diff --git a/crates/clawhdf5-py/tests/test_edit.py b/crates/clawhdf5-py/tests/test_edit.py
index c0b7ba6..ac28607 100644
--- a/crates/clawhdf5-py/tests/test_edit.py
+++ b/crates/clawhdf5-py/tests/test_edit.py
@@ -896,3 +896,38 @@ def test_path_replaced_between_edits(h5py, tmp_path):
     from pathlib import Path
     _held_file_edited(h5py, moved, Path(path), other_bytes)
 
+
+def test_an_edit_releases_the_gil(h5py, tmp_path):
+    """Another Python thread keeps running while a large edit is written:
+    had the edit held the GIL, the other thread would stall for the whole
+    edit (Rust code never yields it)."""
+    import time
+    path = str(tmp_path / "big.h5")
+    with h5py.File(path, "w") as f:
+        f.create_dataset("d", shape=(1024, 1024), dtype="<f8", chunks=(64, 64), compression="gzip")
+    value = np.random.default_rng(0).standard_normal((1024, 1024))
+    stamps = []
+    stop = threading.Event()
+
+    def spin():
+        while not stop.is_set():
+            stamps.append(time.perf_counter())
+
+    with clawhdf5.File(path, "r+") as f:
+        ds = f["d"]
+        t = threading.Thread(target=spin)
+        t.start()
+        try:
+            time.sleep(0.05)
+            t0 = time.perf_counter()
+            ds[...] = value
+            t1 = time.perf_counter()
+        finally:
+            stop.set()
+            t.join()
+        np.testing.assert_array_equal(ds[...], value)
+    during = [x for x in stamps if t0 <= x <= t1]
+    gaps = np.diff([t0] + during + [t1])
+    assert t1 - t0 > 0.1, "the edit is too quick to tell"
+    assert gaps.max() < 0.5 * (t1 - t0), (
+        f"the other thread stalled for {gaps.max():.3f} s of a {t1 - t0:.3f} s edit")