I Built a Multi-Agent Coding System From Scratch in Python (No Frameworks)

Most multi-agent AI tutorials hand you LangChain, AutoGen, or CrewAI and say “here you go.” You wire a few abstractions together, get something running, and never really understand what’s happening under the hood. I wanted to understand what’s actually happening under the hood. So I built one from scratch. This is the story of multi-agent-coder which is a system where a Planner decides at runtime which AI agent to call next, and a team of specialized agents (Architect, Engineer, Critic, TestRunner, Refactorer) collaborates to turn a plain English request into working, tested Python code. No LangChain. No AutoGen. Pure Python. The Core Idea The central insight is simple: one LLM trying to do everything is worse than multiple LLMs each doing one thing well. A single prompt asking an AI to “plan the architecture, write the code, review it for bugs, and refactor it” produces mediocre results across all four. But if you give each job to a separate agent with a focu