diff --git a/runtime/CMakeLists.txt b/runtime/CMakeLists.txt
index 70786db9..6641373a 100644
--- a/runtime/CMakeLists.txt
+++ b/runtime/CMakeLists.txt
@@ -118,6 +118,13 @@ if(BUILD_TESTING)
         target_link_options(boot_state_handoff_test PRIVATE -flto -Wl,--gc-sections)
         target_link_libraries(boot_state_handoff_test PRIVATE ZLIB::ZLIB)
         add_test(NAME boot_state_handoff_test COMMAND boot_state_handoff_test)
+        add_executable(boot_state_impl_tag_test tests/test_boot_state_handoff.c)
+        target_include_directories(boot_state_impl_tag_test PRIVATE include src)
+        target_compile_definitions(boot_state_impl_tag_test PRIVATE PSX_COSIM=1 PSX_SAVESTATE_IMPL_TAG=0x54474801u)
+        target_compile_options(boot_state_impl_tag_test PRIVATE -O2 -flto -ffunction-sections -fdata-sections -UNDEBUG)
+        target_link_options(boot_state_impl_tag_test PRIVATE -flto -Wl,--gc-sections)
+        target_link_libraries(boot_state_impl_tag_test PRIVATE ZLIB::ZLIB)
+        add_test(NAME boot_state_impl_tag_test COMMAND boot_state_impl_tag_test)
 
         add_executable(static_match_cache_test tests/test_static_match_cache.c
             src/overlay_loader.c src/crc32.c)
diff --git a/runtime/include/boot_state.h b/runtime/include/boot_state.h
index 27a94b44..ee25845d 100644
--- a/runtime/include/boot_state.h
+++ b/runtime/include/boot_state.h
@@ -66,7 +66,7 @@ typedef struct {
     uint32_t entry_pc;       /* game PS-EXE entry PC                              */
     uint32_t codegen_hash;   /* PSX_OVERLAY_CODEGEN_HASH (auto-gen by cmake)      */
     int32_t  abi_tag;        /* PSX_OVERLAY_ABI_TAG (abi version | flavor<<16)    */
-    uint32_t codegen_ver;    /* PSX_OVERLAY_CODEGEN_VER                           */
+    uint32_t codegen_ver;    /* codegen version XOR build PSX_SAVESTATE_IMPL_TAG  */
     /* ---- layout ---- */
     uint32_t section_count;  /* number of sections that follow                    */
     uint32_t reserved;       /* v8: enhancement-memory layout cookie; vanilla 0   */
diff --git a/runtime/src/boot_state.c b/runtime/src/boot_state.c
index c48e929e..08b731ef 100644
--- a/runtime/src/boot_state.c
+++ b/runtime/src/boot_state.c
@@ -2,6 +2,13 @@
 #include "fntrace.h"
 #include "mod_memory.h"
 #include "overlay_api.h"   /* PSX_OVERLAY_CODEGEN_HASH / _ABI_TAG / _CODEGEN_VER */
+
+/* Build-selected HLE may use private execution boundaries. Zero preserves
+ * the existing snapshot key for titles that do not select an implementation. */
+#ifndef PSX_SAVESTATE_IMPL_TAG
+#define PSX_SAVESTATE_IMPL_TAG 0u
+#endif
+#define BOOT_STATE_IMPLEMENTATION_KEY ((uint32_t)PSX_OVERLAY_CODEGEN_VER ^ (uint32_t)PSX_SAVESTATE_IMPL_TAG)
 #include "dirty_ram_interp.h"
 #include "gpu.h"           /* gpu_get_vram — CPU-auth mirror under dual-raster   */
 #include "gpu_render.h"    /* gr_vram_transfer_in / gr_vram_transfer_out          */
@@ -390,7 +397,7 @@ static int boot_state_save_to(BsOut* o, const CPUState* cpu,
     h.entry_pc      = entry_pc;
     h.codegen_hash  = (uint32_t)PSX_OVERLAY_CODEGEN_HASH;
     h.abi_tag       = (int32_t)PSX_OVERLAY_ABI_TAG;
-    h.codegen_ver   = (uint32_t)PSX_OVERLAY_CODEGEN_VER;
+    h.codegen_ver   = BOOT_STATE_IMPLEMENTATION_KEY;
     h.section_count = 17 + (psx_mod_memory_snapshot_bytes() ? 1u : 0u);
 
     ok = write_header_le(o, &h);
@@ -870,9 +877,9 @@ int boot_state_check_buffer(const uint8_t* file, size_t file_len,
                  (int)h.abi_tag, (int)PSX_OVERLAY_ABI_TAG);
         boot_state_append_reason(reason, reason_cap, part);
     }
-    if (h.codegen_ver != (uint32_t)PSX_OVERLAY_CODEGEN_VER) {
+    if (h.codegen_ver != BOOT_STATE_IMPLEMENTATION_KEY) {
         snprintf(part, sizeof(part), "codegen_ver=%u(want %u)",
-                 (unsigned)h.codegen_ver, (unsigned)PSX_OVERLAY_CODEGEN_VER);
+                 (unsigned)h.codegen_ver, (unsigned)BOOT_STATE_IMPLEMENTATION_KEY);
         boot_state_append_reason(reason, reason_cap, part);
     }
 
diff --git a/runtime/tests/test_boot_state_handoff.c b/runtime/tests/test_boot_state_handoff.c
index 74040d65..c6493085 100644
--- a/runtime/tests/test_boot_state_handoff.c
+++ b/runtime/tests/test_boot_state_handoff.c
@@ -68,6 +68,16 @@ int main(void) {
         /* Malformed, missing and duplicate phase sections must reject before
          * changing CPU, RAM or the previously active handoff latch. */
         cpu.pc=0xdeadbeef; ram[0x100]=0x55; fntrace_restore_game_started(!started);
+        /* A different build-selected implementation must reject atomically,
+         * including when its only difference is an HLE execution boundary. */
+        PstR key_reader; uint32_t implementation_key;
+        pst_r_init(&key_reader, saved + 24, 4);
+        assert(pst_r_u32(&key_reader, &implementation_key));
+        assert(implementation_key == BOOT_STATE_IMPLEMENTATION_KEY);
+        u32(saved + 24, implementation_key ^ 0x54474801u);
+        assert(!boot_state_load_buffer(saved,len,123,0x801b0000,&cpu));
+        assert(cpu.pc==0xdeadbeef && ram[0x100]==0x55 && fntrace_is_game_started()==!started);
+        u32(saved + 24, implementation_key);
         saved[len-4]=2;
         assert(!boot_state_load_buffer(saved,len,123,0x801b0000,&cpu));
         saved[len-4]=started;