diff --git a/idl/GCObject.json5 b/idl/GCObject.json5
index f86e6ca..cd320d0 100644
--- a/idl/GCObject.json5
+++ b/idl/GCObject.json5
@@ -48,6 +48,11 @@
             doc: ["fomo collector type"],
             definition: "xo::mm::AGCObjectVisitor",
         },
+        {
+            name: "VisitReason",
+            doc: ["hint arg when navigating object graph"],
+            definition: "xo::mm::VisitReason",
+        },
     ],
     const_methods: [
         // size_type shallow_size() const noexcept
@@ -77,7 +82,7 @@
             noexcept: true,
             attributes: [],
         },
-        // size_type visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        // size_type visit_gco_children(VisitReason reason, obj<AGCObjectVisitor>) noexcept
         {
             name: "visit_gco_children",
             doc: [
@@ -87,6 +92,7 @@
             ],
             return_type: "void",
             args: [
+                {type: "VisitReason", name: "reason"},
                 {type: "obj<AGCObjectVisitor>", name: "fn"},
             ],
             const: true,
diff --git a/idl/GCObjectVisitor.json5 b/idl/GCObjectVisitor.json5
index 854287e..fe6fbb1 100644
--- a/idl/GCObjectVisitor.json5
+++ b/idl/GCObjectVisitor.json5
@@ -6,6 +6,7 @@
     includes: [
         "<xo/alloc2/Generation.hpp>",
         "<xo/alloc2/role.hpp>",
+        "<xo/alloc2/VisitReason.hpp>",
         "<xo/arena/AllocInfo.hpp>",
     ],
     // extra includes in GCObject.hpp, if any
@@ -85,12 +86,13 @@
             noexcept: false,
             attributes: [],
         },
-        // void visit_child(AGCObject * iface, void ** pp_data) noexcept;
+        // void visit_child(VisitReason reason, AGCObject * iface, void ** pp_data) noexcept;
         {
             name: "visit_child",
             doc: ["visit child of a gc-aware object. May update child in-place!"],
             return_type: "void",
             args:[
+                {type: "VisitReason", name: "reason"},
                 {type: "AGCObject *", name: "iface"},
                 {type: "void **", name: "pp_data"},
             ],
@@ -121,20 +123,20 @@
         "    (for historical reasons - coul d be in RGCObject_aux.hpp?)",
         " **/",
         "template <typename DRepr>",
-        "void visit_child(xo::facet::obj<AGCObject,DRepr> * p_obj);",
+        "void visit_child(VisitReason reason, xo::facet::obj<AGCObject,DRepr> * p_obj);",
         "",
         "/** visit typed child data pointer in place.",
         "    Defined in RGCObject.hpp to avoid #include cycle",
         " **/",
         "template <typename DRepr>",
-        "void visit_child(DRepr ** pp_data);",
+        "void visit_child(VisitReason reason, DRepr ** pp_data);",
         "",
         "/** visit faceted object pointer stored using some facet",
         "    other than AGCObject",
         " **/",
         "template <typename AFacet, typename DRepr>",
         "requires (!std::is_same_v<AFacet, AGCObject>)",
-        "void visit_poly_child(obj<AFacet,DRepr> * p_pivot);",
+        "void visit_poly_child(VisitReason reason, obj<AFacet,DRepr> * p_pivot);",
         "",
     ]
 }
diff --git a/include/xo/alloc2/VisitReason.hpp b/include/xo/alloc2/VisitReason.hpp
new file mode 100644
index 0000000..569b02d
--- /dev/null
+++ b/include/xo/alloc2/VisitReason.hpp
@@ -0,0 +1,51 @@
+/** @file VisitReason.hpp
+ *
+ *  @author Roland Conybeare, Apr 2026
+ **/
+
+#pragma once
+
+namespace xo {
+    namespace mm {
+
+        /** @brief tag when navigating object graph
+         *
+         *  Used with
+         *    @ref DX1Collector::visit_child
+         *    @ref GCObject::visit_gco_children
+         **/
+        class VisitReason {
+        public:
+            enum class code {
+                invalid = -1,
+
+                /** color not needed **/
+                unspecified,
+
+                /** Forward child pointers inplace for GC.
+                 *  See @ref GCObjectStore::forward_inplace_aux
+                 **/
+                forward,
+                /** verify GC store consistency
+                 *  See @ref DX1Collector::_verify_aux
+                 **/
+                verify,
+
+                N,
+            };
+
+            explicit VisitReason(code x) : code_{x} {}
+
+            static VisitReason unspecified() { return VisitReason(code::unspecified); }
+            static VisitReason forward() { return VisitReason(code::forward); }
+            static VisitReason verify() { return VisitReason(code::verify); }
+
+            code code() const noexcept { return code_; }
+
+            enum code code_;
+        };
+
+    } /*namespace mm*/
+} /*namespace xo*/
+
+/* end VisitReason.hpp */
diff --git a/include/xo/alloc2/gc/AGCObject.hpp b/include/xo/alloc2/gc/AGCObject.hpp
index 6ed3ee7..fbb5f61 100644
--- a/include/xo/alloc2/gc/AGCObject.hpp
+++ b/include/xo/alloc2/gc/AGCObject.hpp
@@ -51,6 +51,8 @@ public:
     using ACollector = xo::mm::ACollector;
     /** fomo collector type **/
     using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
+    /** hint arg when navigating object graph **/
+    using VisitReason = xo::mm::VisitReason;
     ///@}
 
