@@ -174,6 +174,15 @@ def record_build_major() -> None:
     _BUILD_MAJOR_STAMP.write_text(_determine_cuda_major_version() + "\n", encoding="utf-8")
 
 
+def _relativize_extension_sources(extensions) -> None:
+    """Keep absolute source paths out of setuptools' temporary build tree."""
+    for extension in extensions:
+        extension.sources = [
+            os.path.relpath(source, start=Path.cwd()) if os.path.isabs(source) else source
+            for source in extension.sources
+        ]
+
+
 def _build_cuda_core(debug=False):
     # Customizing the build hooks is needed because we must defer cythonization until cuda-bindings,
     # now a required build-time dependency that's dynamically installed via the other hook below,
@@ -292,6 +301,10 @@ def get_sources(mod_name):
         compile_time_env=compile_time_env,
         **extra_cythonize_kwargs,
     )
+    # Cython returns generated sources under the absolute build_dir above.
+    # setuptools mirrors absolute source paths into build/temp, which can push
+    # MSVC linker output paths past MAX_PATH in deeper Windows checkouts.
+    _relativize_extension_sources(_extensions)
 
     return
 
 
@@ -268,6 +268,19 @@ def test_dir_is_anchored_not_relative_to_cwd(self, monkeypatch):
         assert build_dir.parent.parent == build_hooks._BUILD_MAJOR_STAMP.parent
 
 
+class TestSetuptoolsSourcePaths:
+    @pytest.mark.agent_authored(model="gpt-5.6-sol")
+    def test_absolute_sources_are_made_relative(self, tmp_path, monkeypatch):
+        monkeypatch.chdir(tmp_path)
+        generated = tmp_path / "build" / "cython" / "cu13" / "cuda" / "core" / "_device.cpp"
+        relative = "cuda/core/_cpp/helper.cpp"
+        extension = build_hooks.Extension("cuda.core._device", [str(generated), relative])
+
+        build_hooks._relativize_extension_sources([extension])
+
+        assert extension.sources == [os.path.relpath(generated, start=tmp_path), relative]
+
+
 def _load_setup_py(monkeypatch):
     """Import setup.py for its command classes.