From 72fb8d4ce9bf51b166d6807ba49e2bc3710e2db6 Mon Sep 17 00:00:00 2001
From: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Date: Tue, 22 Jul 2025 09:14:15 -0400
Subject: [PATCH] Ensure default fields are not marked nullable (#1224)

---
 src/fastmcp/utilities/json_schema_type.py |  4 +-
 tests/utilities/test_json_schema_type.py  | 55 +++++++++++++++++++++++
 2 files changed, 56 insertions(+), 3 deletions(-)

diff --git a/src/fastmcp/utilities/json_schema_type.py b/src/fastmcp/utilities/json_schema_type.py
index ff5ab4102..62577402b 100644
--- a/src/fastmcp/utilities/json_schema_type.py
+++ b/src/fastmcp/utilities/json_schema_type.py
@@ -561,9 +561,7 @@ def _create_dataclass(
             else:
                 field_def = field(default=None, metadata=meta)
 
-        if is_required and default_val is not MISSING:
-            fields.append((field_name, field_type, field_def))
-        elif is_required:
+        if is_required or default_val is not MISSING:
             fields.append((field_name, field_type, field_def))
         else:
             fields.append((field_name, Union[field_type, type(None)], field_def))  # type: ignore[misc]  # noqa: UP007
diff --git a/tests/utilities/test_json_schema_type.py b/tests/utilities/test_json_schema_type.py
index ca8cd11b6..8cb9ca90a 100644
--- a/tests/utilities/test_json_schema_type.py
+++ b/tests/utilities/test_json_schema_type.py
@@ -1523,3 +1523,58 @@ class TestAdditionalProperties:
         # Should be the same cached class
         assert Type1 is Type2
         assert issubclass(Type1, BaseModel)
+
+
+class TestFieldsWithDefaults:
+    """Test suite for fields with default values not being made nullable."""
+
+    def test_field_with_default_preserves_type(self):
+        """Test that fields with defaults preserve their original type."""
+        schema = {
+            "type": "object",
+            "properties": {"flag": {"type": "boolean", "default": False}},
+        }
+
+        generated_type = json_schema_to_type(schema)
+        regenerated_schema = TypeAdapter(generated_type).json_schema()
+
+        assert regenerated_schema["properties"]["flag"]["type"] == "boolean"
+
+    def test_field_with_default_not_nullable(self):
+        """Test that fields with defaults are not made nullable."""
+        schema = {
+            "type": "object",
+            "properties": {"flag": {"type": "boolean", "default": False}},
+        }
+
+        generated_type = json_schema_to_type(schema)
+        regenerated_schema = TypeAdapter(generated_type).json_schema()
+
+        flag_prop = regenerated_schema["properties"]["flag"]
+        assert "anyOf" not in flag_prop
+
+    def test_field_with_default_uses_default(self):
+        """Test that fields with defaults use their default values."""
+        schema = {
+            "type": "object",
+            "properties": {"flag": {"type": "boolean", "default": False}},
+        }
+
+        generated_type = json_schema_to_type(schema)
+        validator = TypeAdapter(generated_type)
+        result = validator.validate_python({})
+
+        assert result.flag is False
+
+    def test_field_with_default_accepts_explicit_value(self):
+        """Test that fields with defaults accept explicit values."""
+        schema = {
+            "type": "object",
+            "properties": {"flag": {"type": "boolean", "default": False}},
+        }
+
+        generated_type = json_schema_to_type(schema)
+        validator = TypeAdapter(generated_type)
+        result = validator.validate_python({"flag": True})
+
+        assert result.flag is True