diff --git a/poetry.lock b/poetry.lock
index a9ccef2..46b98fd 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -115,6 +115,18 @@ files = [
     {file = "iniconfig-2.1.0.tar.gz", hash = "sha256:3abbd2e30b36733fee78f9c7f7308f2d0050e88f0087fd25c2645f63c773e1c7"},
 ]
 
+[[package]]
+name = "legacy-cgi"
+version = "2.6.4"
+description = "Fork of the standard library cgi and cgitb modules removed in Python 3.13"
+optional = false
+python-versions = ">=3.8"
+groups = ["dev"]
+files = [
+    {file = "legacy_cgi-2.6.4-py3-none-any.whl", hash = "sha256:7e235ce58bf1e25d1fc9b2d299015e4e2cd37305eccafec1e6bac3fc04b878cd"},
+    {file = "legacy_cgi-2.6.4.tar.gz", hash = "sha256:abb9dfc7835772f7c9317977c63253fd22a7484b5c9bbcdca60a29dcce97c577"},
+]
+
 [[package]]
 name = "lxml"
 version = "6.1.1"
@@ -441,4 +453,4 @@ files = [
 [metadata]
 lock-version = "2.1"
 python-versions = ">=3.8,<4.0"
-content-hash = "d54bcac8b306449a805fd6f145f6d910dcbebbccc907b790b028c988c7ef1985"
+content-hash = "a7172bce7871b58ba975c81418c1477022b8cd6ce275dcca76bcedcb766764e4"
diff --git a/pyproject.toml b/pyproject.toml
index 36e6b98..33ceb62 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -41,6 +41,9 @@ apsw = ">=3.46"
 
 [tool.poetry.group.dev.dependencies]
 pytest = ">=8.0"
+# Backport of the stdlib `cgi` module (removed in Python 3.13) so erdr2pml and
+# other modules that still import it can be loaded and tested.
+legacy-cgi = ">=2.6"
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]
diff --git a/tests/test_erdr2pml.py b/tests/test_erdr2pml.py
index 984a452..3b6c4f8 100644
--- a/tests/test_erdr2pml.py
+++ b/tests/test_erdr2pml.py
@@ -1,8 +1,9 @@
-"""Tests for erdr2pml helpers.
+"""Tests for erdr2pml (eReader) helpers.
 
 erdr2pml imports the stdlib ``cgi`` module, which was removed in Python 3.13.
-On such interpreters the whole module is unimportable, so these tests are
-skipped (calibre currently ships Python 3.8-3.11, where they run).
+The ``legacy-cgi`` backport is a dev dependency so the module imports on modern
+interpreters; the skip guard below keeps the suite green if it is ever run
+without that backport (e.g. a bare Python 3.13+ with no dev extras).
 """
 
 import pytest
@@ -12,7 +13,7 @@ import dedrm_test_utils as U
 try:
     erdr2pml = U.load("erdr2pml", package="dedrm")
     _skip_reason = None
-except Exception as exc:  # pragma: no cover - depends on interpreter version
+except Exception as exc:  # pragma: no cover - depends on interpreter / deps
     erdr2pml = None
     _skip_reason = "erdr2pml not importable: {0}".format(exc)
 
@@ -27,7 +28,32 @@ def test_dexor_is_involutive():
     assert erdr2pml.deXOR(once, 0, table) == text
 
 
-def test_sanitize_filename_strips_separators():
-    name = erdr2pml.sanitizeFileName("a/b\\c")
-    assert "/" not in name
-    assert "\\" not in name
+def test_fix_key_known_vector():
+    assert erdr2pml.fixKey(bytes([0, 1, 2, 3, 255, 128, 64])) == bytes(
+        [0x80, 0x01, 0x02, 0x83, 0x7F, 0x80, 0x40]
+    )
+
+
+def test_fix_key_preserves_length():
+    assert len(erdr2pml.fixKey(b"\x00" * 16)) == 16
+
+
+def test_clean_pml_escapes_high_ascii():
+    # bytes 128..255 become \aNNN PML codes; bytes < 128 are left untouched.
+    assert erdr2pml.cleanPML(b"abc\x80\xff\x7f") == b"abc\\a128\\a255\x7f"
+
+
+def test_sanitize_filename_replaces_separators():
+    assert erdr2pml.sanitizeFileName("a/b\\c|d") == "a_b_c_d"
+
+
+def test_sanitize_filename_removes_colon():
+    assert ":" not in erdr2pml.sanitizeFileName("foo: bar")
+
+
+def test_sanitize_filename_strips_control_chars_and_dots():
+    assert erdr2pml.sanitizeFileName("...\x01na\tme.") == "name"
+
+
+def test_sanitize_filename_replaces_angle_brackets():
+    assert erdr2pml.sanitizeFileName("<tag>") == "[tag]"