

Customizing the Default Visualizer

Creating Custom Visualizers

Key Methods

Next Steps

​Basic Example

​Visualizer Configuration Options

​Customizing the Default Visualizer

​Creating Custom Visualizers

​Key Methods

​Next Steps

Basic Example

Visualizer Configuration Options

Customizing the Default Visualizer

Creating Custom Visualizers

Key Methods

Next Steps