fix(task-16): custom keys mirror AutoCAD's full AddCustomInfo rule, deletes are exempt, keys match case-insensitively

The 2b9791f fix mirrored only the characters the first review happened to
measure ('=' and ';'), so `custom={'AAA': 'first', 'A:B': 'x', 'ZZZ': 'never'}`
still raised from AddCustomInfo('A:B') on the live engine after subject and
AAA were applied -- the same half-write for 11 other keys. Re-measured on
AutoCAD 2026 by sweeping every character 0x20-0x7E through a scratch
document: exactly thirteen printable characters are 'Invalid key' anywhere
in a key (" * , / : ; < = > ? \ ` |); control characters, internal spaces,
a no-break space and unicode are accepted; leading/trailing whitespace
(space, tab, NBSP) is refused. `_CUSTOM_KEY_FORBIDDEN` is now that set,
the internal-NBSP refusal (wrong: measured accepted) is gone, and the
validator / tool docstrings state the measured rule instead of a closed
enumeration that was false.

A delete (`{key: None}`) is exempt from the AddCustomInfo syntax rules:
RemoveCustomByKey never validates syntax (measured: 'Key not found', never
'Invalid key'), and 2b9791f ran the check before the None branch, so a key
an ezdxf script wrote ('A=B', ' PAD ') showed in drawing_properties_get and
could never be removed. The line-break rule still applies both ways.

Also measured: custom keys are case-insensitive in AutoCAD (AddCustomInfo
over an existing case-variant is 'Duplicate key'; SetCustomByKey and
RemoveCustomByKey match either spelling and keep the stored one). The COM
routing was case-sensitive, so `{'PROJECT': ...}` over 'Project' went to
AddCustomInfo and failed mid-write -- the same class. Both engines now
match existing keys casefolded (update in place under the stored spelling,
never a second tag), and a request naming two case-variants is refused.

Tests: parametrized validator sweep over all thirteen characters, the
accepted set (control chars, NBSP, unicode, the other printables), the
delete exemption headlessly on a DXF carrying such keys and on the fake
SummaryInfo, case-variant refusal, and case-insensitive routing against a
fake that raises 'Duplicate key' / 'Key not found' like AutoCAD. Confirmed
live through the real ComBackend on a scratch document (Close(False)).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

backends/com_backend.py

@@ -4293,22 +4293,29 @@ class ComBackend(AutoCADBackend):
 
         def _sync():
             info = _acad_doc().SummaryInfo
+            # AutoCAD's custom keys are case-insensitive (measured on 2026:
+            # AddCustomInfo("PROJECT") over an existing "Project" raises
+            # 'Duplicate key'; SetCustomByKey / RemoveCustomByKey match either
+            # spelling and the stored spelling is kept), so existence is
+            # checked casefolded — a case-sensitive check routed "PROJECT" to
+            # AddCustomInfo and failed mid-write after the summary fields and
+            # the earlier keys were already applied.
             existing: set[str] = set()
             for index in range(int(info.NumCustomInfo())):
                 key, _value = info.GetCustomByIndex(index)
-                existing.add(str(key))
+                existing.add(str(key).casefold())
             for field, value in written.items():
                 setattr(info, _SUMMARY_ATTRS[field], value)
             for key, value in to_write.items():
                 # AddCustomInfo on an existing key raises in AutoCAD; SetCustomByKey
                 # on a missing one does too — route by what is really there.
-                if key in existing:
+                if key.casefold() in existing:
                     info.SetCustomByKey(key, value)
                 else:
                     info.AddCustomInfo(key, value)
             deleted = []
             for key in to_delete:
-                if key in existing:
+                if key.casefold() in existing:
                     info.RemoveCustomByKey(key)
                     deleted.append(key)
             return {

backends/contracts/settings.py

@@ -515,10 +515,14 @@ def _decode_setting(key: str, kind: str, raw: Any) -> Any:
 SUMMARY_FIELDS = ("title", "subject", "author", "keywords", "comments")
 
