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Author: Viicos

src/mcp/server/fastmcp/utilities/func_metadata.py

@@ -17,7 +17,7 @@
 from pydantic.fields import FieldInfo
 from pydantic.json_schema import GenerateJsonSchema, JsonSchemaWarningKind
 from typing_extensions import is_typeddict
-from typing_inspection.introspection import UNKNOWN, AnnotationSource, inspect_annotation, is_union_origin
+from typing_inspection.introspection import UNKNOWN, ForbiddenQualifier, AnnotationSource, inspect_annotation, is_union_origin
 
 from mcp.server.fastmcp.exceptions import InvalidSignature
 from mcp.server.fastmcp.utilities.logging import get_logger
@@ -262,12 +262,16 @@ def func_metadata(
     if sig.return_annotation is inspect.Parameter.empty and structured_output is True:
         raise InvalidSignature(f"Function {func.__name__}: return annotation required for structured output")
 
-    inspected_return_ann = inspect_annotation(sig.return_annotation, annotation_source=AnnotationSource.FUNCTION)
+    try:
+        inspected_return_ann = inspect_annotation(sig.return_annotation, annotation_source=AnnotationSource.FUNCTION)
+    except ForbiddenQualifier as e:
+        raise InvalidSignature(f"Function {func.__name__}: return annotation contains an invalid type qualifier") from e
+
     return_type_expr = inspected_return_ann.type
-    if return_type_expr is UNKNOWN and structured_output is True:
-        # `return_type_expr` is `UNKNOWN` when a bare type qualifier is used (unlikely to happen as a return annotation
-        # because it doesn't make any sense, but technically possible).
-        raise InvalidSignature(f"Function {func.__name__}: return annotation required for structured output")
+
+    # `AnnotationSource.FUNCTION` allows no type qualifier to be used, so `return_type_expr` is guaranteed to *not* be
+    # unknown (i.e. a bare `Final`).
+    assert return_type_expr is not UNKNOWN
 
     if is_union_origin(get_origin(return_type_expr)):
         args = get_args(return_type_expr)

tests/server/fastmcp/test_func_metadata.py

@@ -5,7 +5,7 @@
 # pyright: reportUnknownLambdaType=false
 from collections.abc import Callable
 from dataclasses import dataclass
-from typing import Annotated, Any, TypedDict
+from typing import Annotated, Any, TypedDict, Final
 
 import annotated_types
 import pytest
@@ -1182,3 +1182,23 @@ def func_with_reserved_json(
     assert result["json"] == {"nested": "data"}
     assert result["model_dump"] == [1, 2, 3]
     assert result["normal"] == "plain string"
+
+
+def test_disallowed_type_qualifier():
+    from mcp.server.fastmcp.exceptions import InvalidSignature
+
+    def func_disallowed_qualifier() -> Final[int]:  # type: ignore
+        pass
+
+    with pytest.raises(InvalidSignature) as exc_info:
+        func_metadata(func_disallowed_qualifier)
+    assert "return annotation contains an invalid type qualifier" in str(exc_info.value)
+
+
+def test_preserves_pydantic_metadata():
+    def func_with_metadata() -> Annotated[int, Field(gt=1)]: ...
+
+    meta = func_metadata(func_with_metadata)
+
+    assert meta.output_schema is not None
+    assert meta.output_schema["properties"]["result"] == {"exclusiveMinimum": 1, "title": "Result", "type": "integer"}