METR ran a proper RCT experiment seeing how much access to Cursor (using Sonnet 3.7) would accelerate coders working on their own open source repos.

Everyone surveyed expected a substantial speedup. The developers thought they were being substantially sped up.

Instead, it turned out that using Cursor slowed them down.

That surprised everyone, raising the question of why.

Currently our best guess is this comes down to a combination of two factors:

Deeply understood open source repos are close to a worst-case scenario for AI tools, because they require bespoke outputs in various ways and the coder has lots of detailed local knowledge of the codebase that the AI lacks.
The coders in question mostly did not have experience with similar AI tools. The lack of a learning curve during the experiment challenges this, but the tools very clearly have a [...]