diff --git a/src/agents/strict_schema.py b/src/agents/strict_schema.py
index 3f37660a0..22366e629 100644
--- a/src/agents/strict_schema.py
+++ b/src/agents/strict_schema.py
@@ -51,6 +51,8 @@ def _ensure_strict_json_schema(
     typ = json_schema.get("type")
     if typ == "object" and "additionalProperties" not in json_schema:
         json_schema["additionalProperties"] = False
+    elif typ == "object" and json_schema.get("additionalProperties") is None:
+        json_schema["additionalProperties"] = False
     elif (
         typ == "object"
         and "additionalProperties" in json_schema
@@ -107,6 +109,16 @@ def _ensure_strict_json_schema(
     if json_schema.get("default", NOT_GIVEN) is None:
         json_schema.pop("default")
 
+    # Remove all null values to comply with JSON Schema Draft 2020-12
+    # This prevents LLM providers from rejecting schemas with null values
+    keys_to_remove = []
+    for key, value in json_schema.items():
+        if value is None:
+            keys_to_remove.append(key)
+
+    for key in keys_to_remove:
+        json_schema.pop(key)
+
     # we can't use `$ref`s if there are also other properties defined, e.g.
     # `{"$ref": "...", "description": "my description"}`
     #
diff --git a/tests/test_strict_schema.py b/tests/test_strict_schema.py
index c35e9adf7..4d63a823c 100644
--- a/tests/test_strict_schema.py
+++ b/tests/test_strict_schema.py
@@ -124,3 +124,52 @@ def test_invalid_ref_format():
     schema = {"type": "object", "properties": {"a": {"$ref": "invalid", "description": "desc"}}}
     with pytest.raises(ValueError):
         ensure_strict_json_schema(schema)
+
+
+def test_null_values_removal():
+    """
+    Test that null values are removed from JSON schemas to comply with JSON Schema Draft 2020-12.
+    """
+    schema_with_nulls = {
+        "type": "object",
+        "properties": {
+            "param": {
+                "type": "boolean",
+                "default": True,
+                "enum": None,
+                "minimum": None,
+                "maximum": None,
+                "items": None,
+                "properties": None,
+            }
+        },
+    }
+
+    result = ensure_strict_json_schema(schema_with_nulls)
+
+    # The result should not contain any null values
+    param_schema = result["properties"]["param"]
+    assert param_schema["type"] == "boolean"
+    assert param_schema["default"] is True
+    assert "enum" not in param_schema
+    assert "minimum" not in param_schema
+    assert "maximum" not in param_schema
+    assert "items" not in param_schema
+    assert "properties" not in param_schema
+
+
+def test_additional_properties_none_handling():
+    """
+    Test that additionalProperties: None is properly converted to False to maintain strictness.
+    """
+    schema_with_none_additional_properties = {
+        "type": "object",
+        "properties": {"param": {"type": "string"}},
+        "additionalProperties": None,
+    }
+
+    result = ensure_strict_json_schema(schema_with_none_additional_properties)
+
+    # additionalProperties should be False, not removed
+    assert result["additionalProperties"] is False
+    assert "additionalProperties" in result