diff --git a/optimizer/src/main/java/dev/cel/optimizer/AstMutator.java b/optimizer/src/main/java/dev/cel/optimizer/AstMutator.java
index 2e3027dcc..778d041fc 100644
--- a/optimizer/src/main/java/dev/cel/optimizer/AstMutator.java
+++ b/optimizer/src/main/java/dev/cel/optimizer/AstMutator.java
@@ -889,14 +889,35 @@ private CelMutableSource normalizeMacroSource(
         }
       }
 
-      if (exprIdToReplace > 0) {
+      // Handle replacing the synthetic single-element list inside a `map` or `filter` loop_step.
+      //
+      // Example: `[1].map(x, 10)`
+      //   - In the main AST, `loop_step` is `@result + [10]` (where `[10]` is a synthetic LIST
+      //     node wrapping the body `10`).
+      //   - In `macro_calls`, the call is `[1].map(x, 10)`, which only references the inner `10`
+      //     node—NOT the synthetic `[10]` LIST node.
+      //
+      // If a mutation replaces the `[10]` LIST node itself (e.g. with `[20]`), the loop above
+      // won't find `[10]`'s ID in `macro_calls`, so we unwrap `20` from `[20]` into the macro
+      // call (`[1].map(x, 20)`).
+      //
+      // We must first verify that:
+      //   1. This macro is actually a COMPREHENSION (e.g. `has(msg.f)` is in `macro_calls` too,
+      //      but expands to a SELECT node, not a COMPREHENSION).
+      //   2. The replaced LIST node is inside *this* comprehension's `loop_step` (not an unrelated
+      //      list replacement elsewhere in the AST, such as folding `[1, 2] + [3, 4]`).
+      if (exprIdToReplace > 0
+          && allExprs.get(callId).getKind().equals(ExprKind.Kind.COMPREHENSION)) {
         long replacedId = idGenerator.generate(exprIdToReplace);
+        CelMutableComprehension comprehension = allExprs.get(callId).comprehension();
         boolean isListExprBeingReplaced =
             allExprs.containsKey(replacedId)
-                && allExprs.get(replacedId).getKind().equals(ExprKind.Kind.LIST);
+                && allExprs.get(replacedId).getKind().equals(ExprKind.Kind.LIST)
+                && CelNavigableMutableExpr.fromExpr(comprehension.loopStep())
+                    .allNodes()
+                    .anyMatch(node -> node.id() == replacedId);
         if (isListExprBeingReplaced) {
-          unwrapListArgumentsInMacroCallExpr(
-              allExprs.get(callId).comprehension(), newMacroCallExpr);
+          unwrapListArgumentsInMacroCallExpr(comprehension, newMacroCallExpr);
         }
       }
 
diff --git a/optimizer/src/test/java/dev/cel/optimizer/AstMutatorTest.java b/optimizer/src/test/java/dev/cel/optimizer/AstMutatorTest.java
index 2138ff142..0a168eaab 100644
--- a/optimizer/src/test/java/dev/cel/optimizer/AstMutatorTest.java
+++ b/optimizer/src/test/java/dev/cel/optimizer/AstMutatorTest.java
@@ -579,6 +579,25 @@ public void list_replaceElement() throws Exception {
     assertThat(CEL_UNPARSER.unparse(result.toParsedAst())).isEqualTo("[2, 3, 5]");
   }
 
+  @Test
+  public void list_replaceSubtreeWithListInAstWithHasMacro_success() throws Exception {
+    CelAbstractSyntaxTree ast = CEL.compile("has(msg.single_int64) && 1 in [2]").getAst();
+    CelMutableAst mutableAst = CelMutableAst.fromCelAst(ast);
+    CelMutableExpr foldedList =
+        CelMutableExpr.ofList(
+            CelMutableList.create(
+                CelMutableExpr.ofConstant(CelConstant.ofValue(1)),
+                CelMutableExpr.ofConstant(CelConstant.ofValue(2))));
+
+    // Node 8 is `[2]`; replacing it with a LIST triggers normalizeMacroSource while `has(...)` is
+    // present in macroCalls as a SELECT node rather than a COMPREHENSION node.
+    CelAbstractSyntaxTree replacedAst =
+        AST_MUTATOR.replaceSubtree(mutableAst, foldedList, 8).toParsedAst();
+
+    assertThat(CEL_UNPARSER.unparse(replacedAst)).isEqualTo("has(msg.single_int64) && 1 in [1, 2]");
+    assertConsistentMacroCalls(replacedAst);
+  }
+
   @Test
   public void struct_replaceValue() throws Exception {
     // Tree shape (brackets are expr IDs):