Tool calling
The ability of a language model to invoke an external function or API, with structured arguments, and use the result, so it can act rather than only generate text.
Tool calling (also called function calling) is the mechanism by which a language model triggers an external action. The model is told which tools exist and what arguments they take; when a task needs one, it emits a structured call naming the tool and its parameters. The application runs the tool, returns the result to the model, and the model continues with that result in context. This is what lets a model fetch live data, perform a calculation, or carry out an operation instead of only producing prose.
Tool calling is the building block of an agent. A loop of "decide which tool to call, call it, read the result, decide again" is how an agent makes progress on a multi-step task. The quality of the tools, how clearly they are described, how reliable their outputs are, and whether they have side effects, directly shapes how well the agent performs.
On Proxygate, buying data is itself a tool call. Through the MCP server, an agent calls call_api with a listing, endpoint, and method, and receives the upstream response, the exact amount charged, and a signed receipt. Discovery and payment are also tools (browse_apis, check_balance), so the whole purchase flow happens inside the agent's normal tool-calling loop.
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Related concepts
Tool calling: frequently asked questions
Tool calling is when a language model invokes an external function or API with structured arguments and uses the result. It lets the model act, by fetching data or performing operations, rather than only generating text.
Tool calling is the building block of an agent. An agent loops through deciding which tool to call, calling it, reading the result, and deciding again, which is how it makes progress on a multi-step task.
Yes. Through the Proxygate MCP server, an agent buys data with the call_api tool and discovers and funds it with tools like browse_apis and check_balance, so purchasing happens inside its normal tool-calling loop.