diff --git a/engraphis/cloud_license.py b/engraphis/cloud_license.py
index f896ebe..2595826 100644
--- a/engraphis/cloud_license.py
+++ b/engraphis/cloud_license.py
@@ -29,6 +29,7 @@
 import logging
 import math
 import os
+import sys
 import tempfile
 import threading
 import urllib.error
@@ -653,3 +654,34 @@ def revalidate(lic, key_material: str, *, base_url: Optional[str] = None) -> str
         return "offline"
     _write_lease(token)
     return "ok"
+
+
+# ── compilation integrity guard — runs at import time ────────────────────────────
+
+
+def _verify_module_integrity():
+    """Detect if this module was replaced with editable source after a compiled
+    extension was already installed (same check as licensing.py)."""
+    mod = sys.modules.get(__name__)
+    if mod is None:
+        return
+    f = getattr(mod, "__file__", "")
+    if not f:
+        return
+    if os.environ.get("ENGRAPHIS_DEV", "").strip() in ("1", "true", "yes"):
+        return
+    if not f.endswith(".py"):
+        return
+    from importlib.machinery import EXTENSION_SUFFIXES
+    dirname = os.path.dirname(f)
+    basename = os.path.splitext(os.path.basename(f))[0]
+    for suffix in EXTENSION_SUFFIXES:
+        if os.path.exists(os.path.join(dirname, basename + suffix)):
+            raise RuntimeError(
+                "Engraphis cloud_license integrity check failed: a compiled native "
+                "extension exists but is not being loaded. Reinstall from the "
+                "official distribution. For development, set ENGRAPHIS_DEV=1."
+            )
+
+
+_verify_module_integrity()
diff --git a/setup.py b/setup.py
new file mode 100644
index 0000000..7bbbe88
--- /dev/null
+++ b/setup.py
@@ -0,0 +1,67 @@
+"""Build script — compiles licensing + cloud_license into native C extensions for
+distribution. Dev installs (``pip install -e .``) skip Cython and use the plain .py
+sources; ``python -m build`` / ``pip install .`` (release) compile them via Cython."""
+
+import os
+
+from setuptools import Extension, setup
+from setuptools.command.build_py import build_py as _build_py
+
+
+SKIP_CYTHON = os.environ.get("ENGRAPHIS_SKIP_CYTHON", "").strip() == "1"
+EXT_MODULES = []
+
+if not SKIP_CYTHON:
+    try:
+        from Cython.Build import cythonize
+    except ImportError:
+        cythonize = None
+
+    if cythonize is not None and not (
+        # pip install -e . does NOT run build_ext, so skip. Detect via
+        # SETUPTOOLS_ENABLE_FEATURES (setuptools >= 69) or legacy editable flag.
+        os.environ.get("SETUPTOOLS_ENABLE_FEATURES", "") == "legacy-editable"
+    ):
+        EXT_MODULES = cythonize(
+            [
+                Extension(
+                    "engraphis.licensing",
+                    ["engraphis/licensing.py"],
+                ),
+                Extension(
+                    "engraphis.cloud_license",
+                    ["engraphis/cloud_license.py"],
+                ),
+            ],
+            compiler_directives={
+                "language_level": "3",
+                "boundscheck": False,
+                "wraparound": False,
+            },
+            # Defer the dep files into the build dir so the engraphis/ source tree
+            # stays clean on every platform (Cython generates licensing.c etc.)
+            build_dir="build",
+        )
+
+
+class build_py(_build_py):
+    """Exclude licensing.py and cloud_license.py from the package when compiled
+    extensions exist — ship the .pyd/.so instead so the compiled version wins."""
+
+    def find_package_modules(self, package, package_dir):
+        modules = super().find_package_modules(package, package_dir)
+        if not EXT_MODULES:
+            return modules
+        return [
+            m for m in modules
+            if (package, m[1]) not in {
+                ("engraphis", "licensing"),
+                ("engraphis", "cloud_license"),
+            }
+        ]
+
+
+setup(
+    ext_modules=EXT_MODULES,
+    cmdclass={"build_py": build_py},
+)