diff --git a/docs/accuracy/bios_exit_critical_section.md b/docs/accuracy/bios_exit_critical_section.md
index fde76f4d..1a6ba780 100644
--- a/docs/accuracy/bios_exit_critical_section.md
+++ b/docs/accuracy/bios_exit_critical_section.md
@@ -16,6 +16,15 @@ The specification allows K0 to change; this direct handler preserves it too.
 The host-only `cpu->pc = 0` continuation and C return value remain unchanged.
 
 `exit_critical_section_test` compiles the real `traps.c` implementation with
-GNU LTO and tests 16 combinations of incoming `v0` and SR. It checks the full
-CPU state, allowing only the documented SR update and host continuation change.
+LTO and tests 16 combinations of incoming `v0` and SR. It checks the full CPU
+state, allowing only the documented SR update and host continuation change.
 The test uses no BIOS, generated retail code, or game data.
+
+The fixture links only the real trap unit and its test driver. It relies on
+interprocedural optimization removing syscall paths that the driver never
+calls, rather than providing fake scheduler or exception implementations.
+This link contract is verified with Windows x64 MinGW GCC 16.1.0 and is enabled
+only for that toolchain. Other configurations still register the test, but
+CTest reports it as **Skipped** with the unsupported compiler and reason.
+They do not provide register-preservation coverage until the fixture's link
+contract is validated there or a portable real-dependency harness replaces it.
diff --git a/runtime/CMakeLists.txt b/runtime/CMakeLists.txt
index 3796fcd1..e0101ec8 100644
--- a/runtime/CMakeLists.txt
+++ b/runtime/CMakeLists.txt
@@ -183,9 +183,12 @@ if(BUILD_TESTING)
     add_test(NAME mod_gpu_dma_aperture_test
              COMMAND mod_gpu_dma_aperture_test)
 
-    # Compile the real syscall branch. GNU LTO discards the unrelated scheduler
-    # and exception paths, avoiding stubs which could hide register corruption.
-    if(CMAKE_C_COMPILER_ID STREQUAL "GNU" AND NOT APPLE)
+    # The real-handler fixture relies on LTO pruning its unused syscall paths.
+    # Enable only the toolchain on which that link contract is verified; keep
+    # the coverage gap visible in CTest for every other compiler/platform.
+    if(WIN32 AND MINGW AND CMAKE_SIZEOF_VOID_P EQUAL 8
+       AND CMAKE_C_COMPILER_ID STREQUAL "GNU"
+       AND CMAKE_C_COMPILER_VERSION VERSION_EQUAL "16.1.0")
         add_executable(exit_critical_section_test
             tests/test_exit_critical_section.c src/traps.c)
         target_include_directories(exit_critical_section_test PRIVATE include)
@@ -193,6 +196,12 @@ if(BUILD_TESTING)
         target_compile_options(exit_critical_section_test PRIVATE -O3 -flto -ffunction-sections)
         target_link_options(exit_critical_section_test PRIVATE -flto -Wl,--gc-sections)
         add_test(NAME exit_critical_section_test COMMAND exit_critical_section_test)
+    else()
+        add_test(NAME exit_critical_section_test
+            COMMAND "${CMAKE_COMMAND}" -E echo
+                "SKIP: real-handler LTO fixture is verified only on Windows x64 MinGW GCC 16.1.0; current compiler is ${CMAKE_C_COMPILER_ID} ${CMAKE_C_COMPILER_VERSION}")
+        set_tests_properties(exit_critical_section_test PROPERTIES
+            SKIP_REGULAR_EXPRESSION "^SKIP:")
     endif()
 
     add_executable(dma_gpu_linked_list_timing_test