Suppose you ask an assistant to suggest an outfit for today. To make a useful suggestion, it may need the local weather and your calendar. An application programming interface, or API, is a defined way for software to access another system’s capabilities. If the assistant is given access to weather and calendar information, it can use those sources as steps toward an answer; without that access, it cannot retrieve them. section 6.7
An API can also give an AI agent a tool to act beyond conversation. A travel assistant might use external services to find recommendations, while a payment API could enable a purchase. Access alone does not make the action reliable: an assistant might suggest a flight that does not exist or misunderstand what the traveler wants. Those risks matter more when it can make a payment than when it merely offers advice. section 6.7
Using an API well involves choosing the right tool, supplying an input it accepts, and making use of its result. In Toolformer, a language model was trained with examples of API calls and their responses. Calls were marked with special tokens, and training examples were filtered to favor calls that helped predict an answer. This taught the model when and how to use a small set of tools; it did not make every possible tool call necessary or trustworthy. section 8.11
Table 8.1 Sample API tool calls and output formats [49]
| API name |
Example input |
Example output |
| Question answering |
“Where was the Knights of Columbus founded?” |
“New Haven, Connecticut” |
| Wikipedia search |
“Fishing Reel Types” |
“Spin fishing > Spin fishing is distinguished between fly fishing and bait cast fishing by the type of rod and reel used. There are two types of reels used when spin fishing, the open-faced reel and the closed-faced reel.” |
| Calculator |
“27 + 4 * 2” |
“35” |
| Calendar |
(none) |
Today is Monday, June 30, 2025. |
| Machine translation |
“sûreté nucléaire” |
“nuclear safety” |
The sample calls show that an API’s expected input and returned output depend on the tool: a calculator accepts an expression, while the calendar example takes no input and returns a date. section 8.11
