You'll finish with: The right source assets, not a pile of files.
The last chapter got a design system built and published. This one is about the thing that decides whether it was worth publishing: what you fed it. Claude extracts — it doesn't invent.
What works, best first
Anthropic's own guidance is blunt about this: include real examples, not just specs. A colour palette tells Claude three hex codes. A finished landing page tells it how much white space you use, how big your headings run, how heavy your buttons are, and how the three colours actually relate to each other.
Your best existing page is your best asset — even if you didn't design it. If someone built you a good site three years ago, that site is a design system waiting to be extracted.
During onboarding, or afterwards from your Claude Design organisation settings — you are not stuck with what you gave it on day one.
If your brand is scattered across twenty files, don't dump them all in. Put them in one folder, open it in Cowork, and ask for a single design system document covering fonts, colours, graphical style, component patterns and layout conventions — with anything missing flagged.
Then feed that one clean document to Design. It works better with one coherent input than a pile of raw assets.
Two-step: Cowork reads across all your scattered files and writes one tidy summary. Design reads that summary. You get a much better result than uploading twenty random files.
The generated design system lists your actual colours and fonts — not approximations, and not a palette you don't recognise.
If the colours are close-but-wrong, your source asset probably had compressed screenshots in it. Feed it the real file or the live URL instead.