Very interesting new paper from Microsoft and colleagues.

(bookmark it)

Skill libraries are used in every major harness on the assumption that more guidance is free. This work measures what a bad skill actually costs you.

They attribute 307 agent failures to specific loaded skills, 125 functional failures and 182 efficiency regressions, by comparing each skill-guided run against a matched reference run that solves the same task.

The failures rarely come from irrelevant skills. Seemingly relevant skills push the agent to incorrectly implement or omit something the task required.

Cost regressions are not explained by prompt length either. The largest source is excessive verification at 67 cases, followed by heavy implementation pipelines at 30 cases. It turns out that skills quietly turn validation checklists into mandatory work.

Paper: https://arxiv.org/abs/2608.11888

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