     /** @defgroup mm-gcobject-methods **/
@@ -73,7 +75,7 @@ Arguably abusing the word 'visitor' here **/
     /** Invoke fn.visit_child(iface,data) for each child GCObject pointer.
 Context: provides address of data pointer so it can be updated in place
 when @p fn invokes garbage collector reentry point **/
-    virtual void visit_gco_children(Opaque data, obj<AGCObjectVisitor> fn)  const  noexcept = 0;
+    virtual void visit_gco_children(Opaque data, VisitReason reason, obj<AGCObjectVisitor> fn)  const  noexcept = 0;
     ///@}
 }; /*AGCObject*/
 
diff --git a/include/xo/alloc2/gc/AGCObjectVisitor.hpp b/include/xo/alloc2/gc/AGCObjectVisitor.hpp
index a9d0093..86cd9b9 100644
--- a/include/xo/alloc2/gc/AGCObjectVisitor.hpp
+++ b/include/xo/alloc2/gc/AGCObjectVisitor.hpp
@@ -16,6 +16,7 @@
 // includes (via {facet_includes})
 #include <xo/alloc2/Generation.hpp>
 #include <xo/alloc2/role.hpp>
+#include <xo/alloc2/VisitReason.hpp>
 #include <xo/arena/AllocInfo.hpp>
 #include <xo/facet/obj.hpp>
 #include <xo/facet/facet_implementation.hpp>
@@ -71,7 +72,7 @@ Source must be owned by this collector.
 Increments object age **/
     virtual void * alloc_copy(Opaque data, std::byte * src)  const = 0;
     /** visit child of a gc-aware object. May update child in-place! **/
-    virtual void visit_child(Opaque data, AGCObject * iface, void ** pp_data)  const  noexcept = 0;
+    virtual void visit_child(Opaque data, VisitReason reason, AGCObject * iface, void ** pp_data)  const  noexcept = 0;
     ///@}
 }; /*AGCObjectVisitor*/
 
diff --git a/include/xo/alloc2/gc/IGCObjectVisitor_Any.hpp b/include/xo/alloc2/gc/IGCObjectVisitor_Any.hpp
index 53515e2..8dbece0 100644
--- a/include/xo/alloc2/gc/IGCObjectVisitor_Any.hpp
+++ b/include/xo/alloc2/gc/IGCObjectVisitor_Any.hpp
@@ -63,7 +63,7 @@ namespace mm {
 
