diff --git a/backends/xdata_specs.py b/backends/xdata_specs.py
index a0e3dd1..c6b3240 100644
--- a/backends/xdata_specs.py
+++ b/backends/xdata_specs.py
@@ -3,7 +3,11 @@
 DXF XDATA is a list of (group code, value) pairs under a registered APPID.
 This module is the one place that decides how a JSON value becomes a group
 code and enforces the two limits AutoCAD enforces silently: 255 characters
-per string and 16 KB per entity.
+per string and 16 KB per entity. It also refuses the one thing neither engine
+can store: a line separator inside a string. DXF is a text format of
+alternating "group code" / "value" lines, and ezdxf writes a 1000 tag verbatim,
+so a LF or CR in the value splits the tag pair and the saved file -- and every
+undo/transaction snapshot, which is also a DXF save -- becomes unreadable.
 """
 
 from __future__ import annotations
@@ -15,6 +19,7 @@ RESERVED_APPS = {"ACAD"}
 MAX_STRING = 255
 MAX_BYTES = 16 * 1024
 INT_MIN, INT_MAX = -(2**31), 2**31 - 1
+LINE_SEPARATORS = ("\n", "\r")
 
 
 def validate_app_name(app) -> str:
@@ -40,6 +45,11 @@ def encode_values(values) -> list[tuple[int, object]]:
                 raise ValueError(
                     f"xdata: {where} is {len(value)} characters; the limit is {MAX_STRING}"
                 )
+            if any(sep in value for sep in LINE_SEPARATORS):
+                raise ValueError(
+                    f"xdata: {where} contains a newline; "
+                    "line separators are not allowed in XDATA strings"
+                )
             tags.append((1000, value))
             size += len(value.encode("utf-8")) + 2
         elif isinstance(value, int):
diff --git a/tests/test_xdata.py b/tests/test_xdata.py
index e78d195..e60cee6 100644
--- a/tests/test_xdata.py
+++ b/tests/test_xdata.py
@@ -48,6 +48,23 @@ def test_string_over_255_is_refused():
         encode_values(["x" * (MAX_STRING + 1)])
 
 
+@pytest.mark.parametrize("sep", ["\n", "\r", "\r\n"])
+def test_line_separators_in_strings_are_refused(sep):
+    # DXF is "code\nvalue\n" lines and ezdxf writes 1000 tags verbatim, so a
+    # separator inside the value splits the tag pair and corrupts the file.
+    with pytest.raises(ValueError, match=r"values\[1\].*line separators are not allowed"):
+        encode_values(["fine", f"Inspect weld{sep}before paint"])
+
+
+def test_other_control_characters_are_still_allowed():
+    # Only LF/CR split a DXF tag; tabs and the exotic separators round-trip.
+    assert encode_values(["a\tb", "a\x0bb", "a
b"]) == [
+        (1000, "a\tb"),
+        (1000, "a\x0bb"),
+        (1000, "a
b"),
+    ]
+
+
 def test_total_size_over_16k_is_refused():
     with pytest.raises(ValueError, match="16"):
         encode_values(["x" * 255] * (MAX_BYTES // 255 + 1))
@@ -108,6 +125,32 @@ async def test_limits_are_enforced_before_writing(backend):
     assert (await backend.entity_get_xdata(line.handle))["xdata"] == {}
 
 
+async def test_newline_is_refused_before_writing_and_snapshots_still_load(backend, tmp_path):
+    # Every undo/transaction snapshot is a DXF save, so an accepted newline
+    # would break rollback, not just the file on disk.
+    line = await backend.entity_create_line(0, 0, 1, 1)
+    with pytest.raises(ValueError, match="line separators are not allowed"):
+        await backend.entity_set_xdata(line.handle, "NOTES", ["Inspect weld\nbefore paint"])
+    assert (await backend.entity_get_xdata(line.handle))["xdata"] == {}
+
+    await backend.entity_set_xdata(line.handle, "NOTES", ["Inspect weld before paint"])
+    await backend.transaction_begin()
+    circle = await backend.entity_create_circle(5, 5, 1)
+    await backend.transaction_rollback()
+    assert (await backend.entity_get_xdata(line.handle, "NOTES"))["xdata"] == {
+        "NOTES": ["Inspect weld before paint"]
+    }
+    with pytest.raises(RuntimeError, match="not found"):
+        await backend.entity_get(circle.handle)
+
+    path = str(tmp_path / "notes.dxf")
+    await backend.drawing_save(path)
+    await backend.drawing_open(path)
+    assert (await backend.entity_get_xdata(line.handle, "NOTES"))["xdata"] == {
+        "NOTES": ["Inspect weld before paint"]
+    }
+
+
 # ── live engine, fake ActiveX ────────────────────────────────────────────────
 
 
@@ -170,3 +213,10 @@ async def test_com_empty_values_write_only_the_marker(com_backend):
     result = await backend.entity_set_xdata("2F", "ACADMCP_PID", [])
     assert result["removed"] is True
     assert entity.stored == ([1001], ["ACADMCP_PID"])
+
+
+async def test_com_newline_is_refused_before_any_activex_call(com_backend):
+    backend, entity, registered = com_backend
+    with pytest.raises(ValueError, match="line separators are not allowed"):
+        await backend.entity_set_xdata("2F", "NOTES", ["a\r\nb"])
+    assert entity.stored is None and registered == []