---
title: "System Prompt"
description: "Customize the system prompt to control agent behavior and capabilities"
icon: "message"
---

## Overview

You can customize the system prompt by extending the `SystemPrompt` class. Internally, this adds extra instructions to the default system prompt.

<Note>
  Custom system prompts allow you to modify the agent's behavior at a
  fundamental level. Use this feature carefully as it can significantly impact
  the agent's performance and reliability.
</Note>

## Basic Customization

Create a custom system prompt by inheriting from the base class.

```python
from browser_use import Agent, SystemPrompt

class MySystemPrompt(SystemPrompt):
    def important_rules(self) -> str:
        # Get existing rules from parent class
        existing_rules = super().important_rules()

        # Add your custom rules
        new_rules = """
9. MOST IMPORTANT RULE:
- ALWAYS open first a new tab and go to wikipedia.com no matter the task!!!
"""

        # Make sure to use this pattern otherwise the exiting rules will be lost
        return f'{existing_rules}\n{new_rules}'
```

## Using Custom System Prompt

Apply your custom system prompt when creating an agent:

```python
from langchain_openai import ChatOpenAI

# Initialize the model
model = ChatOpenAI(model='gpt-4o')

# Create agent with custom system prompt
agent = Agent(
    task="Your task here",
    llm=model,
    system_prompt_class=MySystemPrompt
)
```
