diff --git a/src/scikit_build_core/build/_file_processor.py b/src/scikit_build_core/build/_file_processor.py
index f50e1408..a422daf8 100644
--- a/src/scikit_build_core/build/_file_processor.py
+++ b/src/scikit_build_core/build/_file_processor.py
@@ -89,7 +89,7 @@ def each_unignored_file(
                     is_path=True,
                 ):
                     # Check to see if any include rules start with this
-                    dstr = str(dirpath / dname).strip("/") + "/"
+                    dstr = (dirpath / dname).as_posix().strip("/") + "/"
                     if not any(p.lstrip("/").startswith(dstr) for p in include):
                         dirs.remove(dname)
 
diff --git a/tests/test_file_processor.py b/tests/test_file_processor.py
index 1de1423c..aaa39a7b 100644
--- a/tests/test_file_processor.py
+++ b/tests/test_file_processor.py
@@ -201,6 +201,30 @@ def test_include_patterns(
     assert result == expected
 
 
+def test_include_pattern_with_nested_path_and_broad_exclude(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """
+    Test that nested include patterns are not pruned by directory traversal,
+    even when exclude patterns match all paths.
+    """
+    monkeypatch.chdir(tmp_path)
+    nested_file = Path("a") / "b" / "c.txt"
+    nested_file.parent.mkdir(parents=True)
+    nested_file.write_text("content")
+
+    result = set(
+        each_unignored_file(
+            Path(),
+            include=["a/b/c.txt"],
+            exclude=["*"],
+            mode="manual",
+        )
+    )
+    assert result == {nested_file}
+
+
 def test_exclude_patterns(
     tmp_path: Path,
     monkeypatch: pytest.MonkeyPatch,