diff --git a/source_modelling/gsf.py b/source_modelling/gsf.py
index 7dcd260..ec44ed7 100644
--- a/source_modelling/gsf.py
+++ b/source_modelling/gsf.py
@@ -112,12 +112,15 @@ def write_gsf(gsf_df: pd.DataFrame, gsf_filepath: Path):
         The path to the GSF file to write.
     """
 
+    if "loc_rake" not in gsf_df:
+        raise ValueError("The DataFrame must have a 'loc_rake' column.")
+    # Defaults are filled on a copy: write_gsf serialises the frame it is
+    # given and must not modify the caller's DataFrame.
+    gsf_df = gsf_df.copy(deep=False)
     if "init_time" not in gsf_df:
         gsf_df["init_time"] = -1
     if "slip" not in gsf_df:
         gsf_df["slip"] = -1
-    if "loc_rake" not in gsf_df:
-        raise ValueError("The DataFrame must have a 'loc_rake' column.")
     with open(gsf_filepath, "w") as gsf_file:
         gsf_file.write(f"{len(gsf_df)}\n")
         gsf_df.to_csv(
diff --git a/tests/test_gsf.py b/tests/test_gsf.py
index 864baa9..ad71091 100644
--- a/tests/test_gsf.py
+++ b/tests/test_gsf.py
@@ -359,3 +359,39 @@ def test_read_gsf(tmp_path: Path):
             ]
         ),
     )
+
+
+def test_write_gsf_does_not_mutate_input(tmp_path: Path):
+    """write_gsf must serialise its input without modifying it.
+
+    Regression test: the ``init_time`` and ``slip`` defaults were assigned
+    directly into the caller's DataFrame, so a frame handed to ``write_gsf``
+    came back carrying two columns of ``-1`` sentinels. The assignments also
+    sat above the ``loc_rake`` validation, so the mutation happened even when
+    ``write_gsf`` went on to raise and write nothing.
+    """
+    gsf_df = pd.DataFrame(
+        {
+            "lon": [172.6],
+            "lat": [-43.5],
+            "dep": [1.0],
+            "sub_dx": [1.0],
+            "sub_dy": [1.0],
+            "loc_stk": [0.0],
+            "loc_dip": [0.0],
+            "loc_rake": [0.0],
+            "seg_no": [0],
+        }
+    )
+    expected_columns = list(gsf_df.columns)
+
+    gsf.write_gsf(gsf_df, tmp_path / "out.gsf")
+
+    assert list(gsf_df.columns) == expected_columns
+
+    # the failure path must not mutate either
+    missing_rake = gsf_df.drop(columns=["loc_rake"])
+    expected_columns = list(missing_rake.columns)
+    with pytest.raises(ValueError, match="loc_rake"):
+        gsf.write_gsf(missing_rake, tmp_path / "out2.gsf")
+    assert list(missing_rake.columns) == expected_columns