From 3be803dd0bb1914270aed1ccd6d9a22f462219d9 Mon Sep 17 00:00:00 2001
From: Pratik Dhanave <i.pratikdhanave@gmail.com>
Date: Wed, 4 Nov 2026 23:24:22 +0530
Subject: [PATCH] fix(mcpserver): keep Annotated field constraints on
 ordinary-class structured output (#3712)

_create_output_model resolved an ordinary class's field hints with
get_type_hints(type_annotation), which drops Annotated metadata. A field
declared Annotated[int, Field(gt=0)] collapsed to a bare int: the generated
output schema lost exclusiveMinimum and convert_result accepted out-of-bound
values such as 0 without raising ValidationError.

Pass include_extras=True so the Annotated metadata is retained and carried
into the synthesized Pydantic model, matching the TypedDict path which
already resolves hints with extras.

Fixes #3712

Signed-off-by: Pratik Dhanave <i.pratikdhanave@gmail.com>
---
 .../mcpserver/utilities/func_metadata.py      |  4 ++-
 tests/server/mcpserver/test_func_metadata.py  | 35 +++++++++++++++++++
 2 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/src/mcp/server/mcpserver/utilities/func_metadata.py b/src/mcp/server/mcpserver/utilities/func_metadata.py
index e5721488f5..c1ed42a0a6 100644
--- a/src/mcp/server/mcpserver/utilities/func_metadata.py
+++ b/src/mcp/server/mcpserver/utilities/func_metadata.py
@@ -552,7 +552,9 @@ def _create_output_model(original_annotation: Any, type_expr: Any, func_name: st
 
         # Case 5: Other class types (dataclasses, regular classes with annotations)
         else:
-            type_hints = get_type_hints(type_annotation)
+            # include_extras keeps `Annotated[...]` metadata (e.g. Field(gt=0)) so field
+            # constraints survive into the synthesized model instead of being silently dropped.
+            type_hints = get_type_hints(type_annotation, include_extras=True)
             if type_hints:
                 # Classes with type hints can be converted to Pydantic models
                 model = _create_model_from_class(type_annotation, type_hints)
diff --git a/tests/server/mcpserver/test_func_metadata.py b/tests/server/mcpserver/test_func_metadata.py
index f929fb3954..7de210ea9a 100644
--- a/tests/server/mcpserver/test_func_metadata.py
+++ b/tests/server/mcpserver/test_func_metadata.py
@@ -973,6 +973,41 @@ def func_returning_class() -> PersonClass:  # pragma: no cover
     }
 
 
+def test_structured_output_ordinary_class_keeps_annotated_field_constraints():
+    """Annotated field constraints on an ordinary class must survive into the output model.
+
+    Regression test for #3712: ``get_type_hints`` was called without ``include_extras=True``,
+    so ``Annotated[int, Field(gt=0)]`` collapsed to a bare ``int`` - the constraint vanished
+    from the schema and output conversion accepted out-of-bound values.
+    """
+
+    class Result:
+        value: Annotated[int, Field(gt=0)]
+
+        def __init__(self, value: int) -> None:
+            self.value = value
+
+    def produce() -> Result:  # pragma: no cover - never called
+        return Result(0)
+
+    meta = func_metadata(produce)
+
+    # The constraint must appear in the generated schema.
+    assert meta.output_schema == {
+        "type": "object",
+        "properties": {"value": {"type": "integer", "exclusiveMinimum": 0}},
+        "required": ["value"],
+        "title": "Result",
+    }
+
+    # A valid value round-trips.
+    assert meta.convert_result(Result(1)).structured_content == {"value": 1}
+
+    # An out-of-bound value is rejected rather than silently accepted.
+    with pytest.raises(ValidationError):
+        meta.convert_result(Result(0))
+
+
 def test_unstructured_output_unannotated_class():
     # Test with class that has no annotations
     class UnannotatedClass: