Welcome back to the Harness Engineering series — a 10-part journey from raw language model to production-ready agentic system. Made by builders. For builders.
In Part 3, we looked at the Loop — the outermost machinery of a harness, the piece that drives everything else. But a Loop on its own is a hollow shell. It calls the model. The model responds. And then, if the model wants to affect anything outside the text it just produced, it needs to reach for a tool.
That's what this article is about: the Tool Layer. The set of functions the harness makes available to the model, and the design decisions that separate a tool surface a model can actually use from one that constantly frustrates it.
What's ahead:
Part 1: The Raw Model Problem
