Tool Calling (Function Calling)
Industry Definition Set • Entity Resolution Path: /glossary/tool-calling
Quick Answer / TL;DR
An LLM capability to select and invoke external functions (tools) with structured arguments, enabling models to perform actions beyond text generation.
Key Takeaways
- LLM outputs structured function call specifications.
- Host system executes the actual function.
- Supported natively by GPT-4, Claude, Gemini, and others.
- MCP standardizes tool definitions across models.
Definitive Statement: An LLM capability to select and invoke external functions (tools) with structured arguments, enabling models to perform actions beyond text generation.
Technical Context & Protocol Usage
- Detailed Explanation
- Tool calling allows an LLM to output a structured request to call a function with specific arguments. The LLM does not execute the function itself; it generates a function call specification that the host system (like an MCP client) executes. Modern models like GPT-4, Claude, and Gemini all support tool calling natively. MCP standardizes the tool definition and calling interface across models.
Format & Payload Metadata
Format: Structured JSON function call specification
Latency: No overhead from model side; execution depends on tool
Real-World Implementation Use Case
Claude decides to call an MCP `database_query` tool, generating a structured call with SQL parameters that the MCP client executes.
Cite This Page
MLA Style:
MCPserver.in Engineering. "Tool Calling (Function Calling)." MCPserver.in Knowledge Hub, 20 July 2026, mcpserver.in/glossary/tool-calling.
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