diff --git a/idl/GCObjectVisitor.json5 b/idl/GCObjectVisitor.json5
index efe14a7..ce7efdc 100644
--- a/idl/GCObjectVisitor.json5
+++ b/idl/GCObjectVisitor.json5
@@ -4,6 +4,7 @@
     output_hpp_dir: "include/xo/alloc2",
     output_impl_subdir: "gc",
     includes: [
+        "<xo/arena/AllocInfo.hpp>",
 //        "<xo/alloc2/Allocator.hpp>",
 //        "<xo/alloc2/Collector.hpp>",
 //        "<cstdint>",
@@ -18,6 +19,7 @@
     pretext: [
         "// see GCObject.hpp, also in xo-alloc2/",
         "namespace xo { namespace mm { class AGCObject; }}",
+        "namespace xo { namespace mm { class AllocInfo; }}",
     ],
     facet: "GCObjectVisitor",
     detail_subdir: "gc",
@@ -45,16 +47,21 @@
 //        },
     ],
     const_methods: [
-        // size_type shallow_size() const noexcept
-//        {
-//            name: "shallow_size",
-//            doc: ["memory consumption for this instance"],
-//            return_type: "size_type",
-//            args: [],
-//            const: true,
-//            noexcept: true,
-//            attributes: [],
-//        },
+        // AllocInfo alloc_info(void * gco);
+        {
+            name: "alloc_info",
+            doc: [
+                "allocation metadata for gc-aware data at address @p gco.",
+                "@p gco must be the result of a call to collector's alloc() function",
+            ],
+            return_type: "AllocInfo",
+            args: [
+                {type: "void *", name: "addr"},
+            ],
+            const: true,
+            noexcept: false,
+            attributes: [],
+        },
     ],
     nonconst_methods: [
         // void alloc_copy(void * src)
@@ -69,7 +76,7 @@
             args: [
                 {type: "std::byte *", name: "src"},
             ],
-            const: false,
+            const: true,  // refers to interface.
             noexcept: false,
             attributes: [],
         },
diff --git a/include/xo/alloc2/gc/AGCObjectVisitor.hpp b/include/xo/alloc2/gc/AGCObjectVisitor.hpp
index efedbfe..71603d1 100644
--- a/include/xo/alloc2/gc/AGCObjectVisitor.hpp
+++ b/include/xo/alloc2/gc/AGCObjectVisitor.hpp
@@ -14,12 +14,14 @@
 #pragma once
 
 // includes (via {facet_includes})
+#include <xo/arena/AllocInfo.hpp>
 #include <xo/facet/obj.hpp>
 #include <xo/facet/facet_implementation.hpp>
 #include <xo/facet/typeseq.hpp>
 
 // see GCObject.hpp, also in xo-alloc2/
 namespace xo { namespace mm { class AGCObject; }}
+namespace xo { namespace mm { class AllocInfo; }}
 
 namespace xo {
 namespace mm {
@@ -53,12 +55,15 @@ public:
     virtual typeseq _typeseq() const noexcept = 0;
     /** destroy instance @p d; calls c++ dtor only for actual runtime type; does not recover memory **/
     virtual void _drop(Opaque d) const noexcept = 0;
+    /** allocation metadata for gc-aware data at address @p gco.
+@p gco must be the result of a call to collector's alloc() function **/
+    virtual AllocInfo alloc_info(Copaque data, void * addr)  const = 0;
 
     // nonconst methods
     /** allocate copy of source object at address @p src.
 Source must be owned by this collector.
 Increments object age **/
-    virtual void * alloc_copy(Opaque data, std::byte * src)  = 0;
+    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;
     ///@}
diff --git a/include/xo/alloc2/gc/IGCObjectVisitor_Any.hpp b/include/xo/alloc2/gc/IGCObjectVisitor_Any.hpp
index 7297239..5e48ef4 100644
--- a/include/xo/alloc2/gc/IGCObjectVisitor_Any.hpp
+++ b/include/xo/alloc2/gc/IGCObjectVisitor_Any.hpp
@@ -58,9 +58,10 @@ namespace mm {
         [[noreturn]] void _drop(Opaque) const noexcept override { _fatal(); }
 
         // const methods
+        [[noreturn]] AllocInfo alloc_info(Copaque, void *)  const override { _fatal(); }
 
         // nonconst methods
-        [[noreturn]] void * alloc_copy(Opaque, std::byte *)  override;
+        [[noreturn]] void * alloc_copy(Opaque, std::byte *)  const override;
         [[noreturn]] void visit_child(Opaque, AGCObject *, void **)  const  noexcept override;
 
         ///@}
diff --git a/include/xo/alloc2/gc/IGCObjectVisitor_Xfer.hpp b/include/xo/alloc2/gc/IGCObjectVisitor_Xfer.hpp
index 2786ed4..8daaee4 100644
--- a/include/xo/alloc2/gc/IGCObjectVisitor_Xfer.hpp
+++ b/include/xo/alloc2/gc/IGCObjectVisitor_Xfer.hpp
@@ -13,6 +13,7 @@
 
 #pragma once
 
+#include <xo/arena/AllocInfo.hpp>
 
 namespace xo {
 namespace mm {
@@ -42,9 +43,12 @@ namespace mm {
         void _drop(Opaque d) const noexcept override { _dcast(d).~DRepr(); }
 
         // const methods
+        AllocInfo alloc_info(Copaque data, void * addr)  const override {
+            return I::alloc_info(_dcast(data), addr);
+        }
 
         // non-const methods
-        void * alloc_copy(Opaque data, std::byte * src)  override {
+        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 {
diff --git a/include/xo/alloc2/gc/RGCObjectVisitor.hpp b/include/xo/alloc2/gc/RGCObjectVisitor.hpp
index 0b120a8..461790a 100644
--- a/include/xo/alloc2/gc/RGCObjectVisitor.hpp
+++ b/include/xo/alloc2/gc/RGCObjectVisitor.hpp
@@ -88,6 +88,9 @@ public:
     void _drop() const noexcept { O::iface()->_drop(O::data()); }
 
     // const methods
+    AllocInfo alloc_info(void * addr)  const {
+        return O::iface()->alloc_info(O::data(), addr);
+    }
 
     // non-const methods (still const in router!)
     void * alloc_copy(std::byte * src)  {
diff --git a/src/alloc2/facet/IGCObjectVisitor_Any.cpp b/src/alloc2/facet/IGCObjectVisitor_Any.cpp
index e60cc31..0946acd 100644
--- a/src/alloc2/facet/IGCObjectVisitor_Any.cpp
+++ b/src/alloc2/facet/IGCObjectVisitor_Any.cpp
@@ -36,7 +36,7 @@ IGCObjectVisitor_Any::_valid
 // nonconst methods
 
 auto
-IGCObjectVisitor_Any::alloc_copy(Opaque, std::byte *)  -> void *
+IGCObjectVisitor_Any::alloc_copy(Opaque, std::byte *)  const -> void *
 {
     _fatal();
 }