diff --git a/masque/test/test_file_roundtrip.py b/masque/test/test_file_roundtrip.py
new file mode 100644
index 0000000..fbadd7b
--- /dev/null
+++ b/masque/test/test_file_roundtrip.py
@@ -0,0 +1,149 @@
+from pathlib import Path
+import pytest
+from numpy.testing import assert_allclose
+
+from ..pattern import Pattern
+from ..library import Library
+from ..file import gdsii, oasis
+from ..shapes import Path as MPath, Circle, Polygon
+from ..repetition import Grid, Arbitrary
+
+def create_test_library(for_gds: bool = False) -> Library:
+    lib = Library()
+
+    # 1. Polygons
+    pat_poly = Pattern()
+    pat_poly.polygon((1, 0), vertices=[[0, 0], [10, 0], [5, 10]])
+    lib["polygons"] = pat_poly
+
+    # 2. Paths with different endcaps
+    pat_paths = Pattern()
+    # Flush
+    pat_paths.path((2, 0), vertices=[[0, 0], [20, 0]], width=2, cap=MPath.Cap.Flush)
+    # Square
+    pat_paths.path((2, 1), vertices=[[0, 10], [20, 10]], width=2, cap=MPath.Cap.Square)
+    # Circle (Only for GDS)
+    if for_gds:
+        pat_paths.path((2, 2), vertices=[[0, 20], [20, 20]], width=2, cap=MPath.Cap.Circle)
+    # SquareCustom
+    pat_paths.path((2, 3), vertices=[[0, 30], [20, 30]], width=2, cap=MPath.Cap.SquareCustom, cap_extensions=(1, 5))
+    lib["paths"] = pat_paths
+
+    # 3. Circles (only for OASIS or polygonized for GDS)
+    pat_circles = Pattern()
+    if for_gds:
+        # GDS writer calls to_polygons() for non-supported shapes,
+        # but we can also pre-polygonize
+        pat_circles.shapes[(3, 0)].append(Circle(radius=5, offset=(10, 10)).to_polygons()[0])
+    else:
+        pat_circles.shapes[(3, 0)].append(Circle(radius=5, offset=(10, 10)))
+    lib["circles"] = pat_circles
+
+    # 4. Refs with repetitions
+    pat_refs = Pattern()
+    # Simple Ref
+    pat_refs.ref("polygons", offset=(0, 0))
+    # Ref with Grid repetition
+    pat_refs.ref("polygons", offset=(100, 0), repetition=Grid(a_vector=(20, 0), a_count=3, b_vector=(0, 20), b_count=2))
+    # Ref with Arbitrary repetition
+    pat_refs.ref("polygons", offset=(0, 100), repetition=Arbitrary(displacements=[[0, 0], [10, 20], [30, -10]]))
+    lib["refs"] = pat_refs
+
+    # 5. Shapes with repetitions (OASIS only, must be wrapped for GDS)
+    pat_rep_shapes = Pattern()
+    poly_rep = Polygon(vertices=[[0, 0], [5, 0], [5, 5], [0, 5]], repetition=Grid(a_vector=(10, 0), a_count=5))
+    pat_rep_shapes.shapes[(4, 0)].append(poly_rep)
+    lib["rep_shapes"] = pat_rep_shapes
+
+    if for_gds:
+        lib.wrap_repeated_shapes()
+
+    return lib
+
+def test_gdsii_full_roundtrip(tmp_path: Path) -> None:
+    lib = create_test_library(for_gds=True)
+    gds_file = tmp_path / "full_test.gds"
+    gdsii.writefile(lib, gds_file, meters_per_unit=1e-9)
+
+    read_lib, _ = gdsii.readfile(gds_file)
+
+    # Check existence
+    for name in lib:
+        assert name in read_lib
+
+    # Check Paths
+    read_paths = read_lib["paths"]
+    # Check caps (GDS stores them as path_type)
+    # Order might be different depending on how they were written,
+    # but here they should match the order they were added if dict order is preserved.
+    # Actually, they are grouped by layer.
+    p_flush = read_paths.shapes[(2, 0)][0]
+    assert p_flush.cap == MPath.Cap.Flush
+
+    p_square = read_paths.shapes[(2, 1)][0]
+    assert p_square.cap == MPath.Cap.Square
+
+    p_circle = read_paths.shapes[(2, 2)][0]
+    assert p_circle.cap == MPath.Cap.Circle
+
+    p_custom = read_paths.shapes[(2, 3)][0]
+    assert p_custom.cap == MPath.Cap.SquareCustom
+    assert_allclose(p_custom.cap_extensions, (1, 5))
+
+    # Check Refs with repetitions
+    read_refs = read_lib["refs"]
+    assert len(read_refs.refs["polygons"]) >= 3 # Simple, Grid (becomes 1 AREF), Arbitrary (becomes 3 SREFs)
+
+    # AREF check
+    arefs = [r for r in read_refs.refs["polygons"] if r.repetition is not None]
+    assert len(arefs) == 1
+    assert isinstance(arefs[0].repetition, Grid)
+    assert arefs[0].repetition.a_count == 3
+    assert arefs[0].repetition.b_count == 2
+
+    # Check wrapped shapes
+    # lib.wrap_repeated_shapes() created new patterns
+    # Original pattern "rep_shapes" now should have a Ref
+    assert len(read_lib["rep_shapes"].refs) > 0
+
+def test_oasis_full_roundtrip(tmp_path: Path) -> None:
+    pytest.importorskip("fatamorgana")
+    lib = create_test_library(for_gds=False)
+    oas_file = tmp_path / "full_test.oas"
+    oasis.writefile(lib, oas_file, units_per_micron=1000)
+
+    read_lib, _ = oasis.readfile(oas_file)
+
+    # Check existence
+    for name in lib:
+        assert name in read_lib
+
+    # Check Circle
+    read_circles = read_lib["circles"]
+    assert isinstance(read_circles.shapes[(3, 0)][0], Circle)
+    assert read_circles.shapes[(3, 0)][0].radius == 5
+
+    # Check Path caps
+    read_paths = read_lib["paths"]
+    assert read_paths.shapes[(2, 0)][0].cap == MPath.Cap.Flush
+    assert read_paths.shapes[(2, 1)][0].cap == MPath.Cap.Square
+    # OASIS HalfWidth is Square. masque's Square is also HalfWidth extension.
+    # Wait, Circle cap in OASIS?
+    # masque/file/oasis.py:
+    # path_cap_map = {
+    #     PathExtensionScheme.Flush: Path.Cap.Flush,
+    #     PathExtensionScheme.HalfWidth: Path.Cap.Square,
+    #     PathExtensionScheme.Arbitrary: Path.Cap.SquareCustom,
+    #     }
+    # It seems Circle cap is NOT supported in OASIS by masque currently.
+    # Let's verify what happens with Circle cap in OASIS write.
+    # _shapes_to_elements in oasis.py:
+    # path_type = next(k for k, v in path_cap_map.items() if v == shape.cap)
+    # This will raise StopIteration if Circle is not in path_cap_map.
+
+    # Check Shape repetition
+    read_rep_shapes = read_lib["rep_shapes"]
+    poly = read_rep_shapes.shapes[(4, 0)][0]
+    assert poly.repetition is not None
+    assert isinstance(poly.repetition, Grid)
+    assert poly.repetition.a_count == 5