From 58e25ccf47429b295e6325b06e27dacf785ae5f8 Mon Sep 17 00:00:00 2001
From: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Date: Sat, 7 Mar 2026 12:10:01 -0500
Subject: [PATCH] Preserve kw-only defaults when rebuilding functions for
 resolved annotations (#3429)
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* Preserve kw-only defaults in cloned adapters (🤖 GPT-5.2-Codex)

* Fix ruff format violation in test_types.py
---
 src/fastmcp/utilities/types.py |  3 +++
 tests/utilities/test_types.py  | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/src/fastmcp/utilities/types.py b/src/fastmcp/utilities/types.py
index 91377df45..f8e4f91af 100644
--- a/src/fastmcp/utilities/types.py
+++ b/src/fastmcp/utilities/types.py
@@ -88,12 +88,14 @@ def get_cached_typeadapter(cls: T) -> TypeAdapter[T]:
                     globals_dict = actual_func.__globals__  # ty: ignore[unresolved-attribute]
                     name = actual_func.__name__  # ty: ignore[unresolved-attribute]
                     defaults = actual_func.__defaults__  # ty: ignore[unresolved-attribute]
+                    kwdefaults = actual_func.__kwdefaults__  # ty: ignore[unresolved-attribute]
                     closure = actual_func.__closure__  # ty: ignore[unresolved-attribute]
                 else:
                     code = cls.__code__
                     globals_dict = cls.__globals__
                     name = cls.__name__
                     defaults = cls.__defaults__
+                    kwdefaults = cls.__kwdefaults__
                     closure = cls.__closure__
 
                 new_func = types.FunctionType(
@@ -107,6 +109,7 @@ def get_cached_typeadapter(cls: T) -> TypeAdapter[T]:
                 new_func.__module__ = cls.__module__
                 new_func.__qualname__ = getattr(cls, "__qualname__", cls.__name__)
                 new_func.__annotations__ = processed_hints
+                new_func.__kwdefaults__ = kwdefaults
 
                 if inspect.ismethod(cls):
                     new_method = types.MethodType(new_func, cls.__self__)
diff --git a/tests/utilities/test_types.py b/tests/utilities/test_types.py
index 7901caff9..1f349ebdb 100644
--- a/tests/utilities/test_types.py
+++ b/tests/utilities/test_types.py
@@ -671,3 +671,21 @@ class TestAnnotationStringDescriptions:
 
         # Should keep the Field description
         assert schema["properties"]["name"]["description"] == "Field desc"
+
+    def test_kwonly_defaults_preserved_when_annotations_are_processed(self):
+        """Keyword-only defaults should survive function cloning during annotation processing."""
+
+        def func(*, limit: Annotated[int, "Maximum number of results"] = 5) -> int:
+            return limit
+
+        adapter = get_cached_typeadapter(func)
+        schema = adapter.json_schema()
+
+        assert "required" not in schema
+        assert schema["properties"]["limit"]["default"] == 5
+        assert (
+            schema["properties"]["limit"]["description"] == "Maximum number of results"
+        )
+
+        validated = adapter.validate_python({})
+        assert validated == 5