diff --git a/workspace-server/internal/registry/access.go b/workspace-server/internal/registry/access.go
index 56a9c419e..a36c9d809 100644
--- a/workspace-server/internal/registry/access.go
+++ b/workspace-server/internal/registry/access.go
@@ -62,7 +62,7 @@ func isAncestorOf(ancestorID, childID string) bool {
 // the org hierarchy. The rules:
 //
 //   - self → self
-//   - siblings (same parent, including both root-level)
+//   - siblings (same parent; root-level workspaces in different orgs are NOT siblings)
 //   - any ancestor → any descendant (e.g. PM → Backend Engineer)
 //   - any descendant → any ancestor (e.g. Security Auditor → PM)
 //
@@ -94,15 +94,13 @@ func CanCommunicate(callerID, targetID string) bool {
 		return false
 	}
 
-	// Siblings — same parent (including root-level where both have no parent)
+	// Siblings — same parent. Root-level workspaces in different orgs
+	// are NOT siblings (they have no shared parent_id because each org
+	// root is its own tree; parent_id IS NULL does not mean "same org").
 	if caller.ParentID != nil && target.ParentID != nil &&
 		*caller.ParentID == *target.ParentID {
 		return true
 	}
-	// Root-level siblings — both have no parent
-	if caller.ParentID == nil && target.ParentID == nil {
-		return true
-	}
 
 	// Direct parent → child (fast path; avoids the ancestor walk)
 	if target.ParentID != nil && caller.ID == *target.ParentID {
diff --git a/workspace-server/internal/registry/access_test.go b/workspace-server/internal/registry/access_test.go
index b01a14691..963e1af5a 100644
--- a/workspace-server/internal/registry/access_test.go
+++ b/workspace-server/internal/registry/access_test.go
@@ -63,14 +63,15 @@ func TestCanCommunicate_Siblings(t *testing.T) {
 	}
 }
 
-func TestCanCommunicate_RootSiblings(t *testing.T) {
+func TestCanCommunicate_Denied_CrossTenantRoots(t *testing.T) {
 	mock := setupMockDB(t)
-	// Both at root level (no parent)
+	// Two different org roots (both parent_id IS NULL) must NOT
+	// communicate — parent_id IS NULL does not mean "same org".
 	expectLookup(mock, "ws-a", nil)
 	expectLookup(mock, "ws-b", nil)
 
-	if !CanCommunicate("ws-a", "ws-b") {
-		t.Error("root-level siblings should communicate")
+	if CanCommunicate("ws-a", "ws-b") {
+		t.Error("cross-tenant org roots should NOT communicate")
 	}
 }