> ## 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.

# Advanced Features

> Caching, auto-selection, dependencies, callbacks, safety policies, and more

## Overview

Beyond basic skill loading, the Skills system provides advanced features for production use:

| Feature                                                    | Description                                        |
| ---------------------------------------------------------- | -------------------------------------------------- |
| [Caching](/concepts/skills/advanced/caching)               | In-memory TTL cache for skill content              |
| [Auto-Selection](/concepts/skills/advanced/auto-selection) | Embedding-based skill relevance filtering          |
| [Dependencies](/concepts/skills/advanced/dependencies)     | Declare and validate inter-skill dependencies      |
| [Versioning](/concepts/skills/advanced/versioning)         | Semantic versioning and constraint filtering       |
| [Callbacks](/concepts/skills/advanced/callbacks)           | Hook into skill load, reference, and script events |
| [Safety Policies](/concepts/skills/advanced/safety)        | Content filtering via the safety engine            |
| [Knowledge Base](/concepts/skills/advanced/knowledge-base) | Semantic search across skill references via RAG    |

## Skills Constructor Reference

All advanced features are configured via the `Skills` constructor:

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

skills = Skills(
    loaders=[LocalSkills("./my-skills")],     # Required — skill sources
    strict_deps=False,                         # Raise on dependency issues
    cache_ttl=300,                             # Cache TTL in seconds (None = disabled)
    on_load=my_load_callback,                  # Callback on skill load
    on_script_execute=my_script_callback,      # Callback on script execution
    on_reference_access=my_ref_callback,       # Callback on reference access
    auto_select=True,                          # Enable embedding-based selection
    max_skills=5,                              # Max skills in system prompt
    embedding_provider=my_provider,            # Provider for auto-select
    policy=my_safety_policy,                   # Safety policy or list of policies
)
```

## Merging Skills

Combine multiple `Skills` instances into one:

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

base = Skills(loaders=[BuiltinSkills()])
custom = Skills(loaders=[LocalSkills("./custom")])

merged = Skills.merge(base, custom)
# Later instances override on name conflict
```

## Tool Binding

Skills can declare which external tools they work best with:

```yaml theme={null}
# In SKILL.md frontmatter
allowed-tools:
  - code_execution
  - file_search
```

Access the union of allowed tools from all actively used skills:

```python theme={null}
active_tools = skills.get_active_skill_tools()
# Returns: {'code_execution', 'file_search'}
```

A skill becomes "active" when the agent calls `get_skill_instructions()` for it.