diff --git a/examples/demo/fibonacci_tco.js b/examples/demo/fibonacci_tco.js
index 8623f1a..69dfdd0 100644
--- a/examples/demo/fibonacci_tco.js
+++ b/examples/demo/fibonacci_tco.js
@@ -3,9 +3,27 @@ function fib(n, a = 0, b = 1) {
   return fib(n - 1, b, a + b);
 }
 
-const start = performance.now();
+let start = performance.now();
 const result = fib(35);
-const end = performance.now();
+const singleShotMs = performance.now() - start;
+
+const warmup = 10000;
+const iterations = 200000;
+
+for (let i = 0; i < warmup; i++) fib(35);
+
+let sink = 0;
+start = performance.now();
+for (let i = 0; i < iterations; i++) sink += i & 1;
+const loopOverhead = performance.now() - start;
+
+start = performance.now();
+for (let i = 0; i < iterations; i++) sink += fib(35);
+const elapsed = performance.now() - start;
+
+const net = Math.max(0, elapsed - loopOverhead);
+const perCallUs = (net * 1000) / iterations;
 
 console.log(`fibonacci(35) = ${result}`);
-console.log(`Time: ${(end - start).toFixed(4)} ms (${((end - start) * 1000).toFixed(2)} µs)`);
+console.log(`Single-shot: ${singleShotMs.toFixed(4)} ms (${(singleShotMs * 1000).toFixed(2)} µs)`);
+console.log(`Time: ${perCallUs.toFixed(4)} µs/call (${iterations} iterations)`);
diff --git a/include/silver/engine.h b/include/silver/engine.h
index 99e821d..c1d09e6 100644
--- a/include/silver/engine.h
+++ b/include/silver/engine.h
@@ -73,10 +73,16 @@ typedef struct {
   uint32_t cached_index;
   uint32_t epoch;
   uintptr_t cached_aux;
-  ant_shape_t *add_from_shape;
-  ant_shape_t *add_to_shape;
-  uint32_t add_slot;
-  uint32_t add_epoch;
+  // keep this union valid: each IC slot must belong to exactly one bytecode site/op
+  union {
+    ant_value_t receiver_proto;
+    struct {
+      ant_shape_t *from_shape;
+      ant_shape_t *to_shape;
+      uint32_t slot;
+      uint32_t epoch;
+    } add;
+  } guard;
   bool cached_is_own;
 } sv_ic_entry_t;
 
diff --git a/src/silver/ops/property.h b/src/silver/ops/property.h
index ed51791..ad7c105 100644
--- a/src/silver/ops/property.h
+++ b/src/silver/ops/property.h
@@ -169,6 +169,7 @@ static inline bool sv_ic_try_get_hit(
   } else {
     ant_object_t *holder = ic->cached_holder;
     if (!holder || holder->is_exotic || !holder->shape) return false;
+    if (receiver->proto != ic->guard.receiver_proto) return false;
     source = holder;
     prop_shape = holder->shape;
   }
@@ -423,6 +424,7 @@ static inline ant_value_t sv_prop_get_field_ic(
     if (sv_ic_probe_get_chain(obj, a->str, &holder, &prop_idx, &out)) {
       ic->cached_shape = ptr->shape;
       ic->cached_holder = holder;
+      ic->guard.receiver_proto = ptr->proto;
       ic->cached_index = prop_idx;
       ic->cached_is_own = (holder == ptr);
       ic->epoch = ant_ic_epoch_counter;
@@ -504,26 +506,26 @@ static inline void sv_ic_set_add_transition(
 ) {
   if (!ic) return;
 
-  if (ic->add_from_shape != from) {
-    if (ic->add_from_shape) ant_shape_release(ic->add_from_shape);
-    ic->add_from_shape = NULL;
+  if (ic->guard.add.from_shape != from) {
+    if (ic->guard.add.from_shape) ant_shape_release(ic->guard.add.from_shape);
+    ic->guard.add.from_shape = NULL;
     if (from) {
       ant_shape_retain(from);
-      ic->add_from_shape = from;
+      ic->guard.add.from_shape = from;
     }
   }
 
