Compose Atoms

A heavy domain is a kit, not a viewer with modes. Each host is an import list. An atom owns the data, actions, and dependencies of one capability. The assembler only chooses what to mount.

This is a module-graph split. Hiding UI does not drop a module. A host that never imports a file is the only host that does not ship it.

Do not use this skill to slice one component into smaller files. That is react.

When

The folder is a product surface (Conversation, Acceptance, Document, Verify, Task, …) and at least one of these is true:

• One entry owns read + write + workflow + host integration.
• A second host already exists or is the next change (page, portal, share, mobile, popup, micro-app).
• The next feature is another boolean / mode / variant on the fat tree.
• Reusing a header, list, or card would import trays, stores, or chat.

If the file is large but has one capability and one host, stop. Use react.

Grain

An atom is the smallest unit a host is allowed not to mount.

Ask, for every chunk: would any host ship the rest and skip this? If yes, it is an atom. If no, it stays inside its parent.

That grain is coarser than a visual section and finer than the whole page.

Too coarse	Right grain	Too fine
One Viewer with readOnly	Identity, goal, list, decision, …	Every badge, row, and icon as a public export
Chat as a single import	List, composer, intervention, …	Every message sub-row the page then rewires
Document as editor + every panel	Canvas, header, comments as omitted	Internal toolbar buttons