Why AI Engineers Are Moving Beyond Prompts to 'Harness Engineering'
A software engineer reflects on how three years of AI development revealed that prompt quality is rarely the reason agents fail in production. According to a Y Combinator survey of CTOs and CPOs from March 2026, around 40% of AI agent projects collapse after deployment, with leaders consistently noting that the model itself is not the differentiating factor. The real bottleneck, the author argues, lies in the infrastructure surrounding the prompt — including tool access, memory, context management, and loop control. This shift has driven the field from prompt engineering to context engineering, where the entire context window is treated as the core artifact to be optimised. With 75% of YC enterprise companies already deploying coding agents and workflows being redesigned around them, understanding the full agent harness is increasingly seen as essential to production success.
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