Tool Calling
The model never executes anything. It only requests a call — your code decides whether to honour it, runs it, and reports the result back.
Custom Tools
The @tool decorator turns a typed, documented function into something a model can call:
Agent Concepts
A chain has edges you fixed at design time: step A always leads to step B. An agent hands that decision to the model — the next step is chosen at runtime, from the model's own output.
React Agent
createagent builds a ReAct-style agent — reason, act, observe, repeat — from a model and a tool list. The constructor itself lives in LangGraph's prebuilt layer; langchain.agents.createagent is the stable entry point.
Multi-Tool Routing
Tool selection quality degrades as the tool list grows — with a handful of well-named tools the model picks correctly almost every time; past a few dozen, overlapping descriptions start colliding and the wrong tool gets called more often.