diff --git a/src/agents/strict_schema.py b/src/agents/strict_schema.py
index 15b4c652fd..d8956a28e3 100644
--- a/src/agents/strict_schema.py
+++ b/src/agents/strict_schema.py
@@ -361,9 +361,8 @@ def _ensure_strict_json_schema(
                 for i, entry in enumerate(all_of)
             ]
 
-    # strip `None` defaults as there's no meaningful distinction here
-    # the schema will still be `nullable` and the model will default
-    # to using `None` anyway
+    # Preserve the historical early handling for `default: None`: it is non-constraining and
+    # should not force a `$ref` expansion by itself.
     if json_schema.get("default", NOT_GIVEN) is None:
         json_schema.pop("default")
 
@@ -399,6 +398,10 @@ def _ensure_strict_json_schema(
             inside_nested_resource=inside_nested_resource,
         )
 
+    # Strict schemas mark every property as required, so API-level defaults are not used and
+    # must be omitted because Structured Outputs rejects `default` in property definitions.
+    json_schema.pop("default", None)
+
     if budget.reject_open_objects and "$ref" in json_schema:
         raise UserError(_UNVALIDATED_REF_ERROR)
 
diff --git a/tests/mcp/test_strict_schema_ref_defaults.py b/tests/mcp/test_strict_schema_ref_defaults.py
new file mode 100644
index 0000000000..516faccff9
--- /dev/null
+++ b/tests/mcp/test_strict_schema_ref_defaults.py
@@ -0,0 +1,37 @@
+from agents.mcp import MCPUtil
+
+from .helpers import FakeMCPServer
+
+
+def test_mcp_ref_with_non_null_default_remains_strict() -> None:
+    server = FakeMCPServer()
+    server.add_tool(
+        "currency_tool",
+        {
+            "type": "object",
+            "properties": {
+                "currency": {
+                    "$ref": "#/$defs/Currency",
+                    "default": "EUR",
+                }
+            },
+            "$defs": {
+                "Currency": {
+                    "type": "string",
+                    "enum": ["EUR", "USD"],
+                }
+            },
+        },
+    )
+
+    function_tool = MCPUtil.to_function_tool(server.tools[0], server, True)
+
+    assert function_tool.strict_json_schema is True
+    assert function_tool.params_json_schema["required"] == ["currency"]
+    assert function_tool.params_json_schema["additionalProperties"] is False
+
+    currency_schema = function_tool.params_json_schema["properties"]["currency"]
+    assert currency_schema == {
+        "type": "string",
+        "enum": ["EUR", "USD"],
+    }
diff --git a/tests/test_function_schema.py b/tests/test_function_schema.py
index 1b261ce9b5..7abc5104d1 100644
--- a/tests/test_function_schema.py
+++ b/tests/test_function_schema.py
@@ -501,7 +501,7 @@ def func_with_optional_field(
     optional_schema = properties.get("optional_param", {})
     assert optional_schema.get("type") == "number"
     assert optional_schema.get("minimum") == 0.0  # ge=0.0
-    assert optional_schema.get("default") == 5.0
+    assert "default" not in optional_schema
 
     # Valid input with default
     valid_input = {"required_param": "test"}
@@ -675,13 +675,13 @@ def func_with_multiple_field_constraints(
     assert name_schema.get("type") == "string"
     assert name_schema.get("minLength") == 1
     assert name_schema.get("maxLength") == 50
-    assert name_schema.get("default") == "Unknown"
+    assert "default" not in name_schema
 
     # Check factor field
     factor_schema = properties.get("factor", {})
     assert factor_schema.get("type") == "number"
     assert factor_schema.get("exclusiveMinimum") == 0.0
-    assert factor_schema.get("default") == 1.0
+    assert "default" not in factor_schema
     assert factor_schema.get("description") == "Positive multiplier"
 
     # Valid input with defaults
@@ -761,7 +761,7 @@ def func_with_annotated_optional_field(
     optional_schema = properties.get("optional_param", {})
     assert optional_schema.get("type") == "number"
     assert optional_schema.get("minimum") == 0.0  # ge=0.0
-    assert optional_schema.get("default") == 5.0
+    assert "default" not in optional_schema
 
     # Valid input with default
     valid_input = {"required_param": "test"}
@@ -854,13 +854,13 @@ def func_with_annotated_multiple_field_constraints(
     assert name_schema.get("type") == "string"
     assert name_schema.get("minLength") == 1
     assert name_schema.get("maxLength") == 50
-    assert name_schema.get("default") == "Unknown"
+    assert "default" not in name_schema
 
     # Check factor field
     factor_schema = properties.get("factor", {})
     assert factor_schema.get("type") == "number"
     assert factor_schema.get("exclusiveMinimum") == 0.0
-    assert factor_schema.get("default") == 1.0
+    assert "default" not in factor_schema
     assert factor_schema.get("description") == "Positive multiplier"
 
     # Valid input with defaults
diff --git a/tests/test_strict_schema_defaults.py b/tests/test_strict_schema_defaults.py
new file mode 100644
index 0000000000..6ecfa56d8a
--- /dev/null
+++ b/tests/test_strict_schema_defaults.py
@@ -0,0 +1,19 @@
+from agents.strict_schema import ensure_strict_json_schema
+
+
+def test_strict_schema_removes_all_property_defaults() -> None:
+    schema = {
+        "type": "object",
+        "properties": {
+            "currency": {"type": "string", "default": "EUR"},
+            "retries": {"type": "integer", "default": 3},
+            "enabled": {"type": "boolean", "default": False},
+            "note": {"type": ["string", "null"], "default": None},
+        },
+    }
+
+    result = ensure_strict_json_schema(schema)
+
+    assert result["required"] == ["currency", "retries", "enabled", "note"]
+    for property_schema in result["properties"].values():
+        assert "default" not in property_schema