 
-#: Characters AutoCAD's ``SummaryInfo.AddCustomInfo`` rejects inside a custom
-#: key with ``Invalid key`` (measured on AutoCAD 2026; internal spaces, tabs
-#: and other unicode are accepted). ``\xa0`` is the no-break space.
-_CUSTOM_KEY_FORBIDDEN = {"=": "'='", ";": "';'", "\xa0": "a no-break space (U+00A0)"}
+#: The printable characters AutoCAD's ``SummaryInfo.AddCustomInfo`` rejects
+#: anywhere inside a custom key with ``Invalid key``. Measured on AutoCAD 2026
+#: by sweeping every character 0x20-0x7E through ``AddCustomInfo`` on a scratch
+#: document: exactly these thirteen are refused; every other printable ASCII
+#: character, every control character (0x01-0x1F, including tab), internal
+#: spaces, a no-break space and other unicode are accepted. Leading or
+#: trailing whitespace (space, tab or no-break space) is refused separately.
+_CUSTOM_KEY_FORBIDDEN = frozenset('"*,/:;<=>?\\`|')
 
 
 def _check_custom_text(key: str, text: str, what: str) -> None:
@@ -538,24 +542,30 @@ def _check_custom_text(key: str, text: str, what: str) -> None:
         )
 
 
-def _check_custom_key(key: str) -> None:
-    """Mirror AutoCAD's own custom-key rules so the live engine cannot refuse
-    mid-write. Measured: ``AddCustomInfo`` raises ``Invalid key`` for a key
-    with leading/trailing whitespace and for ``=``, ``;`` or a no-break space
-    anywhere in it — after the summary fields and earlier keys were already
-    applied — while the headless engine happily writes the same key."""
+def _check_custom_key_for_write(key: str) -> None:
+    """Mirror AutoCAD's ``AddCustomInfo`` key syntax so the live engine cannot
+    refuse mid-write. Measured on AutoCAD 2026: ``Invalid key`` for a key with
+    leading/trailing whitespace (space, tab, no-break space) or any of
+    ``_CUSTOM_KEY_FORBIDDEN`` anywhere in it — raised after the summary fields
+    and the earlier keys were already applied, while the headless engine
+    writes the same key without complaint. Only a *write* goes through
+    ``AddCustomInfo``; ``RemoveCustomByKey`` does not validate syntax (measured:
+    ``Key not found``, never ``Invalid key``), so a delete is not held to these
+    rules — a key that reached the drawing by other means (a DXF written by
+    ezdxf, say) must stay removable."""
     if key != key.strip():
         raise ValueError(
             f"custom: key {key!r} has leading or trailing whitespace — AutoCAD's "
             "AddCustomInfo rejects it as 'Invalid key'. Strip the key."
         )
-    for char, name in _CUSTOM_KEY_FORBIDDEN.items():
-        if char in key:
-            raise ValueError(
-                f"custom: key {key!r} contains {name} — AutoCAD's AddCustomInfo rejects "
-                "it as 'Invalid key'. Use another separator."
-            )
-    _check_custom_text(key, key, "key")
+    bad = sorted(set(key) & _CUSTOM_KEY_FORBIDDEN)
+    if bad:
+        shown = " ".join(repr(c) for c in bad)
+        raise ValueError(
+            f"custom: key {key!r} contains {shown} — AutoCAD's AddCustomInfo rejects "
+            f"it as 'Invalid key' (refused characters: {''.join(sorted(_CUSTOM_KEY_FORBIDDEN))}). "
+            "Use another separator."
+        )
 
 
 def validate_drawing_properties(
@@ -574,11 +584,20 @@ def validate_drawing_properties(
     enforce or the file format would break on, so the same call cannot succeed
     on one engine and half-apply on the other (``ValueError`` naming the key):
 
