> ## Documentation Index
> Fetch the complete documentation index at: https://docs.upsonic.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Callbacks

> Hook into skill usage events for logging and monitoring

## Overview

Register callback functions to be notified when agents interact with skills. Useful for logging, analytics, and custom monitoring.

## Available Callbacks

| Callback              | Triggered When                     | Arguments                                            |
| --------------------- | ---------------------------------- | ---------------------------------------------------- |
| `on_load`             | Agent loads a skill's instructions | `skill_name: str, description: str`                  |
| `on_script_execute`   | Agent executes a skill script      | `skill_name: str, script_path: str, returncode: int` |
| `on_reference_access` | Agent reads a skill reference      | `skill_name: str, reference_path: str`               |

## Usage

```python theme={null}
from upsonic import Agent
from upsonic.skills import Skills, LocalSkills


def on_skill_loaded(skill_name: str, description: str):
    print(f"[SKILL LOADED] {skill_name}: {description}")


def on_script_executed(skill_name: str, script_path: str, returncode: int):
    print(f"[SCRIPT RUN] {skill_name}/{script_path} -> exit code {returncode}")


def on_reference_accessed(skill_name: str, reference_path: str):
    print(f"[REFERENCE READ] {skill_name}/{reference_path}")


skills = Skills(
    loaders=[LocalSkills("./my-skills")],
    on_load=on_skill_loaded,
    on_script_execute=on_script_executed,
    on_reference_access=on_reference_accessed,
)

agent = Agent(
    model="anthropic/claude-sonnet-4-6",
    name="Agent",
    role="Developer Assistant",
    goal="Assist with development tasks",
    skills=skills,
)

result = agent.print_do("Review this code using the code-review skill.")
# Output:
# [SKILL LOADED] code-review: Code quality analysis and best practices
# [REFERENCE READ] code-review/style-guide.md
```

## Error Handling

Callback exceptions are caught and logged as warnings — they never interrupt the agent's execution:

```python theme={null}
def buggy_callback(skill_name: str, description: str):
    raise ValueError("oops")

skills = Skills(
    loaders=[LocalSkills("./my-skills")],
    on_load=buggy_callback,  # Logs warning, doesn't crash
)
```