Ch 4: Tools & Function Calling

Ch 4 — Tools & Function Calling

How LLMs invoke external functions, APIs, and databases — the bridge between reasoning and action

Index Under the Hood →

High Level

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Step- / 8

AWhy Tools?LLMs can think — tools let them act

psychology

LLM AloneCan reason but
can't act on the world

+ tools

build

LLM + ToolsCan search, call APIs,
query DBs, run code

smart_toy

AgentReasons about what
tool to use and when

arrow_downward How function calling works at a high level

BThe Function Calling FlowLLM decides → you execute → LLM sees result

chat

User Message"What's the weather
in Tokyo?"

+ tools

psychology

LLM DecidesReturns tool_call:
get_weather("Tokyo")

execute

api

You Run ItCall the actual
weather API

result

LLM Responds"It's 22°C and
sunny in Tokyo"

arrow_downward Defining tools with the @tool decorator

CDefining ToolsThe @tool decorator turns any function into a tool

code

Python Functiondef get_weather
(city: str) -> str

@tool

build

LangChain ToolName, description,
args schema

bind

smart_toy

Model + Toolsmodel.bind_tools
([get_weather])

arrow_downward The message protocol: AIMessage → ToolMessage

DThe Tool Message ProtocolHow tool calls and results flow as messages

psychology

AIMessage.tool_calls = [{
name, args, id}]

execute

build

Run Functionget_weather("Tokyo")
→ "22°C, sunny"

wrap

mail

ToolMessagecontent="22°C, sunny"
tool_call_id=matching

arrow_downward Multiple tools and parallel calling

EMultiple Tools & Parallel CallsLLMs can call several tools at once

cloud

get_weatherWeather API
for any city

web_searchSearch the web
for information

calculate

calculatorMath operations
and conversions

parallel

dynamic_feed

Parallel CallsLLM can call 2+
tools in one turn

arrow_downward MCP: the standard protocol for tool access

FMCP — Model Context ProtocolThe USB-C of AI tools: one standard, any tool

smart_toy

Any AgentLangChain, CrewAI,
any framework

MCP

hub

MCP ServerStandardized
tool interface

exposes

database

Any ResourceFiles, DBs, APIs,
GitHub, Slack...