Define a tool for a model to call

I want the model to be able to <what the tool does>.

Write the tool definition:
- A name that says what it does.
- A description written for a reader who has never seen my system:
  what it does, when to use it, and explicitly when NOT to.
- Every parameter typed, described, and marked required or optional.
  No parameter whose meaning needs outside knowledge.
- What it returns, including what it returns when there's no result.

Then the handler:
- Validate every argument. The model can and will send nonsense.
- Never let a tool argument reach a query, a shell, or a file path
  unescaped.
- Return errors as data the model can act on, not exceptions.

Tell me the three ways the model is most likely to misuse this.