fix: preserve nullability for null-only anyOf/oneOf schemas (#3171)

python/composio/utils/schema_converter.py

@@ -234,6 +234,11 @@ def _handle_toplevel_combiner(
             allow_undefined_array_items=True,
             allow_undefined_type=True,
         )
+        # A null-only anyOf/oneOf (e.g. {"anyOf": [{"type": "null"}]}) collapses to
+        # NoneType in the library, which loses the nullable-but-untyped intent.
+        # Mirror the direct {"type": "null"} mapping and return Optional[Any].
+        if result is type(None):
+            return PYDANTIC_TYPE_TO_PYTHON_TYPE["null"]
         # If result is a type (like a Union or Optional), return it directly
         # If result is a model class, return it
         return result
@@ -270,6 +275,10 @@ def _build_union_from_options(options: t.List[t.Dict[str, t.Any]]) -> t.Type:
         pydantic_types.append(ptype)
 
     if len(pydantic_types) == 0:
+        # Preserve nullability when every branch was a null branch
+        # (e.g. {"anyOf": [{"type": "null"}]}) instead of downgrading to str.
+        if has_null:
+            return t.cast(t.Type, PYDANTIC_TYPE_TO_PYTHON_TYPE["null"])
         return str  # Fallback
 
     if len(pydantic_types) == 1:

python/tests/test_schema_parser.py

@@ -615,6 +615,35 @@ def test_anyof_single_type(self):
         assert result is str
         assert not hasattr(result, "__origin__")
 
+    @pytest.mark.unit
+    @pytest.mark.schema
+    def test_anyof_null_only_preserves_nullability(self):
+        """
+        Regression test for #3171: anyOf containing only {"type": "null"}
+        must yield Optional[Any], matching the direct {"type": "null"} mapping.
+        Previously this was downgraded to str (or NoneType depending on the
+        underlying library version), erasing nullability.
+        """
+        json_schema = {"anyOf": [{"type": "null"}]}
+        result = json_schema_to_pydantic_type(json_schema)
+        assert result == t.Optional[t.Any]
+
+    @pytest.mark.unit
+    @pytest.mark.schema
+    def test_oneof_null_only_preserves_nullability(self):
+        """oneOf containing only {"type": "null"} should also yield Optional[Any]."""
+        json_schema = {"oneOf": [{"type": "null"}]}
+        result = json_schema_to_pydantic_type(json_schema)
+        assert result == t.Optional[t.Any]
+
+    @pytest.mark.unit
+    @pytest.mark.schema
+    def test_anyof_repeated_null_branches(self):
+        """Multiple null branches still collapse to a single Optional[Any]."""
+        json_schema = {"anyOf": [{"type": "null"}, {"type": "null"}]}
+        result = json_schema_to_pydantic_type(json_schema)
+        assert result == t.Optional[t.Any]
+
     def test_allof_single_option(self):
         """Test allOf with single option."""
         json_schema = {