diff --git a/include/xo/alloc/GC.hpp b/include/xo/alloc/GC.hpp
index a0bcef2b..64cf198d 100644
--- a/include/xo/alloc/GC.hpp
+++ b/include/xo/alloc/GC.hpp
@@ -7,6 +7,7 @@
 
 #include "ListAlloc.hpp"
 #include "GcStatistics.hpp"
+#include "xo/callback/UpCallbackSet.hpp"
 #include "xo/indentlog/print/array.hpp"
 #include <vector>
 #include <array>
@@ -105,12 +106,31 @@ namespace xo {
 
         using MutationLog = std::vector<MutationLogEntry>;
 
+        /** @class GcCopyCallback
+         *  @brief optional callback to observe individual copy operations during GC
+         *
+         *  For viz
+         **/
+        class GcCopyCallback {
+        public:
+            virtual void notify_gc_copy(std::size_t z, const void * src_addr, const void * dest_addr,
+                                        generation src_gen, generation dest_gen) = 0;
+            /** invoked when added to callback set (i.e. @ref GC::GcCopyCallbackSet) **/
+            void notify_add_callback() {}
+            /** invoked when removed from callback set **/
+            void notify_remove_callback() {}
+        };
+
         /** @class GC
          *  @brief generational garbage collector
          *
          *  Works with objects of type @ref xo::Object
          **/
         class GC : public IAlloc {
+        public:
+            using CallbackId = xo::fn::CallbackId;
+            using GcCopyCallbackSet = xo::fn::UpCallbackSet<GcCopyCallback>;
+
         public:
             /** create new GC instance with configuration @p config **/
             explicit GC(const Config & config);
@@ -160,6 +180,11 @@ namespace xo {
              *  from @c *addr
              **/
             void add_gc_root(Object ** addr);
+            /** may optionally use this to observe GC copy phase.
+             *  Will be invoked once _per surviving object_, so not cheap.
+             *  Intended for GC visualization.
+             **/
+            CallbackId add_gc_copy_callback(up<GcCopyCallback> fn);
             /** request garbage collection. **/
             void request_gc(generation g);
             /** disable garbage collection until matching call to @ref enable_gc.
@@ -335,6 +360,9 @@ namespace xo {
 
             /** enabled when 0.  disabled when <0 **/
             int gc_enabled_ = 0;
+
+            /** for (optional) viz: invoke when copying individual objects **/
+            GcCopyCallbackSet gc_copy_cbset_;
         };
     } /*namespace gc*/
 
diff --git a/src/alloc/CMakeLists.txt b/src/alloc/CMakeLists.txt
index 589f3b90..07f87784 100644
--- a/src/alloc/CMakeLists.txt
+++ b/src/alloc/CMakeLists.txt
@@ -15,5 +15,6 @@ set(SELF_SRCS
 
 xo_add_shared_library4(${SELF_LIB} ${PROJECT_NAME}Targets ${PROJECT_VERSION} 1 ${SELF_SRCS})
 xo_dependency(${SELF_LIB} reflect)
+xo_dependency(${SELF_LIB} callback)
 
 #end CMakeLists.txt
diff --git a/src/alloc/GC.cpp b/src/alloc/GC.cpp
index bcb8118b..d0aaa660 100644
--- a/src/alloc/GC.cpp
+++ b/src/alloc/GC.cpp
@@ -289,6 +289,12 @@ namespace xo {
             gc_root_v_.push_back(addr);
         }
 
+        auto
+        GC::add_gc_copy_callback(up<GcCopyCallback> fn) -> CallbackId
+        {
+            return gc_copy_cbset_.add_callback(std::move(fn));
+        }
+
         void
         GC::checkpoint()
         {
@@ -322,16 +328,19 @@ namespace xo {
         {
             scope log(XO_DEBUG(config_.debug_flag_), xtag("z", z), xtag("+pad", IAlloc::alloc_padding(z)));
 
-            generation_result gr = this->fromspace_generation_of(src);
+            generation_result src_gr = this->fromspace_generation_of(src);
 
             std::byte * retval = nullptr;
 
-            switch (gr) {
+            switch (src_gr) {
             case generation_result::tenured:
                 {
                     log && log("tenured");
 
                     retval = this->tenured_to()->alloc(z);
+
+                    gc_copy_cbset_.invoke(&GcCopyCallback::notify_gc_copy,
+                                          z, src, retval, generation::tenured, generation::tenured);
                 }
                 break;
             case generation_result::nursery:
@@ -347,11 +356,18 @@ namespace xo {
 
                         assert(this->tospace_generation_of(retval) == generation_result::tenured);
 
+                        gc_copy_cbset_.invoke(&GcCopyCallback::notify_gc_copy,
+                                              z, src, retval, generation::nursery, generation::tenured);
+
                         this->gc_statistics_.total_promoted_ += IAlloc::with_padding(z);
+
                     } else {
                         log && log("nursery");
 
                         retval = this->nursery_to()->alloc(z);
+
+                        gc_copy_cbset_.invoke(&GcCopyCallback::notify_gc_copy,
+                                              z, src, retval, generation::nursery, generation::nursery);
                     }
                 }
                 break;