diff --git a/lib/spack/spack/test/conftest.py b/lib/spack/spack/test/conftest.py
index c8c300ca95a292..75e7840343fa6e 100644
--- a/lib/spack/spack/test/conftest.py
+++ b/lib/spack/spack/test/conftest.py
@@ -763,6 +763,27 @@ def mutable_empty_config(tmpdir_factory, configuration_dir):
         yield cfg
 
 
+@pytest.fixture(scope="session", autouse=True)
+def mock_wsdk_externals(mock_configuration_scopes):
+    with spack.config.use_configuration(*mock_configuration_scopes) as config:
+        mock_pkgs = spack.util.spack_yaml.syaml_dict()
+        def make_external_entry(name):
+            external_pkg_items = spack.util.spack_yaml.syaml_dict([
+                ("spec", f"{name}@10.0.20348 plat=x64"),
+                ("prefix", "C:/Program Files (x86)/Windows Kits/10")
+            ])
+            external_pkg = spack.util.spack_yaml.syaml_dict()
+            external_pkg["externals"] = [external_pkg_items]
+            external_pkg["buildable"] = False
+            mock_pkgs[name] = external_pkg
+
+        make_external_entry("win-sdk")
+        make_external_entry("wgl")
+        pkgs_cfg = config.get("packages")
+        pkgs_cfg = spack.config.merge_yaml(pkgs_cfg, mock_pkgs)
+        config.set("packages", pkgs_cfg)
+
+
 @pytest.fixture(scope="function")
 def concretize_scope(mutable_config, tmpdir):
     """Adds a scope for concretization preferences"""