Signal Over Noise

Signal Over Noise

Claude Code’s Harness Architecture

What the Reference Implementation Teaches Us

Justin Wilson's avatar
Justin Wilson
Aug 13, 2026
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The most instructive agent architecture in the world is not open source, and that is precisely why it is worth studying.

Last week I wrote about the MBZUAI study that decompiled Claude Code and found that 98.4% of it is harness infrastructure, with only 1.6% left for the model’s actual decision logic. The number made for a good provocation. But a ratio i…

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