-    * a key with leading or trailing whitespace, or containing ``=``, ``;`` or
-      a no-break space — AutoCAD's ``AddCustomInfo`` rejects these as
-      ``Invalid key`` mid-write (measured on AutoCAD 2026), after the summary
-      fields and the earlier keys were already applied; the headless engine
-      writes them without complaint, and AutoCAD then loads them from the DXF;
+    * a key to *write* with leading or trailing whitespace, or containing any
+      of the thirteen characters ``" * , / : ; < = > ? \\ ` |`` — AutoCAD's
+      ``AddCustomInfo`` rejects exactly these as ``Invalid key`` mid-write
+      (measured on AutoCAD 2026 over every printable ASCII character), after
+      the summary fields and the earlier keys were already applied; the
+      headless engine writes them without complaint, and AutoCAD then loads
+      them from the DXF. A *delete* (``None``) is exempt: ``RemoveCustomByKey``
+      never validates syntax, and such keys do reach drawings by other routes,
+      so they must stay removable;
+    * two keys in one request that differ only by case — AutoCAD's custom keys
+      are case-insensitive (measured: ``AddCustomInfo("PROJECT")`` over an
+      existing ``Project`` raises ``Duplicate key``; ``SetCustomByKey`` /
+      ``RemoveCustomByKey`` match either spelling), so the second would fail
+      mid-write on the live engine and write a second tag headlessly;
     * a key or value containing a line break (LF or CR) — ezdxf emits custom
       properties unescaped, one header line per text, so the saved DXF is
       corrupt and the whole drawing fails to reopen (``DXFStructureError``),
@@ -599,13 +618,23 @@ def validate_drawing_properties(
         raise TypeError("custom: must be an object of {key: text | null}")
     to_write: dict[str, str] = {}
     to_delete: list[str] = []
+    seen: dict[str, str] = {}  # casefolded key -> the spelling seen first
     for key, value in (custom or {}).items():
         if not isinstance(key, str) or not key.strip():
             raise TypeError(f"custom: key {key!r} must be a non-empty string")
-        _check_custom_key(key)
+        _check_custom_text(key, key, "key")
+        folded = key.casefold()
+        if folded in seen:
+            raise ValueError(
+                f"custom: keys {seen[folded]!r} and {key!r} differ only by case — "
+                "AutoCAD's custom keys are case-insensitive (AddCustomInfo raises "
+                "'Duplicate key'). Mention the key once."
+            )
+        seen[folded] = key
         if value is None:
             to_delete.append(key)
         elif isinstance(value, str):
+            _check_custom_key_for_write(key)
             _check_custom_text(key, value, "value")
             to_write[key] = value
         else:

backends/ezdxf_backend.py

@@ -6142,15 +6142,26 @@ class EzdxfBackend(AutoCADBackend):
                     "Save as R2004 or newer first."
                 )
             custom_vars = doc.header.custom_vars
+            # AutoCAD's custom keys are case-insensitive (measured on 2026:
+            # SetCustomByKey("PROJECT") updates an existing "Project" and keeps
+            # that spelling; a second AddCustomInfo is 'Duplicate key'), and
+            # ezdxf's CustomVars are plain case-sensitive tuples, so the match
+            # is done here: an existing tag is updated in place under its
+            # stored spelling, never duplicated under a second one.
+            props = custom_vars.properties
             for key, value in to_write.items():
-                if custom_vars.has_tag(key):
-                    custom_vars.replace(key, value)
+                folded = key.casefold()
+                hits = [i for i, (tag, _v) in enumerate(props) if tag.casefold() == folded]
+                if hits:
+                    props[hits[0]] = (props[hits[0]][0], value)
                 else:
                     custom_vars.append(key, value)
             deleted = []
             for key in to_delete:
-                if custom_vars.has_tag(key):
-                    custom_vars.remove(key, all=True)
+                folded = key.casefold()
+                kept = [(tag, v) for tag, v in props if tag.casefold() != folded]
+                if len(kept) != len(props):
+                    props[:] = kept
                     deleted.append(key)
             if to_write or deleted:
                 self._mark_dirty()

