07Tools & Function Calling

The model never runs anything itself

This surprises most people: the LLM can't actually send an email or open a file. What it can do is ask your code to run a named function with specific arguments. Your code runs it and hands back the result. That's function calling (also called tool use).

Anatomy of a tool definition

A tool is described to the model with a name, a description and a JSON Schema for its parameters. The model reads the description to decide when to use it, so the description is effectively a prompt.

{
  "name": "check_invoice_status",
  "description": "Look up the payment status of a client invoice by its invoice number. Use this whenever the user asks whether an invoice is paid, overdue, or how much is owed. Returns status, amount due in PHP, and days overdue.",
  "input_schema": {
    "type": "object",
    "properties": {
      "invoice_id": {
        "type": "string",
        "description": "The invoice number, digits only, e.g. '1042'"
      }
    },
    "required": ["invoice_id"]
  }
}

Different providers use slightly different field names (input_schema, parameters), and many SDKs generate this from normal functions automatically — as you'll see in Lesson 09.

Writing tools models use correctly

1. Name tools like verbs

   search_products, create_calendar_event, get_order_status. Avoid vague names like helper or process.

2. Write descriptions for a newcomer

   Say what it does, when to use it, what it returns, and any gotchas. Three or four sentences is normal for important tools.

3. Describe every parameter with an example

   Formats matter: “ISO date like 2026-10-10”, “amount in PHP, no commas”.

4. Keep tools focused

   One clear job each. A handful of well-designed tools beats dozens of overlapping ones.

5. Return useful, compact results

   Return what the model needs to decide the next step — not a 5 MB raw API dump. Summarize or paginate large outputs.

6. Return helpful errors

   Instead of crashing, return "error: invoice 1O42 not found — IDs are digits only". The model will often correct itself and retry.

Designing safe tools

Common tool types for business agents

Instead of writing every integration yourself, you can plug in ready-made tool servers through the Model Context Protocol (Lesson 11).