Before it
o1 reasoned in a closed loop with no access to anything outside itself.
The problem
Reasoning without tools cannot look anything up or run code to check itself.
What it introduced
A reasoning model that calls tools, including search and code execution, inside its chain of thought.
Why it matters
Reasoning and tool use merged here, which is the technical basis of the agentic models that followed.
What modern models inherit
Tool calls inside the reasoning loop.
What came next
Folded into GPT-5's unified system later that year.