diff --git a/pyproject.toml b/pyproject.toml
index f247c60..7b7167e 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -7,7 +7,10 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "mthread-draw"
-version = "1.2.0"
+# Read from mthread_draw.__version__ rather than written here as well. It was
+# written here as well, and 1.3.0 shipped a 1.2.0 wheel because a bump touched
+# the two __init__ files and not this line.
+dynamic = ["version"]
 description = "Fast synthetic touch input, gesture recording, replay and image drawing for Android over ADB"
 readme = "README.md"
 requires-python = ">=3.9"
@@ -54,6 +57,9 @@ Terms = "https://github.com/MAXAWER/MThread-Draw/blob/main/TERMS.md"
 [project.scripts]
 mthread = "mthread.cli:main"
 
+[tool.setuptools.dynamic]
+version = { attr = "mthread_draw.__version__" }
+
 [tool.setuptools]
 packages = ["mthread", "mthread_draw"]
 
diff --git a/tools/build_macos.py b/tools/build_macos.py
index fef7c76..826f813 100644
--- a/tools/build_macos.py
+++ b/tools/build_macos.py
@@ -27,6 +27,18 @@
 DIST = ROOT / "dist"
 PROJECT = ROOT / "macos"
 
+# Where build_app.py leaves the engine. Imported rather than repeated: the two
+# scripts disagreeing about it is what shipped an empty application.
+sys.path.insert(0, str(Path(__file__).resolve().parent))
+from build_app import STAGE  # noqa: E402
+
+
+def find_engine() -> Path | None:
+    for candidate in (STAGE / "mthread-draw-engine", DIST / "mthread-draw-engine"):
+        if candidate.is_dir():
+            return candidate
+    return None
+
 BUNDLE_ID = "io.github.maxawer.mthread-draw"
 
 
@@ -86,7 +98,7 @@ def make_icon() -> Path | None:
     return icns
 
 
-def assemble(binary: Path) -> Path:
+def assemble(binary: Path, allow_no_engine: bool = False) -> Path:
     app = DIST / "MThreadDraw.app"
     if app.exists():
         shutil.rmtree(app)
@@ -123,13 +135,19 @@ def assemble(binary: Path) -> Path:
         plist["CFBundleIconFile"] = "MThreadDraw"
     (contents / "Info.plist").write_bytes(plistlib.dumps(plist))
 
-    engine = DIST / "mthread-draw-engine"
-    if engine.is_dir():
+    engine = find_engine()
+    if engine is not None:
         shutil.copytree(engine, contents / "Resources" / "engine")
         print(f"  engine folded in from {engine}")
+    elif allow_no_engine:
+        print("  building without the engine, as asked: this bundle only runs "
+              "from a source checkout")
     else:
-        print("  no engine in dist/: build it first with tools/build_app.py, or the "
-              "app will look for a source checkout")
+        raise SystemExit(
+            "no engine found - run tools/build_app.py first. "
+            "Refusing rather than warning: a bundle without the engine looks "
+            "finished, weighs 300 KB, and does nothing on a machine that has no "
+            "checkout of this repository. Pass --no-engine if that is what you want.")
 
     size = sum(f.stat().st_size for f in app.rglob("*") if f.is_file())
     print(f"built {app} ({size // 1024 // 1024} MB)")
@@ -159,13 +177,15 @@ def main() -> int:
     parser = argparse.ArgumentParser(description=__doc__,
                                      formatter_class=argparse.RawDescriptionHelpFormatter)
     parser.add_argument("--dmg", action="store_true", help="also build the disk image")
+    parser.add_argument("--no-engine", action="store_true",
+                        help="build the window alone, for a source checkout")
     args = parser.parse_args()
 
     if sys.platform != "darwin":
         raise SystemExit("the macOS front end only builds on macOS")
 
     DIST.mkdir(exist_ok=True)
-    app = assemble(build_binary())
+    app = assemble(build_binary(), allow_no_engine=args.no_engine)
     if args.dmg:
         build_dmg(app)
     return 0