         // nonconst methods
         [[noreturn]] void * alloc_copy(Opaque, std::byte *)  const override;
-        [[noreturn]] void visit_child(Opaque, AGCObject *, void **)  const  noexcept override;
+        [[noreturn]] void visit_child(Opaque, VisitReason, AGCObject *, void **)  const  noexcept override;
 
         ///@}
 
diff --git a/include/xo/alloc2/gc/IGCObjectVisitor_Xfer.hpp b/include/xo/alloc2/gc/IGCObjectVisitor_Xfer.hpp
index 00ad359..0cc6087 100644
--- a/include/xo/alloc2/gc/IGCObjectVisitor_Xfer.hpp
+++ b/include/xo/alloc2/gc/IGCObjectVisitor_Xfer.hpp
@@ -15,6 +15,7 @@
 
 #include <xo/alloc2/Generation.hpp>
 #include <xo/alloc2/role.hpp>
+#include <xo/alloc2/VisitReason.hpp>
 #include <xo/arena/AllocInfo.hpp>
 
 namespace xo {
@@ -56,8 +57,8 @@ namespace mm {
         void * alloc_copy(Opaque data, std::byte * src)  const override {
             return I::alloc_copy(_dcast(data), src);
         }
-        void visit_child(Opaque data, AGCObject * iface, void ** pp_data)  const  noexcept override {
-            return I::visit_child(_dcast(data), iface, pp_data);
+        void visit_child(Opaque data, VisitReason reason, AGCObject * iface, void ** pp_data)  const  noexcept override {
+            return I::visit_child(_dcast(data), reason, iface, pp_data);
         }
 
         ///@}
diff --git a/include/xo/alloc2/gc/IGCObject_Any.hpp b/include/xo/alloc2/gc/IGCObject_Any.hpp
index 6297477..f9ff5f8 100644
--- a/include/xo/alloc2/gc/IGCObject_Any.hpp
+++ b/include/xo/alloc2/gc/IGCObject_Any.hpp
@@ -48,6 +48,7 @@ namespace mm {
         using AAllocator = AGCObject::AAllocator;
         using ACollector = AGCObject::ACollector;
         using AGCObjectVisitor = AGCObject::AGCObjectVisitor;
+        using VisitReason = AGCObject::VisitReason;
 
         ///@}
         /** @defgroup mm-gcobject-any-methods **/
@@ -65,7 +66,7 @@ namespace mm {
 
         // nonconst methods
         [[noreturn]] Opaque gco_shallow_move(Opaque, obj<AGCObjectVisitor>)  const  noexcept override;
-        [[noreturn]] void visit_gco_children(Opaque, obj<AGCObjectVisitor>)  const  noexcept override;
+        [[noreturn]] void visit_gco_children(Opaque, VisitReason, obj<AGCObjectVisitor>)  const  noexcept override;
 
         ///@}
 
diff --git a/include/xo/alloc2/gc/IGCObject_Xfer.hpp b/include/xo/alloc2/gc/IGCObject_Xfer.hpp
index 633c103..c5a0a6b 100644
--- a/include/xo/alloc2/gc/IGCObject_Xfer.hpp
+++ b/include/xo/alloc2/gc/IGCObject_Xfer.hpp
@@ -36,6 +36,7 @@ namespace mm {
         using AAllocator = AGCObject::AAllocator;
         using ACollector = AGCObject::ACollector;
         using AGCObjectVisitor = AGCObject::AGCObjectVisitor;
+        using VisitReason = AGCObject::VisitReason;
         ///@}
 