server.py

@@ -6962,13 +6962,20 @@ async def drawing_properties_set(
     engine (`capability: dwgprops` — SummaryInfo lives in the DWG, a DXF has
     no slot for it; custom properties still work headlessly), a non-string
     value (`TypeError` naming the field or key — nothing is coerced with
-    `str()`), an empty custom key, a custom key AutoCAD's AddCustomInfo would
-    reject mid-write (leading/trailing whitespace, `=`, `;`, no-break space —
-    `ValueError` naming the key, on both engines), a line break in a custom
-    key or value (it corrupts the DXF on save), and headlessly a document
-    older than R2004 (`ValueError` — ezdxf only writes the custom-property
-    header pairs for AC1018+, so they would vanish at save; save as R2004 or
-    newer first). On the live engine a custom key is added with AddCustomInfo
+    `str()`), an empty custom key, a custom key to *write* that AutoCAD's
+    AddCustomInfo would reject mid-write (measured on AutoCAD 2026: leading or
+    trailing whitespace, or any of the thirteen characters
+    `" * , / : ; < = > ? \\ ` |` anywhere in the key; internal spaces, tabs
+    and unicode are fine — `ValueError` naming the key, on both engines; a
+    delete is exempt because RemoveCustomByKey never validates syntax, so a
+    key that reached the drawing another way stays removable), two keys in
+    one request that differ only by case (custom keys are case-insensitive in
+    AutoCAD), a line break in a custom key or value (it corrupts the DXF on
+    save), and headlessly a document older than R2004 (`ValueError` — ezdxf
+    only writes the custom-property header pairs for AC1018+, so they would
+    vanish at save; save as R2004 or newer first). A key is matched to the
+    drawing case-insensitively on both engines and keeps its stored spelling
+    when updated. On the live engine a custom key is added with AddCustomInfo
     when new and changed with SetCustomByKey when present.
     """
     summary = {

tests/test_drawing_properties.py

@@ -10,9 +10,14 @@ instead of losing the write at save).
 
 Custom keys and values are validated once, by name, against what would break
 *either* engine: keys AutoCAD's AddCustomInfo rejects mid-write as
-`Invalid key` (measured on AutoCAD 2026: leading/trailing whitespace, `=`,
-`;`, no-break space) and line breaks, which ezdxf writes unescaped and
-which corrupt the whole DXF.
+`Invalid key` (measured on AutoCAD 2026 by sweeping every printable ASCII
+character through a scratch document: leading/trailing whitespace and exactly
+the thirteen characters `" * , / : ; < = > ? \\ ` |` anywhere in the key --
+internal spaces, tabs, control characters, a no-break space and unicode are
+accepted), case-variant duplicates (AutoCAD's keys are case-insensitive:
+`Duplicate key`), and line breaks, which ezdxf writes unescaped and which
+corrupt the whole DXF. A delete is exempt from the AddCustomInfo syntax rules
+(RemoveCustomByKey never validates: `Key not found`, never `Invalid key`).
 """
 
 from __future__ import annotations
@@ -22,7 +27,11 @@ import types
 import pytest
 
 from backends.base import UnsupportedCapabilityError
-from backends.contracts.settings import SUMMARY_FIELDS, validate_drawing_properties
+from backends.contracts.settings import (
+    _CUSTOM_KEY_FORBIDDEN,
+    SUMMARY_FIELDS,
+    validate_drawing_properties,
+)
 
