--- title: "What Is Tool Calling (Function Calling)? | Dawiso" description: "Tool calling lets an LLM produce a structured request to invoke an external function or API. Learn how it works and how it relates to MCP and agents." url: "https://www.dawiso.com/glossary/tool-calling" language: "en" --- June 30, 2026 tool callingfunction callingtool useLLMAI agentsMCP # What Is Tool Calling (Function Calling)? **Tool calling** is the mechanism by which a [large language model](https://www.dawiso.com/glossary/large-language-model) can invoke an external function, API, or service to complete a task. Rather than answering only from what it learned in training, the model generates a structured request, typically JSON, that names a tool and supplies its arguments. The application executes that tool and feeds the result back to the model. Tool calling is what turns a text generator into something that can fetch live data and act on real systems. It matters because the most useful enterprise AI tasks need information the model does not have: a current account balance, a customer's order history, the result of a calculation. Instead of hallucinating an answer, a model with tool calling passes the request to the right tool, a database query, a weather API, a calculator, and incorporates the real result into its response. TL;DR **Tool calling** (also called **function calling**) lets an LLM emit a structured request, usually JSON, to invoke an external function or API. The model decides *which* tool to call and with *what* arguments, but it does not execute the tool; the application runs it and returns the result for the model to use. It is the foundation of AI agents. The [Model Context Protocol (MCP)](https://www.dawiso.com/glossary/model-context-protocol-mcp) standardizes how tools and context are exposed to any model, so they do not have to be wired up per application. Tools are only as trustworthy as the data behind them. Dawiso exposes governed catalog, glossary, and lineage as context any MCP-compatible agent can call. ## What Tool Calling Means In a tool-calling setup, the model is given a prompt alongside a predefined set of tools, each described with a name, a purpose, its arguments, and the shape of its expected output. The model reads the prompt and decides whether reaching for a tool is necessary to fulfill the request. If it is, the model does not run anything itself. It produces a structured object identifying the tool and the parameters to use, and hands that back to the application to execute. ## How It Works The loop is straightforward and iterative: - **Describe the tools.** The application tells the model which tools exist and how to call them. - **The model decides.** Given a request, the model determines whether a tool is needed, picks the right one, and assembles the arguments into structured JSON. - **The application executes.** Critically, the model does not execute the function. The application runs the tool, calls the API, queries the database, and captures the result. - **The result feeds back.** The output is returned to the model, which incorporates it into its final response, or calls another tool, creating a dynamic, multi-step problem-solving loop. That separation, the model proposes, the application executes, is what makes tool calling governable: every action passes through code you control. ## Tool Calling vs. Function Calling The two terms describe the same capability. **Function calling** is the name OpenAI popularized for letting a model interface with external systems; the broader community increasingly says **tool calling** because a "tool" can be any external capability, not just a code function. Some providers use "tool use." For practical purposes, treat them as synonyms: a model emitting a structured request for an external capability that the application then runs. ## Tool Calling and Agents Tool calling is the building block of [AI agents](https://www.dawiso.com/glossary/agentic-ai). An agent is, in large part, a loop of tool calls: the model assesses a goal, calls a tool, reads the result, and decides the next step until the task is done. Without tool calling, a model can only talk; with it, the model can search, retrieve, compute, and act. That power is also why [AI agent governance](https://www.dawiso.com/glossary/ai-agent-governance) matters: each tool call is a point where an agent touches real data or systems and must stay within policy. ## Tool Calling vs. MCP Tool calling and the Model Context Protocol are complementary, not competing. Tool calling is the *mechanism*: the model emits a structured request and the application executes it. [MCP](https://www.dawiso.com/glossary/model-context-protocol-mcp) is the *open standard* for how tools and context are exposed to a model in the first place, so a tool wired up once is available to any MCP-compatible model or agent, rather than being re-integrated for every application. Function calling makes a model able to call a tool; MCP makes the enterprise's tools and governed context discoverable and reusable across all of them. ## How Dawiso Fits Tool calling makes a model able to reach your systems, but the value of any tool depends on the data behind it. A tool that queries the wrong table, or returns numbers an agent cannot interpret, produces a confident wrong answer just as surely as a hallucination would. Dawiso turns the data an agent can call into governed, trustworthy context: - **Governed context as a callable resource.** The [data catalog](https://www.dawiso.com/product/data-catalog) and [business glossary](https://www.dawiso.com/product/business-glossary) expose what terms, metrics, and datasets mean, so a tool call resolves to the right, authoritative data. - **Trust signals on every answer.** [Lineage](https://www.dawiso.com/product/interactive-data-lineage) and [classification](https://www.dawiso.com/glossary/data-classification) travel with the context, so an agent knows where data came from and how sensitive it is before acting on it. - **Available to any MCP-compatible agent.** The [Context Layer](https://www.dawiso.com/context-layer) serves this governed context through the open [MCP Server](https://www.dawiso.com/product/mcp), so it is callable across agents and applications without bespoke integration. Tool calling is the mechanism; Dawiso makes sure the tools resolve to data your agents can trust. ## Conclusion Tool calling, or function calling, is the mechanism that lets an LLM emit a structured request to invoke an external function or API, with the application executing the tool and feeding the result back. It is the foundation of agents and the reason AI can fetch live data and act rather than only generate text. MCP standardizes how those tools and context are exposed so they are reusable across models. The remaining question is whether the data behind each tool is trustworthy, which is exactly the governed context a context layer provides. [See it in action](https://www.dawiso.com/product/mcp) MCP (Model Context Protocol) Connect agents and LLMs directly to your enterprise data and business knowledge. ### Table of contents What It MeansHow It WorksTool vs. Function CallingTool Calling & AgentsTool Calling vs. MCPHow Dawiso Fits ## Related Terms ### [What Is an MCP Server? Meaning, Types and Security](https://www.dawiso.com/glossary/mcp-server) An MCP server exposes tools, resources and prompts to AI agents over the Model Context Protocol. How it works, local vs remote, OAuth and security risks. Read more → ### [What Is Model Context Protocol (MCP)?](https://www.dawiso.com/glossary/model-context-protocol-mcp) Learn what Model Context Protocol (MCP) is, how it connects AI agents to enterprise data, and why it matters for reliable AI applications. Read more → ### [What Are Multi-Agent Systems?](https://www.dawiso.com/glossary/multi-agent-systems) Multi-agent systems are architectures where multiple AI agents collaborate to complete complex tasks. Learn orchestration patterns, enterprise use cases, and the governance they require. Read more → ### [What Is Procedural Memory in AI Agents?](https://www.dawiso.com/glossary/agent-memory-procedural) Procedural memory is the learned skills, workflows, and tool-use routines an AI agent applies to act. Learn how it differs from semantic and episodic memory. Read more → ### [What Are Context Packs?](https://www.dawiso.com/glossary/context-packs) Context packs are reusable, curated bundles of definitions, data, and tools that give AI agents the context for a domain or task. Learn the emerging pattern. Read more → ### [OpenAI Frontier: The Data Governance Problem](https://www.dawiso.com/glossary/openai-frontier-data-governance) OpenAI Frontier lets enterprises deploy AI agents across their data and systems. Learn the data governance problem this creates - and how a context layer solves it. Read more →