# UNDF: UNDF-2026-000000337
--- a/crates/environ/src/component/translate/adapt.rs
+++ b/crates/environ/src/component/translate/adapt.rs
@@ -168,7 +168,8 @@ pub struct AdapterOptions {
     pub instance: RuntimeComponentInstanceIndex,
     /// The ancestors (i.e. chain of instantiating instances) of the instance
     /// specified in the `instance` field.
-    pub ancestors: Vec<RuntimeComponentInstanceIndex>,
+    /// Stored as IndexSet for O(1) contains() — order preserved for serialization.
+    pub ancestors: indexmap::IndexSet<RuntimeComponentInstanceIndex>,

--- a/crates/environ/src/fact.rs
+++ b/crates/environ/src/fact.rs
@@ -127,7 +127,8 @@ struct AdapterOptions {
     instance: RuntimeComponentInstanceIndex,
     /// The ancestors (i.e. chain of instantiating instances) of the instance
     /// specified in the `instance` field.
-    ancestors: Vec<RuntimeComponentInstanceIndex>,
+    /// IndexSet for O(1) contains().
+    ancestors: indexmap::IndexSet<RuntimeComponentInstanceIndex>,

--- a/crates/environ/src/component/translate/inline.rs
+++ b/crates/environ/src/component/translate/inline.rs
@@ -1580,7 +1580,8 @@ fn build_adapter_options(...) -> AdapterOptions {
     AdapterOptions {
         instance: frame.instance,
-        ancestors: frames
+        ancestors: frames
             .iter()
             .rev()
             .skip(1)
             .map(|(frame, _)| frame.instance)
-            .collect(),
+            .collect::<indexmap::IndexSet<_>>(),