 pytestmark = pytest.mark.asyncio
 
@@ -55,9 +64,15 @@ def test_validation_splits_writes_and_deletes():
         # mid-write after the summary and earlier keys were already applied.
         (None, {" BAD ": "x"}, ValueError, "' BAD '"),
         (None, {"PROJECT ": "x"}, ValueError, "trailing whitespace"),
+        (None, {"\tTAB": "x"}, ValueError, "leading or trailing whitespace"),
+        (None, {"NB\xa0": "x"}, ValueError, "leading or trailing whitespace"),
         (None, {"A=B": "x"}, ValueError, "'A=B' contains '='"),
         (None, {"A;B": "x"}, ValueError, "'A;B' contains ';'"),
-        (None, {"NB\xa0SP": "x"}, ValueError, "no-break space"),
+        (None, {"A:B/C": "x"}, ValueError, "'A:B/C' contains '/' ':'"),
+        # Case-variant duplicates: AutoCAD raises 'Duplicate key' on the second.
+        (None, {"Project": "a", "PROJECT": "b"}, ValueError, "differ only by case"),
+        (None, {"Project": "a", "PROJECT": None}, ValueError, "differ only by case"),
+        (None, {"Gr\u00fcn": "a", "GR\u00dcN": "b"}, ValueError, "differ only by case"),
         # A line break is written unescaped by ezdxf and corrupts the DXF.
         (None, {"NOTE": "line1\nline2"}, ValueError, "custom['NOTE']: the value"),
         (None, {"NOTE": "line1\rline2"}, ValueError, "line break"),
@@ -71,13 +86,56 @@ def test_validation_refuses_by_name(summary, custom, exc, fragment):
     assert fragment in str(excinfo.value)
 
 
-@pytest.mark.parametrize("key", ["IN SP", "T\tAB", "\u00dcn\u00efcode", "X" * 300])
+INVALID_KEY_CHARS = '"*,/:;<=>?\\`|'
+
+
+def test_forbidden_set_is_exactly_the_thirteen_measured_characters():
+    assert _CUSTOM_KEY_FORBIDDEN == frozenset(INVALID_KEY_CHARS)
+    assert len(_CUSTOM_KEY_FORBIDDEN) == 13
+
+
+@pytest.mark.parametrize("char", sorted(INVALID_KEY_CHARS))
+def test_validation_refuses_every_addcustominfo_invalid_key_character(char):
+    """Measured on AutoCAD 2026 (`AddCustomInfo(f"A{c}B", "v")` for every
+    c in 0x20-0x7E on a scratch document): exactly these thirteen raise
+    `Invalid key`. The earlier fix mirrored only `=` and `;`, so 'A:B',
+    'REV/2', 'SIZE, mm', 'A"B' ... still half-applied on the live engine."""
+    key = f"A{char}B"
+    with pytest.raises(ValueError, match="Invalid key") as excinfo:
+        validate_drawing_properties(None, {key: "x"})
+    assert repr(key) in str(excinfo.value) and repr(char) in str(excinfo.value)
+
+
+@pytest.mark.parametrize(
+    "key",
+    [
+        "IN SP",
+        "T\tAB",
+        "\u00dcn\u00efcode",
+        "X" * 300,
+        "NB\xa0SP",
+        "A\x01B",
+        "A\x1fB",
+        "A!#$%&'()+-.@[]^_{}~B",
+        "\u4e2d\u2014\u200b",
+    ],
+)
 def test_validation_accepts_keys_autocad_accepts(key):
-    """Measured: internal spaces, tabs, unicode and 300-char keys pass
+    """Measured: internal spaces, tabs, control characters, a no-break space,
+    unicode, every other printable ASCII character and 300-char keys pass
     AddCustomInfo; the validator must not be stricter than AutoCAD."""
     assert validate_drawing_properties(None, {key: "v"}) == ({}, {key: "v"}, [])
 
 
+@pytest.mark.parametrize("key", ["A=B", " PAD ", "A:B", "NB\xa0", 'A"B'])
+def test_validation_exempts_a_delete_from_the_addcustominfo_syntax_rules(key):
+    """A delete never calls AddCustomInfo, and RemoveCustomByKey does not
+    validate syntax (measured: 'Key not found', never 'Invalid key'), so a key
+    a DXF script wrote must stay removable. The line-break rule still holds
+    for both directions (a delete of such a key is pinned refused above)."""
+    assert validate_drawing_properties(None, {key: None}) == ({}, {}, [key])
+
+
 # ── headless engine ─────────────────────────────────────────────────────────
 
