The Claude API multi-agent loop, without the framework

Most Claude API tutorials show a single tool call. Most frameworks hide the loop behind abstractions you can't read. This post shows the loop directly — what actually happens between "Claude requests a tool" and "Claude finishes."

The loop in plain English

When you give Claude tools, a single API call isn't always enough. Claude decides whether to call a tool, you execute it, then you send the result back. Claude might call another tool, or it might answer. That cycle is the agent loop.

user message

    ↓

Claude responds

    ↓

stop_reason == "tool_use"?  →  execute tools  →  back to Claude

    ↓

stop_reason == "end_turn"

    ↓

return final text

The implementation

The entire loop is in

This pattern is what I used as the foundation for — AI engineer, Lisbon. Also wrote about debugging a silent native crash in ctranslate2 + WinRT.