diff --git a/shared/sx/ref/deps.sx b/shared/sx/ref/deps.sx
index 10bc059..78c5546 100644
--- a/shared/sx/ref/deps.sx
+++ b/shared/sx/ref/deps.sx
@@ -314,6 +314,24 @@
     (empty? (transitive-io-refs name env io-names))))
 
 
+;; --------------------------------------------------------------------------
+;; Host obligation: selective expansion in async partial evaluation
+;; --------------------------------------------------------------------------
+;; The spec classifies components as pure or IO-dependent. Each host's
+;; async partial evaluator (the server-side rendering path that bridges
+;; sync evaluation with async IO) must use this classification:
+;;
+;;   IO-dependent component → expand server-side (IO must resolve)
+;;   Pure component         → serialize for client (can render anywhere)
+;;   Layout slot context    → expand all (server needs full HTML)
+;;
+;; The spec provides the data (component-io-refs, component-pure?).
+;; The host provides the async runtime that acts on it.
+;; This is not SX semantics — it is host infrastructure. Every host
+;; with a server-side async evaluator implements the same rule.
+;; --------------------------------------------------------------------------
+
+
 ;; --------------------------------------------------------------------------
 ;; Platform interface summary
 ;; --------------------------------------------------------------------------