diff --git a/.github/workflows/ci_diag_sequential_lib_load.py b/.github/workflows/ci_diag_sequential_lib_load.py
new file mode 100644
index 00000000000..920d4f13ea1
--- /dev/null
+++ b/.github/workflows/ci_diag_sequential_lib_load.py
@@ -0,0 +1,28 @@
+"""CI diagnostic, not for merge.
+
+Imports every grass.lib.* ctypes submodule one at a time in a single
+process, flushing progress after each import, to find exactly how many
+(and which) sequential ctypes library loads it takes to reproduce the
+"Windows fatal exception: code 0xc0000139" crash seen under
+pytest-xdist. A plain `python -c "from grass.pygrass import utils"`
+in a single fresh process (see the sibling diagnostic PR) does not
+crash, so this checks whether the crash is tied to the number of
+distinct DLLs loaded cumulatively within one process.
+"""
+
+import faulthandler
+import pkgutil
+
+faulthandler.enable()
+
+import grass.lib  # noqa: E402
+
+modules = sorted(m.name for m in pkgutil.iter_modules(grass.lib.__path__))
+print(f"Found {len(modules)} grass.lib submodules: {modules}", flush=True)
+
+for i, name in enumerate(modules, 1):
+    print(f"[{i}/{len(modules)}] importing grass.lib.{name} ...", flush=True)
+    __import__(f"grass.lib.{name}")
+    print(f"[{i}/{len(modules)}] OK", flush=True)
+
+print("ALL grass.lib submodules imported successfully", flush=True)
diff --git a/.github/workflows/osgeo4w.yml b/.github/workflows/osgeo4w.yml
index 96b5a6bb9aa..4549ec77417 100644
--- a/.github/workflows/osgeo4w.yml
+++ b/.github/workflows/osgeo4w.yml
@@ -124,6 +124,24 @@ jobs:
         shell: msys2 {0}
         run: .github/workflows/test_simple.sh
 
+      - name: "[CI diagnostic, not for merge] Sequential grass.lib load count"
+        # A plain single import of grass.pygrass.utils (which loads
+        # grass_gis.8.6.dll) does not crash in isolation (see the sibling
+        # standalone-import-test PR), but the same load crashes with
+        # "Windows fatal exception: code 0xc0000139" under pytest-xdist.
+        # This imports every grass.lib.* ctypes submodule one at a time
+        # in a single process to see whether the crash is tied to the
+        # number of distinct DLLs accumulated as loaded in one process
+        # (e.g. a Windows/MinGW TLS-slot exhaustion), and if so, exactly
+        # how many/which imports it takes to trigger it.
+        run: |
+          call %OSGEO4W_ROOT%\opt\grass\etc\env.bat
+          set PYTHONPATH=%GISBASE%\etc\python;%PYTHONPATH%
+          path %GISBASE%\lib;%GISBASE%\bin;%PATH%
+          python .github/workflows/ci_diag_sequential_lib_load.py
+        # Obfuscated usage of GitHub Actions features, Windows CMD shell limits analysis
+        shell: cmd /D /E:ON /V:OFF /S /C "CALL C:/OSGeo4W/OSGeo4W.bat "{0}"" # zizmor: ignore[misfeature]
+
       - name: Install additional python packages
         run: |
           python -m pip install ^