# 05 Custom Tool

Script: `example/05_custom_tool.py`.

This script implements a deterministic calculator by subclassing `FlowToolCall`. The model chooses when to call it; the tool itself is local Python code.

```bash
python example/05_custom_tool.py
```

| Method/member | Role |
| --- | --- |
| `name` | OpenAI function tool name. |
| `description` | Model-visible tool description. |
| `parameters` | JSON Schema, generated from a Pydantic model. |
| `__call__(session, arguments)` | Runtime execution. |
| `parallel_safe=True` | Lets the async runtime group this pure tool separately. |

The script avoids `eval`; it parses arithmetic with `ast` and a small operator whitelist.

[← Runnable Examples](index.md)