Tools follow the Model

Availability is Model-dependent, so the advertised WebMCP tool set may change as the application changes:

RequestedDeleteTodo: {
  name: 'delete_todo',
  description: 'Delete the selected todo',
  available: model => Option.isSome(model.selectedTodoId),
}

no selection       tools: create_todo
       |
user selects todo
       v
selected todo      tools: create_todo, delete_todo

With followModel: true (the default) and a host with subscribe, a Model change reconciles registrations. A capability that disappears is unregistered; a newly available capability is registered.

Availability is stronger than discoverability. A capability absent from the tool list is also refused if a caller somehow invokes its known name.

Reconciliation is serialized, so overlapping Model changes cannot register the same capability twice. A failed browser registration is not recorded as live and is retried on a later reconcile.