Agentic Harness Engineering: LLMs as the New OS
Original excerpt
At the AI start-up I’ve been working at, building a financial personal assistant, we implemented LlamaIndex, added the Model Context Protocol (MCP), and built complex Retrieval-Augmented Generation (RAG) pipelines. Each piece added complexity without adding direct business value.
When we stripped everything back to plain Python, simple API calls, and a custom ReAct engine, things finally worked. What we accidentally built was a harness featuring specialized financial tools, domain-specific guardrails, and purpose-built context engineering.
We did not know the term yet, but the lesson was clear. The model was never the problem. The system and infrastructure around it were.
Most engineering…
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