Not yet. I think Seed has a fairly clear way to extend data and behavior, but the extension model for interfaces is still implicit.

Right now I can see several ingredients:

Documents/blocks have stable identities and schemas.

Tools can expose behavior and invoke other tools.

Queries can produce structured sets of objects.

New blob/schema types can extend the underlying system.

Different views—document, search, graph, board, activity, agents—can project the same underlying objects.

But I don't yet see a crisp rule equivalent to:

“Given a type of object, how can somebody add a new way of viewing or interacting with it without modifying the Seed client?”

That is the missing abstraction.

I would make View a first-class extension point. Something roughly like:

Object → Schema → Views → Actions

For example, a Document could advertise or be matched by:

document → editor | reader | outline | graph | board-card | agent-context

And a custom ontology object:

Person → profile-view | graph-node | table-row

The important part is that the view shouldn't necessarily belong to the object. A separately published component/tool could declare:

“I know how to render objects satisfying schema X.”

That gives you something much closer to Engelbart's view specs and also to Smalltalk's idea that the underlying object isn't identical to its presentation.

I think Seed should probably distinguish three extension layers:

Data extension — schemas / attributes / blob types.

Behavior extension — tools / methods / agents.

Presentation extension — views / renderers / interaction surfaces.

You have the first two in increasingly recognizable form. The third doesn't yet have an equally clear protocol.

A powerful consequence would be that Search, Library, Boards, Graph, Agents, etc. stop being hard-coded "features" and become compositions of views over Seed objects.

The design principle I would aim for is:

Any object can acquire new interfaces without changing the object. Any interface can operate on objects it understands without knowing where they live.

That feels very aligned with the architecture you've already been moving toward.