Ch 4: Tools & Function Calling

Ch 4 — Tools & Function Calling

Inside the @tool decorator, bind_tools serialization, ToolMessage protocol, and MCP internals

Index ← High Level

Under the Hood

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A@tool Decorator InternalsWhat happens when Python loads your function

code

Your Functiondef + type hints
+ docstring

inspect

schema

Pydantic SchemaAuto-generated
args_schema model

wrap

build

StructuredToolBaseTool subclass
name, desc, schema

arrow_downward bind_tools: how tool schemas reach the LLM API

Bbind_tools & API SerializationConverting tools into the format the LLM API expects

build

StructuredTool.name .description
.args_schema

convert

data_object

JSON SchemaOpenAI "function"
format dict

attach

send

HTTP Requesttools=[ ] in
API payload

arrow_downward API response parsing: tool_calls on AIMessage

CResponse Parsing & AIMessage.tool_callsHow the raw API response becomes structured tool_calls

cloud_download

API ResponseJSON with
tool_calls array

parse

psychology

AIMessage.content = ""
.tool_calls = [...]

extract

list_alt

ToolCall Dictname, args (parsed),
id, type

arrow_downward Tool execution and ToolMessage construction

DTool Execution & ToolMessageRunning the function and packaging the result

list_alt

ToolCall Dictname + args
from AIMessage

lookup

play_arrow

Execute Fntool.invoke(args)
runs your code

wrap

mail

ToolMessagecontent + id
→ back to LLM

arrow_downward ToolNode: LangGraph's automatic tool executor

EToolNode in LangGraphAutomatic tool dispatch — the bridge between graph and tools

psychology

Agent NodeLLM returns
AIMessage

has tools?

call_split

Conditional Edgetool_calls?
→ tools : END

yes

build

ToolNodeAuto-dispatches
all tool_calls

loop

replay

Back to AgentToolMessages
added to state

arrow_downward MCP protocol internals: JSON-RPC, transports, discovery

FMCP Protocol InternalsJSON-RPC messages, transports, and tool discovery

smart_toy

MCP Clientinitialize →
tools/list

JSON-RPC

hub

MCP Servertools/call →
execute + return

transport

swap_horiz

stdio / SSELocal process or
remote HTTP