Glossary
function calling
A pattern where a model emits a structured call (function name plus arguments), the runtime executes it, and the result returns as input on the model's next turn.
The protocol that lets a language model trigger code execution. The caller defines a tool schema (name, description, JSON-schema parameter shape); the model either responds with text or emits a structured tool- call object naming the tool and arguments; the host runs the tool and returns the result; the model continues with that result in context.
The technique is what makes agents possible. Web search, code execution, database queries, file reads, MCPprotocolsAn open protocol from Anthropic that standardizes how language models discover and call external tools, data sources, and prompts via a small JSON-RPC interface. Open full entry server actions: all are function calls under the hood. Reliability varies by model: function-calling quality is now a standard axis on which models are benchmarked.
Two flavors coexist. Provider-specific schemas (OpenAI’s tools API,
Anthropic’s tool_use blocks, Gemini’s function declarations) are the
common case. The MCPprotocolsAn open protocol from Anthropic that standardizes how language models discover and call external tools, data sources, and prompts via a small JSON-RPC interface.
Open full entry (MCP) tries to standardize
the tool layer across providers so the same MCP server works regardless
of which model is making the call.