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Function Calling vs. Tool Use: What's the Difference?
Whether function calling and tool use are the same thing, and where the terminology genuinely diverges in practice.
Direct answer
Function calling and tool use refer to the same underlying capability — a model choosing to invoke a defined function with structured arguments — with 'function calling' typically used for the specific API feature a model provider exposes, and 'tool use' as the broader term covering that capability regardless of implementation, including MCP-based tool exposure.
Last reviewed 2026-08-29
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Same capability, different framing
'Function calling' is commonly the vendor-specific term for the API feature that lets a model return a structured request to call a named function with specific arguments. 'Tool use' is the broader, framework-agnostic term for the same underlying behavior — a model deciding to invoke something beyond text generation — and covers function calling as one implementation among others, including MCP's standardized tool exposure.
Where a practical distinction sometimes matters
In practice, 'tool use' is often used when discussing the agent's decision-making about which capability to invoke, while 'function calling' is used when discussing the specific request/response format an API expects — but this is a convention, not a rigid technical boundary, and the terms are frequently used interchangeably in documentation and conversation.