diff --git a/exact_dependencies.py b/exact_dependencies.py
new file mode 100644
index 0000000..4abcde1
--- /dev/null
+++ b/exact_dependencies.py
@@ -0,0 +1,24 @@
+#! /usr/bin/env python
+'''
+Load and save exact-dependencies.json safely.
+
+The format of exact-dependencies is simply a dictionary of
+package name and its exact version:
+
+    {
+        "elm-lang/core": "4.0.5"
+    }
+'''
+
+import json
+
+import elm_package
+
+
+load = elm_package.load
+
+
+# dump(package: Dict, fileobj: FileLike) -> None
+def dump(package, fileobj):
+    to_save = elm_package.sorted_deps(package)
+    json.dump(to_save, fileobj, sort_keys=False, indent=4, separators=(',', ': '))
diff --git a/native_deps_sync.py b/native_deps_sync.py
new file mode 100755
index 0000000..ed74cdf
--- /dev/null
+++ b/native_deps_sync.py
@@ -0,0 +1,54 @@
+#! /usr/bin/env python
+from __future__ import print_function
+
+import sys
+import argparse
+
+import elm_package
+import exact_dependencies
+
+
+def sync_versions(top_level_file, spec_file, quiet=False, dry=False, note_test_deps=True):
+    """ first file should be the top level elm-native-package.json.
+        second file should be the spec level elm-native-package.json.
+    """
+
+    with open(top_level_file) as f:
+        top_level = exact_dependencies.load(f)
+
+    with open(spec_file) as f:
+        spec = exact_dependencies.load(f)
+
+    (messages, new_deps) = elm_package.sync_deps(top_level, spec)
+    spec = new_deps
+
+    if len(messages) > 0:
+        print('{number} packages changed.'.format(number=len(messages)))
+
+        if not dry:
+            with open(spec_file, 'w') as f:
+                exact_dependencies.dump(spec, f)
+        else:
+            print("No changes written.")
+
+        if not quiet:
+            print('\n'.join(messages))
+    else:
+        print('No changes needed.')
+
+
+def main():
+
+    parser = argparse.ArgumentParser(description='Sync deps between a parent and a sub')
+
+    parser.add_argument('--quiet', '-q', action='store_true', help='don\'t print anything', default=False)
+    parser.add_argument('--dry', '-d', action='store_true', help='only print possible changes', default=False)
+    parser.add_argument('top_level_file')
+    parser.add_argument('spec_file')
+    args = parser.parse_args()
+
+    sync_versions(args.top_level_file, args.spec_file, quiet=args.quiet, dry=args.dry)
+
+
+if __name__ == '__main__':
+    main()
diff --git a/tests/test_native_deps_sync.py b/tests/test_native_deps_sync.py
new file mode 100644
index 0000000..9c97235
--- /dev/null
+++ b/tests/test_native_deps_sync.py
@@ -0,0 +1,46 @@
+from collections import OrderedDict
+import json
+
+from hypothesis import given
+import hypothesis.strategies as st
+
+import native_deps_sync
+
+
+top_level_deps = [
+    ('NoRedInk/top-1', '1.0.0 <= v <= 1.0.0'),
+    ('NoRedInk/top-2', '1.0.0 <= v <= 1.0.0'),
+    ('NoRedInk/top-3', '1.0.0 <= v <= 1.0.0'),
+]
+
+spec_deps = [
+    ('NoRedInk/top-1', '1.0.0 <= v <= 1.0.0'),
+    ('NoRedInk/top-2', '1.0.0 <= v <= 1.0.0'),
+    ('NoRedInk/spec-1', '1.0.0 <= v <= 1.0.0'),
+    ('NoRedInk/spec-2', '1.0.0 <= v <= 1.0.0'),
+]
+
+
+@given(top_level_deps=st.permutations(top_level_deps),
+       spec_deps=st.permutations(spec_deps))
+def test_spec_order_is_preserved(
+        tmpdir,
+        top_level_deps,
+        spec_deps):
+    top_level_file = tmpdir.join('elm-native-package.json')
+    spec_file = tmpdir.join('spec-elm-native-package.json')
+
+    top_level = OrderedDict(top_level_deps)
+    top_level_file.write(json.dumps(top_level))
+
+    spec = OrderedDict(spec_deps)
+    spec_file.write(json.dumps(spec))
+
+    native_deps_sync.sync_versions(
+        str(top_level_file),
+        str(spec_file),
+        quiet=False,
+        dry=False)
+
+    new_spec = json.loads(spec_file.read(), object_pairs_hook=OrderedDict)
+    assert list(new_spec.keys()) == ['NoRedInk/spec-1', 'NoRedInk/spec-2', 'NoRedInk/top-1', 'NoRedInk/top-2', 'NoRedInk/top-3']