The Harness Is Everything

Title: The Harness Is Everything

URL Source: https://medium.com/@reliabledataengineering/the-harness-is-everything-a4114e8a54d1?source=rss------data_science-5

Published Time: 2026-04-13T07:03:43Z

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# The Harness Is Everything. What Cursor, Claude Code, and… | by Reliable Data Engineering | Apr, 2026 | Medium

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# The Harness Is Everything

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_What Cursor, Claude Code, and Perplexity actually built — and why the model was never the real product_

**Read the complete technical deep-dive:**[https://reliable-data-engineering.netlify.app/posts/article_harness_is_everything/](https://reliable-data-engineering.netlify.app/posts/article_harness_is_everything/)

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There is a conversation happening right now across every engineering Slack, every AI-adjacent team standup, and every conference hallway that goes something like this: somebody shows off a tool that feels almost magic, a coding agent that ships whole features in minutes, and then somebody else says, “yeah, but with a different model it was useless.” Both people are right. And neither one quite understands why.

The answer isn’t hiding in a model card