diff --git a/src/anthropic/lib/_parse/_transform.py b/src/anthropic/lib/_parse/_transform.py
index ce0c83ac9..fe82a4c9e 100644
--- a/src/anthropic/lib/_parse/_transform.py
+++ b/src/anthropic/lib/_parse/_transform.py
@@ -51,6 +51,27 @@ def get_transformed_string(
     return schema
 
 
+def _dynamic_object_keywords_without_properties(json_schema: dict[str, Any]) -> list[str]:
+    """Return unsupported keywords that are the only way this object can admit keys."""
+    keywords: list[str] = []
+
+    pattern_properties = json_schema.get("patternProperties")
+    if isinstance(pattern_properties, dict) and pattern_properties:
+        keywords.append("patternProperties")
+
+    additional_properties = json_schema.get("additionalProperties")
+    if additional_properties is True or isinstance(additional_properties, dict):
+        keywords.append("additionalProperties")
+
+    if "propertyNames" in json_schema and additional_properties is not False:
+        keywords.append("propertyNames")
+
+    if "unevaluatedProperties" in json_schema and json_schema["unevaluatedProperties"] is not False:
+        keywords.append("unevaluatedProperties")
+
+    return keywords
+
+
 def transform_schema(
     json_schema: type[pydantic.BaseModel] | dict[str, Any],
 ) -> dict[str, Any]:
@@ -126,8 +147,19 @@ def transform_schema(
         strict_schema["title"] = title
 
     if type_ == "object":
+        properties = json_schema.pop("properties", {})
+        if not properties:
+            unsupported_dynamic_keywords = _dynamic_object_keywords_without_properties(json_schema)
+            if unsupported_dynamic_keywords:
+                keywords = ", ".join(unsupported_dynamic_keywords)
+                raise ValueError(
+                    "Structured output schemas cannot safely transform an object that relies on "
+                    f"{keywords} without explicit properties. Transforming this schema would make "
+                    "the object accept no keys."
+                )
+
         strict_schema["properties"] = {
-            key: transform_schema(prop_schema) for key, prop_schema in json_schema.pop("properties", {}).items()
+            key: transform_schema(prop_schema) for key, prop_schema in properties.items()
         }
         json_schema.pop("additionalProperties", None)
         strict_schema["additionalProperties"] = False
diff --git a/tests/lib/_parse/test_dynamic_object_transform.py b/tests/lib/_parse/test_dynamic_object_transform.py
new file mode 100644
index 000000000..edcfb39ab
--- /dev/null
+++ b/tests/lib/_parse/test_dynamic_object_transform.py
@@ -0,0 +1,46 @@
+from typing import Any
+
+import pytest
+from pydantic import BaseModel
+
+from anthropic.lib._parse._transform import transform_schema
+
+
+def test_additional_properties_map_raises_instead_of_becoming_empty_object():
+    schema = {
+        "type": "object",
+        "additionalProperties": {"type": "string"},
+    }
+
+    with pytest.raises(ValueError, match="additionalProperties"):
+        transform_schema(schema)
+
+
+def test_additional_properties_true_raises_instead_of_becoming_empty_object():
+    schema = {
+        "type": "object",
+        "additionalProperties": True,
+    }
+
+    with pytest.raises(ValueError, match="additionalProperties"):
+        transform_schema(schema)
+
+
+def test_pydantic_typed_dict_field_raises_instead_of_becoming_empty_object():
+    class Model(BaseModel):
+        values: dict[str, str]
+
+    schema = Model.model_json_schema()["properties"]["values"]
+
+    with pytest.raises(ValueError, match="additionalProperties"):
+        transform_schema(schema)
+
+
+def test_pydantic_arbitrary_dict_field_raises_instead_of_becoming_empty_object():
+    class Model(BaseModel):
+        values: dict[str, Any]
+
+    schema = Model.model_json_schema()["properties"]["values"]
+
+    with pytest.raises(ValueError, match="additionalProperties"):
+        transform_schema(schema)
diff --git a/tests/lib/_parse/test_transform.py b/tests/lib/_parse/test_transform.py
index 7a2799dce..a5af5fb05 100644
--- a/tests/lib/_parse/test_transform.py
+++ b/tests/lib/_parse/test_transform.py
@@ -229,3 +229,55 @@ def test_original_schema_not_mutated():
     transform_schema(original_schema)
 
     assert original_schema == original_schema_backup
+
+
+@pytest.mark.parametrize(
+    ("keyword", "constraint"),
+    [
+        ("patternProperties", {"^S_": {"type": "string"}}),
+        ("propertyNames", {"pattern": "^S_"}),
+        ("unevaluatedProperties", {"type": "string"}),
+    ],
+)
+def test_dynamic_object_schema_without_properties_raises(keyword: str, constraint: object):
+    schema = {"type": "object", keyword: constraint}
+
+    with pytest.raises(ValueError, match=keyword):
+        transform_schema(schema)
+
+
+def test_pattern_properties_with_explicit_properties_keeps_existing_demotion_behavior():
+    schema = {
+        "type": "object",
+        "properties": {"fixed": {"type": "string"}},
+        "required": ["fixed"],
+        "patternProperties": {"^S_": {"type": "string"}},
+    }
+
+    result = transform_schema(schema)
+
+    assert result == {
+        "type": "object",
+        "properties": {"fixed": {"type": "string"}},
+        "additionalProperties": False,
+        "required": ["fixed"],
+        "description": "{patternProperties: {'^S_': {'type': 'string'}}}",
+    }
+
+
+def test_closed_empty_object_is_not_rejected():
+    schema = {
+        "type": "object",
+        "additionalProperties": False,
+        "propertyNames": {"pattern": "^S_"},
+        "unevaluatedProperties": False,
+    }
+
+    result = transform_schema(schema)
+
+    assert result == {
+        "type": "object",
+        "properties": {},
+        "additionalProperties": False,
+        "description": "{propertyNames: {'pattern': '^S_'}, unevaluatedProperties: False}",
+    }