         /** @defgroup mm-gcobject-xfer-methods **/
@@ -56,8 +57,8 @@ namespace mm {
         Opaque gco_shallow_move(Opaque data, obj<AGCObjectVisitor> gc)  const  noexcept override {
             return I::gco_shallow_move(_dcast(data), gc);
         }
-        void visit_gco_children(Opaque data, obj<AGCObjectVisitor> fn)  const  noexcept override {
-            return I::visit_gco_children(_dcast(data), fn);
+        void visit_gco_children(Opaque data, VisitReason reason, obj<AGCObjectVisitor> fn)  const  noexcept override {
+            return I::visit_gco_children(_dcast(data), reason, fn);
         }
 
         ///@}
diff --git a/include/xo/alloc2/gc/RCollector_aux.hpp b/include/xo/alloc2/gc/RCollector_aux.hpp
index 83bc631..e964a93 100644
--- a/include/xo/alloc2/gc/RCollector_aux.hpp
+++ b/include/xo/alloc2/gc/RCollector_aux.hpp
@@ -24,32 +24,33 @@ namespace xo {
         template <typename Object>
         template <typename DRepr>
         void
-        RGCObjectVisitor<Object>::visit_child(xo::facet::obj<AGCObject,DRepr> * p_obj)
+        RGCObjectVisitor<Object>::visit_child(VisitReason reason,
+                                              xo::facet::obj<AGCObject,DRepr> * p_obj)
         {
-            this->visit_child(p_obj->iface(), (void **)&(p_obj->data_));
+            this->visit_child(reason, p_obj->iface(), (void **)&(p_obj->data_));
         }
 
         template <typename Object>
         template <typename DRepr>
         void
-        RGCObjectVisitor<Object>::visit_child(DRepr ** p_repr)
+        RGCObjectVisitor<Object>::visit_child(VisitReason reason, DRepr ** p_repr)
         {
             // fetch static interface for DRepr (strip const: FacetImplementation specializations use non-const DRepr)
             auto iface = xo::facet::impl_for<AGCObject, std::remove_const_t<DRepr>>();
 
-            this->visit_child(&iface, (void **)p_repr);
+            this->visit_child(reason, &iface, (void **)p_repr);
         }
 
         template <typename Object>
         template <typename AFacet, typename DRepr>
         requires (!std::is_same_v<AFacet, AGCObject>)
         void
-        RGCObjectVisitor<Object>::visit_poly_child(obj<AFacet, DRepr> * p_objs)
+        RGCObjectVisitor<Object>::visit_poly_child(VisitReason reason, obj<AFacet, DRepr> * p_objs)
         {
             if (*p_objs) {
                 auto e = xo::facet::FacetRegistry::instance().variant<AGCObject,AFacet>(*p_objs);
 
-                this->visit_child(e.iface(), (void **)&(p_objs->data_));
+                this->visit_child(reason, e.iface(), (void **)&(p_objs->data_));
             }
         }
 
diff --git a/include/xo/alloc2/gc/RGCObject.hpp b/include/xo/alloc2/gc/RGCObject.hpp
index 1d62559..5b40155 100644
--- a/include/xo/alloc2/gc/RGCObject.hpp
+++ b/include/xo/alloc2/gc/RGCObject.hpp
@@ -35,6 +35,7 @@ public:
     using AAllocator = AGCObject::AAllocator;
     using ACollector = AGCObject::ACollector;
     using AGCObjectVisitor = AGCObject::AGCObjectVisitor;
+    using VisitReason = AGCObject::VisitReason;
     ///@}
 
     /** @defgroup mm-gcobject-router-ctors **/
@@ -61,8 +62,8 @@ public:
     Opaque gco_shallow_move(obj<AGCObjectVisitor> gc)  noexcept {
         return O::iface()->gco_shallow_move(O::data(), gc);
     }
-    void visit_gco_children(obj<AGCObjectVisitor> fn)  noexcept {
-        return O::iface()->visit_gco_children(O::data(), fn);
+    void visit_gco_children(VisitReason reason, obj<AGCObjectVisitor> fn)  noexcept {
+        return O::iface()->visit_gco_children(O::data(), reason, fn);
     }
 
