Both product-design and product-analysis produce the same artifact: product.md, a durable statement of what a product is and why — problem, audience, value proposition, goals, non-goals. They arrive at it from completely different directions, and the difference is deliberate.
Product Design: dialogue, because it's genuinely your idea
When you're establishing a brand-new product from your own idea, product-design runs a structured interview — section by section, not a giant upfront questionnaire. It asks targeted questions, drafts a section from your answer, shows it, and lets you correct it before moving on. When an answer is vague — "it's for people who need to stay organized" — it presses for specifics rather than writing the vague version down. This is deliberately not delegated autonomy. A product's identity is the user's own idea, refined — not a decision an agent should make on their behalf.
Product Analysis: extraction, because the identity already exists somewhere
When you're cloning a competitor, porting an old version, or building on top of something that already has real users, product-analysis performs the same Product Manager-level synthesis, but reads it off evidence instead: the product's own UI, its official site and app-store listing, hosted video, reviews and community discussion. It weighs sources by a clear hierarchy — the product's own material, then official statements, then third-party commentary — and flags explicitly when a product's stated positioning and its actual observed audience diverge, which is itself often the most useful finding in the whole document.
Same document, same downstream treatment
Once either skill produces product.md, every later spec for that product treats it identically: link to it, don't restate it, keep each spec's own summary scoped to what that specific piece of work adds. The distinction between origination and extraction matters entirely at the point of creation and not at all afterward — which is exactly how it should work. The document doesn't need to remember which path produced it; only the process that wrote it does.
Picking the right one
One test settles it: is there a real product already generating evidence you could analyze right now — a URL, a build, a prior version? If yes, product-analysis. If the honest answer is "no, this is a new idea in my head," product-design. Getting this pick right is what keeps the very first document in your spec folder from being either a fabricated guess or an unnecessarily interrogative dialogue about something you could have just shown the agent.