-  if (ic->add_to_shape != to) {
-    if (ic->add_to_shape) ant_shape_release(ic->add_to_shape);
-    ic->add_to_shape = NULL;
+  if (ic->guard.add.to_shape != to) {
+    if (ic->guard.add.to_shape) ant_shape_release(ic->guard.add.to_shape);
+    ic->guard.add.to_shape = NULL;
     if (to) {
       ant_shape_retain(to);
-      ic->add_to_shape = to;
+      ic->guard.add.to_shape = to;
     }
   }
 
-  ic->add_slot = slot;
-  ic->add_epoch = epoch;
+  ic->guard.add.slot = slot;
+  ic->guard.add.epoch = epoch;
 }
 
 static inline ant_value_t sv_op_put_field(
@@ -557,23 +559,23 @@ static inline ant_value_t sv_op_put_field(
       !ptr->frozen && !ptr->sealed && ptr->extensible &&
       ptr->type_tag != T_ARR &&
       !sv_is_proto_atom(a) &&
-      ic->add_epoch == ant_ic_epoch_counter &&
-      ic->add_from_shape == ptr->shape &&
-      ic->add_to_shape) {
+      ic->guard.add.epoch == ant_ic_epoch_counter &&
+      ic->guard.add.from_shape == ptr->shape &&
+      ic->guard.add.to_shape) {
     if (ant_shape_lookup_interned(ptr->shape, a->str) < 0) {
       ant_shape_t *old_shape = ptr->shape;
-      ant_shape_retain(ic->add_to_shape);
-      ptr->shape = ic->add_to_shape;
+      ant_shape_retain(ic->guard.add.to_shape);
+      ptr->shape = ic->guard.add.to_shape;
       ant_shape_release(old_shape);
-      if (ic->add_slot >= ptr->prop_count &&
-          !js_obj_ensure_prop_capacity(ptr, ic->add_slot + 1)) {
+      if (ic->guard.add.slot >= ptr->prop_count &&
+          !js_obj_ensure_prop_capacity(ptr, ic->guard.add.slot + 1)) {
         return js_mkerr(js, "oom");
       }
-      ant_object_prop_set_unchecked(ptr, ic->add_slot, val);
+      ant_object_prop_set_unchecked(ptr, ic->guard.add.slot, val);
       gc_write_barrier(js, ptr, val);
       ic->cached_shape = ptr->shape;
       ic->cached_holder = ptr;
-      ic->cached_index = ic->add_slot;
+      ic->cached_index = ic->guard.add.slot;
       ic->epoch = ant_ic_epoch_counter;
       return val;
     }
diff --git a/src/silver/swarm.c b/src/silver/swarm.c
index 259cb72..3291288 100644
--- a/src/silver/swarm.c
+++ b/src/silver/swarm.c
@@ -761,7 +761,7 @@ static bool mir_emit_get_field_ic_fastpath(
   char gf_ot_name[32], gf_op_name[32], gf_os_name[32], gf_ics_name[32];
   char gf_h_name[32], gf_hs_name[32], gf_idx_name[32], gf_pc_name[32];
   char gf_ica_name[32], gf_il_name[32], gf_ovf_name[32], gf_ovi_name[32];
-  char gf_io_name[32], gf_src_name[32];
+  char gf_io_name[32], gf_src_name[32], gf_op_proto_name[32], gf_ic_proto_name[32];
   snprintf(gf_ic_name, sizeof(gf_ic_name), "gf_ic_%d_%u", bc_off, (unsigned)ic_idx);
   snprintf(gf_ice_name, sizeof(gf_ice_name), "gf_ice_%d_%u", bc_off, (unsigned)ic_idx);
   snprintf(gf_ot_name, sizeof(gf_ot_name), "gf_ot_%d_%u", bc_off, (unsigned)ic_idx);
@@ -778,6 +778,8 @@ static bool mir_emit_get_field_ic_fastpath(
   snprintf(gf_ovi_name, sizeof(gf_ovi_name), "gf_ovi_%d_%u", bc_off, (unsigned)ic_idx);
   snprintf(gf_io_name, sizeof(gf_io_name), "gf_io_%d_%u", bc_off, (unsigned)ic_idx);
   snprintf(gf_src_name, sizeof(gf_src_name), "gf_src_%d_%u", bc_off, (unsigned)ic_idx);
+  snprintf(gf_op_proto_name, sizeof(gf_op_proto_name), "gf_opp_%d_%u", bc_off, (unsigned)ic_idx);
+  snprintf(gf_ic_proto_name, sizeof(gf_ic_proto_name), "gf_icp_%d_%u", bc_off, (unsigned)ic_idx);
 
