Acting — How Tools Get Invoked

Tripmate has decided its next step: search trains from Pune to Jaipur for Thursday night. But a language model can only produce text. It cannot open a connection to a railway website. So how does a decision become an action?

The rule

The model writes a request; your code does the deed.

A tool is a function your program offers to the model, described by a name, a purpose and the inputs it takes. When the model wants to act, it does not run anything. It writes a structured tool call — which tool, with which arguments:

Your program then does four things:

1. Reads the call and checks the tool name exists.
2. Validates the arguments — right types, allowed values, permission to act.
3. Runs the real function, such as an API request to a timetable service.
4. Returns the result to the model as a new message, so the next turn can use it.

Many model providers support this directly: you pass tool descriptions with the request, and the model's reply can contain tool calls in a fixed format instead of plain text. The exact field names differ between providers, but the shape is the same everywhere.

The shortcut

Picture a waiter and a kitchen. The model is the waiter: it takes the order and writes a ticket. Your code is the kitchen: it checks the ticket, cooks, and sends the plate back. Whenever you wonder "can the model do X?", ask "is there a ticket for X, and will the kitchen accept it?"

The trap is trusting the ticket blindly. Models sometimes invent a tool that does not exist, pass a malformed date, or request an action they should not take. Validation in your code is the safety net, and anything risky — like book_ticket, which spends money — deserves a confirmation step.

Drills

Answer fast — trust the rule.

Remember

• The model writes a tool call; your code validates it, runs it and returns the result.
• Shortcut: waiter and kitchen — is there a ticket, and will the kitchen accept it?
• Trap: running tool calls without validation, especially ones that spend money or change data.