 
@@ -137,6 +195,60 @@ async def test_autocad_invalid_keys_are_refused_headlessly_too(backend):
     assert (await backend.drawing_properties_get())["custom"] == {}
 
 
+async def test_legacy_invalid_syntax_keys_stay_deletable_headlessly(tmp_path):
+    """Regression of 2b9791f: `_check_custom_key` ran before the `value is
+    None` branch, so `{'A=B': None}` on a document carrying 'A=B' (any ezdxf
+    script writes such keys; AutoCAD loads them from the DXF) was refused with
+    a reason that only applies to AddCustomInfo -- the tool showed a key it
+    could never remove."""
+    import ezdxf
+
+    from backends.ezdxf_backend import EzdxfBackend
+
+    path = tmp_path / "legacy.dxf"
+    doc = ezdxf.new("R2010")
+    for tag in ("A=B", "NB\xa0SP", " PAD ", "A:B"):
+        doc.header.custom_vars.append(tag, "legacy")
+    doc.saveas(str(path))
+    backend = EzdxfBackend()
+    await backend.connect()
+    try:
+        await backend.drawing_open(str(path))
+        assert (await backend.drawing_properties_get())["custom"] == {
+            "A=B": "legacy",
+            "NB\xa0SP": "legacy",
+            " PAD ": "legacy",
+            "A:B": "legacy",
+        }
+        res = await backend.drawing_properties_set(custom={"A=B": None, " PAD ": None})
+        assert res["custom_deleted"] == ["A=B", " PAD "]
+        assert (await backend.drawing_properties_get())["custom"] == {
+            "NB\xa0SP": "legacy",
+            "A:B": "legacy",
+        }
+        # ...but writing such a key is still refused, before anything is touched.
+        with pytest.raises(ValueError, match="'A:B' contains ':'"):
+            await backend.drawing_properties_set(custom={"A:B": "new", "OK": "x"})
+        assert (await backend.drawing_properties_get())["custom"] == {
+            "NB\xa0SP": "legacy",
+            "A:B": "legacy",
+        }
+    finally:
+        await backend.disconnect()
+
+
+async def test_custom_keys_match_case_insensitively_headlessly(backend):
+    """Mirrors AutoCAD (measured on 2026): a write to 'PROJECT' over an
+    existing 'Project' updates it under the stored spelling instead of adding
+    a second tag AutoCAD would call a duplicate; a delete matches either
+    spelling."""
+    await backend.drawing_properties_set(custom={"Project": "X-1", "Rev": "A"})
+    res = await backend.drawing_properties_set(custom={"PROJECT": "X-2", "rev": None})
+    assert res["custom_written"] == ["PROJECT"] and res["custom_deleted"] == ["rev"]
+    assert list(backend._doc.header.custom_vars) == [("Project", "X-2")]
+    assert (await backend.drawing_properties_get())["custom"] == {"Project": "X-2"}
+
+
 async def _backend_for_version(tmp_path, version: str):
     import ezdxf
 
@@ -274,17 +386,35 @@ class _FakeSummaryInfo:
         self.calls.append(("GetCustomByIndex", index))
         return self._custom[index]  # pywin32 hands [out] BSTR pairs back as a tuple
 
+    # Measured on AutoCAD 2026: the three mutators match keys case-insensitively,
+    # an existing key keeps its stored spelling, a second Add is 'Duplicate key',
+    # a Set/Remove of an absent key is 'Key not found'.
+
+    def _index(self, key):
+        for i, (k, _v) in enumerate(self._custom):
+            if k.casefold() == key.casefold():
+                return i
+        return None
+
     def AddCustomInfo(self, key, value):
         self.calls.append(("AddCustomInfo", key, value))
+        if self._index(key) is not None:
+            raise RuntimeError("AutoCAD COM error: Duplicate key")
         self._custom.append((key, value))
 
