Reject tables inserted into inline tables

CHANGELOG.md

@@ -28,6 +28,7 @@
 - Fix `comment()` producing invalid TOML for a multiline string by prefixing every line with `#`, not just the first. ([#449](https://github.com/python-poetry/tomlkit/issues/449))
 - Fix the separator comma being swallowed by a trailing comment when appending a key to a multiline inline table, leaving the new key without a separator so the result no longer round-trips. ([#512](https://github.com/python-poetry/tomlkit/issues/512))
 - Fix a `KeyAlreadyPresent` error when parsing or accessing an out-of-order table whose array-of-tables elements are split across the table's parts. ([#505](https://github.com/python-poetry/tomlkit/issues/505))
+- Reject tables inserted into inline tables instead of serializing invalid TOML. ([#531](https://github.com/python-poetry/tomlkit/issues/531))
 
 ## [0.15.0] - 2026-05-10
 

tests/test_items.py

@@ -887,6 +887,17 @@ def test_trim_comments_when_building_inline_table() -> None:
     assert table.as_string() == '{foo = "bar", baz = "foobaz"}'
 
 
+def test_append_table_to_inline_table_raises() -> None:
+    table = api.table()
+    table.append("a", 1)
+    inline_table = api.inline_table()
+
+    with pytest.raises(ValueError, match="cannot contain a table"):
+        inline_table.append("table", table)
+    with pytest.raises(ValueError, match="cannot contain a table"):
+        inline_table["table"] = table
+
+
 def test_deleting_inline_table_element_does_not_leave_trailing_separator() -> None:
     table = api.inline_table()
     table["foo"] = "bar"

tomlkit/items.py

@@ -2035,6 +2035,8 @@ def append(self, key: Key | str | None, _item: Any) -> InlineTable:
         if not isinstance(_item, Item):
             _item = item(_item, _parent=self)
 
+        self._validate_child(_item)
+
         if not isinstance(_item, (Whitespace, Comment)):
             if not _item.trivia.indent and len(self._value) > 0 and not self._new:
                 _item.trivia.indent = " "
@@ -2051,6 +2053,10 @@ def append(self, key: Key | str | None, _item: Any) -> InlineTable:
 
         return self
 
+    def _validate_child(self, _item: Item) -> None:
+        if isinstance(_item, Table):
+            raise ValueError("Inline tables cannot contain a table")
+
     def as_string(self) -> str:
         buf = "{"
         emitted_key = False
@@ -2174,6 +2180,9 @@ def _render_dotted(self, key: Key, table: Table) -> list[str]:
     def __setitem__(self, key: Key | str, value: Any) -> None:  # type: ignore[override]
         if hasattr(value, "trivia") and value.trivia.comment:
             value.trivia.comment = ""
+        if not isinstance(value, Item):
+            value = item(value, _parent=self)
+        self._validate_child(value)
         super().__setitem__(key, value)
 
     def __copy__(self) -> InlineTable: