Harness Engineering Derived from what Models can’t do alone: It feels like a good time to step back and reshare some basic mental models for…

Harness Engineering Derived from what Models can’t do alone: It feels like a good time to step back and reshare some basic mental models for why harnesses exist in the first place - working backwards from The Model Why the Harness Exists: Harnesses exist to augment and shape models to do useful work for us. Models are fundamentally token input and output machines They cannot do useful things in the world without a harness. This is an important but under appreciated truth about the agentic systems we’re building The whole point of a harness is to extend the abilities of models to do useful work on our behalf Primitives the World Finds Useful Today: The diagrams below describe what capability we want and we work backwards from that capability to design a component of the harness. Filesystems, bash, compaction and Ralph Loops are all derived by working backwards from a goal to what a model can’t do today. We add those to the harness because they’re useful and help our agents more reliably complete work Model-Harness Training Loop: As we find incorporate a useful harness primitive into the model, model labs often directly train on that capability so the next version of the model is better at leveraging that tooling. This isn’t the end all be all, models are good at generalizing to other tools via in-context-learning. But this is a tried and true recipe we’ve seen repeat over the last 3 years and likely into the future. Different labs have different priors on useful harness capabilities. This is the entire reason they share prompting guides that differ dramatically down to their tool design. There’s harness mania happening right now which is cool to see as we’ve been investing in this for a while! :) It helps to step back and think about “why does this thing exist in the first place?”

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Source: Harrison Chase (X) | 2026-04-14