     def SetCustomByKey(self, key, value):
         self.calls.append(("SetCustomByKey", key, value))
-        self._custom = [(k, value if k == key else v) for k, v in self._custom]
+        i = self._index(key)
+        if i is None:
+            raise RuntimeError("AutoCAD COM error: Key not found")
+        self._custom[i] = (self._custom[i][0], value)
 
     def RemoveCustomByKey(self, key):
         self.calls.append(("RemoveCustomByKey", key))
-        self._custom = [(k, v) for k, v in self._custom if k != key]
+        i = self._index(key)
+        if i is None:
+            raise RuntimeError("AutoCAD COM error: Key not found")
+        del self._custom[i]
 
 
 @pytest.fixture
@@ -351,6 +481,53 @@ async def test_com_validates_before_touching_activex(com_backend):
     assert info.calls == [] and info.Title == "Old title"
 
 
+@pytest.mark.parametrize(
+    "key", ["A:B", "REV/2", "SIZE, mm", 'A"B', "A\\B", "A<B>", "A|B", "A*B", "A?B"]
+)
+async def test_com_refuses_every_addcustominfo_invalid_key_before_any_write(com_backend, key):
+    """Re-measured on AutoCAD 2026 after 2b9791f: `summary={'subject':
+    'PARTIAL?'}, custom={'AAA': 'first', 'A:B': 'x', 'ZZZ': 'never'}` still
+    raised from AddCustomInfo('A:B') and left subject='PARTIAL?',
+    custom={'AAA': 'first'} -- the same half-write for every character the
+    first fix did not mirror. Nothing reaches SummaryInfo now."""
+    backend, info = com_backend
+    with pytest.raises(ValueError, match="Invalid key"):
+        await backend.drawing_properties_set(
+            summary={"subject": "PARTIAL?"},
+            custom={"AAA": "first", key: "x", "ZZZ": "never"},
+        )
+    assert info.calls == [] and info.Subject == ""
+    assert info._custom == [("PROJECT", "X-1"), ("OLD", "1")]
+
+
+async def test_com_deletes_a_legacy_invalid_syntax_key(com_backend):
+    """RemoveCustomByKey does not validate syntax (measured: 'Key not found'
+    for an absent 'A=B', never 'Invalid key'), so a key AutoCAD loaded from a
+    DXF is removed, not refused."""
+    backend, info = com_backend
+    info._custom.append(("A=B", "legacy"))
+    res = await backend.drawing_properties_set(custom={"A=B": None})
+    assert res["custom_deleted"] == ["A=B"]
+    assert ("RemoveCustomByKey", "A=B") in info.calls
+    assert info._custom == [("PROJECT", "X-1"), ("OLD", "1")]
+
+
+async def test_com_routes_case_variant_keys_to_set_and_remove(com_backend):
+    """Measured on AutoCAD 2026: AddCustomInfo('PROJECT') over an existing
+    'Project' raises 'Duplicate key' -- a case-sensitive existence check
+    routed it there and failed mid-write. Existence is matched casefolded."""
+    backend, info = com_backend
+    res = await backend.drawing_properties_set(custom={"project": "X-2", "old": None, "New": "n"})
+    assert res["custom_written"] == ["New", "project"] and res["custom_deleted"] == ["old"]
+    mutations = [c for c in info.calls if c[0] not in ("NumCustomInfo", "GetCustomByIndex")]
+    assert mutations == [
+        ("SetCustomByKey", "project", "X-2"),
+        ("AddCustomInfo", "New", "n"),
+        ("RemoveCustomByKey", "old"),
+    ]
+    assert info._custom == [("PROJECT", "X-2"), ("New", "n")], "stored spelling kept"
+
+
 async def test_com_refuses_autocad_invalid_keys_before_any_write(com_backend):
     """Measured on AutoCAD 2026: `summary={'subject': 'PARTIAL?'},
     custom={'AAA': 'first', ' BAD ': 'x', 'ZZZ': 'never'}` raised an unnamed