Great guide — thorough and practical. Two things I'd add from my experience building and testing skills:
1. Baseline comparison across models: The guide suggests comparing with and without a skill (p9), but doesn't mention that the same skill can perform very differently across models. A skill that improves outcomes on a larger model might overwhelm a smaller one with too many competing instructions. Testing against the baseline for each model you intend to support matters.
2. Implicit vs explicit constraints as a design tool: The guide focuses on explicit instructions — tell Claude what to do, what not to do, how to validate. In my testing, implicit constraints (format rules, structural limits) were sometimes more effective at preventing unintended outputs, because they limit the output space structurally rather than relying on the model to interpret conditional logic. Skill authors might benefit from thinking about what their format rules prevent as a side effect, not just what their instructions request.
Curious if others building skills are seeing similar patterns.