     ///@}
diff --git a/include/xo/alloc2/gc/RGCObjectVisitor.hpp b/include/xo/alloc2/gc/RGCObjectVisitor.hpp
index 8b7ab26..4972593 100644
--- a/include/xo/alloc2/gc/RGCObjectVisitor.hpp
+++ b/include/xo/alloc2/gc/RGCObjectVisitor.hpp
@@ -67,20 +67,20 @@ public:
         (for historical reasons - coul d be in RGCObject_aux.hpp?)
      **/
     template <typename DRepr>
-    void visit_child(xo::facet::obj<AGCObject,DRepr> * p_obj);
+    void visit_child(VisitReason reason, xo::facet::obj<AGCObject,DRepr> * p_obj);
     
     /** visit typed child data pointer in place.
         Defined in RGCObject.hpp to avoid #include cycle
      **/
     template <typename DRepr>
-    void visit_child(DRepr ** pp_data);
+    void visit_child(VisitReason reason, DRepr ** pp_data);
     
     /** visit faceted object pointer stored using some facet
         other than AGCObject
      **/
     template <typename AFacet, typename DRepr>
     requires (!std::is_same_v<AFacet, AGCObject>)
-    void visit_poly_child(obj<AFacet,DRepr> * p_pivot);
+    void visit_poly_child(VisitReason reason, obj<AFacet,DRepr> * p_pivot);
     
 
     // builtin methods
@@ -99,8 +99,8 @@ public:
     void * alloc_copy(std::byte * src)  {
         return O::iface()->alloc_copy(O::data(), src);
     }
-    void visit_child(AGCObject * iface, void ** pp_data)  noexcept {
-        return O::iface()->visit_child(O::data(), iface, pp_data);
+    void visit_child(VisitReason reason, AGCObject * iface, void ** pp_data)  noexcept {
+        return O::iface()->visit_child(O::data(), reason, iface, pp_data);
     }
 
     ///@}
diff --git a/include/xo/alloc2/role.hpp b/include/xo/alloc2/role.hpp
index d2b7c2f..9b068a1 100644
--- a/include/xo/alloc2/role.hpp
+++ b/include/xo/alloc2/role.hpp
@@ -12,6 +12,8 @@ namespace xo {
     namespace mm {
         static constexpr uint32_t c_n_role = 2;
 
+        /** @brief identify GC half-spaces
+         **/
         class Role {
         public:
             using value_type = std::uint32_t;
diff --git a/src/alloc2/IGCObject_Any.cpp b/src/alloc2/IGCObject_Any.cpp
index 914123d..4e729e0 100644
--- a/src/alloc2/IGCObject_Any.cpp
+++ b/src/alloc2/IGCObject_Any.cpp
@@ -42,7 +42,7 @@ IGCObject_Any::gco_shallow_move(Opaque, obj<AGCObjectVisitor>)  const  noexcept
 }
 
 auto
-IGCObject_Any::visit_gco_children(Opaque, obj<AGCObjectVisitor>)  const  noexcept -> void
+IGCObject_Any::visit_gco_children(Opaque, VisitReason, obj<AGCObjectVisitor>)  const  noexcept -> void
 {
     _fatal();
 }
diff --git a/src/alloc2/facet/IGCObjectVisitor_Any.cpp b/src/alloc2/facet/IGCObjectVisitor_Any.cpp
index 0946acd..934aa94 100644
--- a/src/alloc2/facet/IGCObjectVisitor_Any.cpp
+++ b/src/alloc2/facet/IGCObjectVisitor_Any.cpp
@@ -42,7 +42,7 @@ IGCObjectVisitor_Any::alloc_copy(Opaque, std::byte *)  const -> void *
 }
 
 auto
-IGCObjectVisitor_Any::visit_child(Opaque, AGCObject *, void **)  const  noexcept -> void
+IGCObjectVisitor_Any::visit_child(Opaque, VisitReason, AGCObject *, void **)  const  noexcept -> void
 {
     _fatal();
 }