diff --git a/include/thespian/handle.hpp b/include/thespian/handle.hpp
index 80cdc98..477ab87 100644
--- a/include/thespian/handle.hpp
+++ b/include/thespian/handle.hpp
@@ -46,13 +46,9 @@ private:
 
 auto operator==(const handle &, const handle &) -> bool;
 
-/// Stable opaque actor identity: the raw instance pointer. Two handles for
-/// the same actor return the same value. Returns 0 for expired handles.
-[[nodiscard]] inline auto instance_id(const handle &h) -> uintptr_t {
-  // NOLINTNEXTLINE(*-reinterpret-cast)
-  if (auto sp = handle_ref(h).lock())
-    return reinterpret_cast<uintptr_t>(sp.get());
-  return 0;
-}
+/// Stable numeric actor identity assigned at spawn (atomic incrementing
+/// counter, starting at 1). Two handles for the same actor return the same
+/// value. Returns 0 for a null or expired handle.
+[[nodiscard]] auto instance_id(const handle &h) -> uintptr_t;
 
 } // namespace thespian
diff --git a/src/instance.cpp b/src/instance.cpp
index acfac01..01e23cc 100644
--- a/src/instance.cpp
+++ b/src/instance.cpp
@@ -85,6 +85,8 @@ using namespace std::chrono_literals;
 
 namespace thespian {
 
+static atomic<uintptr_t> next_instance_id{1}; // NOLINT(*-avoid-non-const-global-variables)
+
 struct context_impl : context {
   explicit context_impl(executor::context executor)
       : executor{move(executor)} {}
@@ -226,6 +228,7 @@ struct instance : std::enable_shared_from_this<instance> {
   instance(context_impl &ctx, behaviour b, exit_handler eh, string_view name,
            env_t env)
       : ctx(ctx), strand_(ctx.executor.create_strand()), name_{name},
+        instance_id_{next_instance_id.fetch_add(1, memory_order_relaxed)},
         env_(move(env)) {
     if (eh)
       exit_handlers_.emplace_front(eh);
@@ -491,6 +494,7 @@ struct instance : std::enable_shared_from_this<instance> {
   context_impl &ctx;
   executor::strand strand_;
   string name_;
+  uintptr_t instance_id_;
   receiver receiver_;
   receiver exited_receiver_;
   sync_receiver sync_receiver_;
@@ -596,6 +600,12 @@ auto context::spawn_link(behaviour b, exit_handler eh, string_view name)
   return spawn_link(move(b), move(eh), name, env_t{});
 }
 
+auto instance_id(const handle &h) -> uintptr_t {
+  if (auto sp = handle_ref(h).lock())
+    return sp->instance_id_;
+  return 0;
+}
+
 auto env() -> env_t & { return private_call().env_; }
 auto self() -> handle { return make_handle(private_call().lifetime_); }
 auto self_ref() -> handle & { return private_call().self_ref_; }