diff --git a/src/cpp/tests/wheel_smoke.py b/src/cpp/tests/wheel_smoke.py
index c81cf5da..a9ed4882 100644
--- a/src/cpp/tests/wheel_smoke.py
+++ b/src/cpp/tests/wheel_smoke.py
@@ -21,7 +21,8 @@ def main() -> None:
 
     # The extension is the one shipped in the wheel, and it carries its own OpenCASCADE (no OCP, no system OCCT).
     here = Path(cpp.__file__).resolve().parent
-    assert any(here.glob("_native*")), f"no compiled extension next to {cpp.__file__}"
+    # a file: a directory `_native/` next to it holds the type stubs and is not the extension
+    assert any(path.is_file() for path in here.glob("_native*")), f"no compiled extension next to {cpp.__file__}"
     from simplecadapi._cpp import occt
 
     print("python", sys.version.split()[0], "| OpenCASCADE", occt.__version__, "|", here)
diff --git a/test/test_source_tree_is_built.py b/test/test_source_tree_is_built.py
new file mode 100644
index 00000000..0a4f6318
--- /dev/null
+++ b/test/test_source_tree_is_built.py
@@ -0,0 +1,41 @@
+"""A checkout is a built source tree only when the compiled module is there; the type stubs do not count.
+
+The checkout carries `src/simplecadapi/_cpp/_native/*.pyi` (a directory).  The validation workers put the checkout's
+source on the path of their subprocess only when it is built, because otherwise it shadows the installed wheel with Python
+files that have no compiled module: the wheel test job of the 2.2.7 dry run failed that way.
+"""
+
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "tools" / "research" / "build123dftc"))
+from b3ftc.validation import _source_tree_is_built  # noqa: E402
+
+
+def make(tmp_path: Path, *names: str, stubs: bool = False) -> Path:
+    cpp = tmp_path / "simplecadapi" / "_cpp"
+    cpp.mkdir(parents=True)
+    if stubs:
+        (cpp / "_native").mkdir()
+        (cpp / "_native" / "__init__.pyi").write_text("")
+    for name in names:
+        (cpp / name).write_bytes(b"")
+    return tmp_path
+
+
+def test_the_type_stub_directory_alone_is_not_a_build(tmp_path) -> None:
+    assert not _source_tree_is_built(make(tmp_path, stubs=True))
+
+
+def test_nothing_is_not_a_build(tmp_path) -> None:
+    assert not _source_tree_is_built(make(tmp_path))
+
+
+@pytest.mark.parametrize("name", ["_native.cpython-312-darwin.so", "_native.cpython-313-x86_64-linux-gnu.so", "_native.cp312-win_amd64.pyd"])
+def test_the_compiled_module_is_a_build_with_or_without_stubs(tmp_path, name) -> None:
+    assert _source_tree_is_built(make(tmp_path, name, stubs=True))
+
diff --git a/tools/research/build123dftc/b3ftc/validation.py b/tools/research/build123dftc/b3ftc/validation.py
index 70bbf727..0c0ae925 100644
--- a/tools/research/build123dftc/b3ftc/validation.py
+++ b/tools/research/build123dftc/b3ftc/validation.py
@@ -159,6 +159,16 @@ def _interpreter(executable: str | None) -> str:
     return os.path.abspath(executable) if os.sep in executable else executable
 
 
+def _source_tree_is_built(source: Path) -> bool:
+    """Whether the compiled module sits next to the Python files of *source* (a development build).
+
+    The module is a file (`_native*.so` / `_native*.pyd`); a *directory* `_native/` next to it holds the type stubs a
+    checkout carries and says nothing about a build.
+    """
+
+    return any(path.is_file() for path in (source / "simplecadapi" / "_cpp").glob("_native*"))
+
+
 def _process(command: list[str], staging: Path, env: Mapping[str, str],
              output: Path, timeout: float) -> dict[str, Any]:
     if timeout <= 0:
@@ -222,7 +232,7 @@ def _worker(ftc_path: Path, workdir: Path | None, timeout: float,
         # The checkout's source tree goes on the path only when it is a built one (the compiled module sits next to
         # its Python files, as in a development build); otherwise it would shadow the installed wheel.
         source = repo_root / "src"
-        built = any((source / "simplecadapi" / "_cpp").glob("_native*"))
+        built = _source_tree_is_built(source)
         env["PYTHONPATH"] = os.pathsep.join(
             [*([str(source)] if built else []), str(tool_dir), env.get("PYTHONPATH", "")])
         started = time.monotonic()
diff --git a/tools/research/cadqueryftc/cadquery_validate.py b/tools/research/cadqueryftc/cadquery_validate.py
index a613aa63..b9e88757 100644
--- a/tools/research/cadqueryftc/cadquery_validate.py
+++ b/tools/research/cadqueryftc/cadquery_validate.py
@@ -32,7 +32,10 @@
 if str(_TOOLS_DIR) not in sys.path:
     sys.path.insert(0, str(_TOOLS_DIR))
 # the checkout's source tree only when it is a built one (it would shadow an installed wheel otherwise)
-if str(_REPO_ROOT / "src") not in sys.path and any((_REPO_ROOT / "src" / "simplecadapi" / "_cpp").glob("_native*")):
+# (the module is a file; a directory `_native/` next to it holds the type stubs and is no sign of a build)
+if str(_REPO_ROOT / "src") not in sys.path and any(
+    path.is_file() for path in (_REPO_ROOT / "src" / "simplecadapi" / "_cpp").glob("_native*")
+):
     sys.path.insert(0, str(_REPO_ROOT / "src"))
 
 from cqftc.tracer.trace_schema import OperationTrace  # noqa: E402