I can say unironically that we haven't even tapped the full potential of GPT-4. The original one, from 2023. With no reasoning, no RL, no tool calling, no structured outputs, etc. (No MCP, ye gods!) Yes, it's possible to build coding agents with it!
I say this because I did!
Forcing yourself to make things work with older models forces you to keep things simple. You don't need 50KB of prompts. You can make a coding agent with GPT-4 and half a page of prompt.
Now, why would we do this? Well, these constraints force you to think differently about the problem. Context management becomes non-optional. Semantic compression (for Python it's as simple as `grep -r def .`) becomes non-optional. Bloating the prompt with infinite detail and noise... you couldn't if you wanted to!
Well, surely none of this is relevant today? Well, it turns out all of it still is! e.g. small fix, the "grep def" (or your language's equivalent) can be trivially added as a startup hook to Claude Code, and suddenly it doesn't have to spend half your token budget poking around the codebase, because -- get this -- it can just see where everything is... (What a concept, right?)
-- We can also get into "If you let the LLM design the API then you don't need a prompt because it already knows how it should work", but... we can talk about that later ;)