   MIR_reg_t r_ic = MIR_new_func_reg(ctx, fn->u.func, MIR_T_I64, gf_ic_name);
   MIR_reg_t r_ic_epoch = MIR_new_func_reg(ctx, fn->u.func, MIR_T_I64, gf_ice_name);
@@ -795,6 +797,8 @@ static bool mir_emit_get_field_ic_fastpath(
   MIR_reg_t r_overflow_idx = MIR_new_func_reg(ctx, fn->u.func, MIR_T_I64, gf_ovi_name);
   MIR_reg_t r_is_own = MIR_new_func_reg(ctx, fn->u.func, MIR_T_I64, gf_io_name);
   MIR_reg_t r_source = MIR_new_func_reg(ctx, fn->u.func, MIR_T_I64, gf_src_name);
+  MIR_reg_t r_obj_proto = MIR_new_func_reg(ctx, fn->u.func, MIR_T_I64, gf_op_proto_name);
+  MIR_reg_t r_ic_proto = MIR_new_func_reg(ctx, fn->u.func, MIR_T_I64, gf_ic_proto_name);
 
   MIR_label_t load_overflow = MIR_new_label(ctx);
   MIR_label_t fast_done = MIR_new_label(ctx);
@@ -869,6 +873,21 @@ static bool mir_emit_get_field_ic_fastpath(
       MIR_new_reg_op(ctx, r_is_own),
       MIR_new_int_op(ctx, 0)));
 
+  MIR_append_insn(ctx, fn,
+    MIR_new_insn(ctx, MIR_MOV,
+      MIR_new_reg_op(ctx, r_obj_proto),
+      MIR_new_mem_op(ctx, MIR_T_I64,
+        (MIR_disp_t)offsetof(ant_object_t, proto), r_obj_ptr, 0, 1)));
+  MIR_append_insn(ctx, fn,
+    MIR_new_insn(ctx, MIR_MOV,
+      MIR_new_reg_op(ctx, r_ic_proto),
+      MIR_new_mem_op(ctx, MIR_T_I64,
+        (MIR_disp_t)offsetof(sv_ic_entry_t, guard.receiver_proto), r_ic, 0, 1)));
+  MIR_append_insn(ctx, fn,
+    MIR_new_insn(ctx, MIR_BNE,
+      MIR_new_label_op(ctx, slow),
+      MIR_new_reg_op(ctx, r_obj_proto),
+      MIR_new_reg_op(ctx, r_ic_proto)));
   MIR_append_insn(ctx, fn,
     MIR_new_insn(ctx, MIR_MOV,
       MIR_new_reg_op(ctx, r_holder),
diff --git a/tests/test_property_ic_proto_guard.cjs b/tests/test_property_ic_proto_guard.cjs
new file mode 100644
index 0000000..a6c84fd
--- /dev/null
+++ b/tests/test_property_ic_proto_guard.cjs
@@ -0,0 +1,20 @@
+const assert = require('assert');
+
+const protoA = { marker: 11 };
+const protoB = { marker: 29 };
+
+function make(proto) {
+  return { __proto__: proto, own: 1 };
+}
+
+function marker(obj) {
+  return obj.marker;
+}
+
+for (let i = 0; i < 64; i++) {
+  assert.strictEqual(marker(make(protoA)), 11);
+}
+
+assert.strictEqual(marker(make(protoB)), 29);
+
+console.log('property-ic-proto-guard:ok');