Fix Pydantic field alias consistency in structured output (#1099)

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

@@ -111,7 +111,7 @@ class FuncMetadata(BaseModel):
 
             assert self.output_model is not None, "Output model must be set if output schema is defined"
             validated = self.output_model.model_validate(result)
-            structured_content = validated.model_dump(mode="json")
+            structured_content = validated.model_dump(mode="json", by_alias=True)
 
             return (unstructured_content, structured_content)
 

tests/server/fastmcp/test_func_metadata.py

@@ -839,3 +839,48 @@ def test_structured_output_unserializable_type_error():
         func_metadata(func_returning_namedtuple, structured_output=True)
     assert "is not serializable for structured output" in str(exc_info.value)
     assert "Point" in str(exc_info.value)
+
+
+def test_structured_output_aliases():
+    """Test that field aliases are consistent between schema and output"""
+
+    class ModelWithAliases(BaseModel):
+        field_first: str | None = Field(default=None, alias="first", description="The first field.")
+        field_second: str | None = Field(default=None, alias="second", description="The second field.")
+
+    def func_with_aliases() -> ModelWithAliases:
+        # When aliases are defined, we must use the aliased names to set values
+        return ModelWithAliases(**{"first": "hello", "second": "world"})
+
+    meta = func_metadata(func_with_aliases)
+
+    # Check that schema uses aliases
+    assert meta.output_schema is not None
+    assert "first" in meta.output_schema["properties"]
+    assert "second" in meta.output_schema["properties"]
+    assert "field_first" not in meta.output_schema["properties"]
+    assert "field_second" not in meta.output_schema["properties"]
+
+    # Check that the actual output uses aliases too
+    result = ModelWithAliases(**{"first": "hello", "second": "world"})
+    unstructured_content, structured_content = meta.convert_result(result)
+
+    # The structured content should use aliases to match the schema
+    assert "first" in structured_content
+    assert "second" in structured_content
+    assert "field_first" not in structured_content
+    assert "field_second" not in structured_content
+    assert structured_content["first"] == "hello"
+    assert structured_content["second"] == "world"
+
+    # Also test the case where we have a model with defaults to ensure aliases work in all cases
+    result_with_defaults = ModelWithAliases()  # Uses default None values
+    unstructured_content_defaults, structured_content_defaults = meta.convert_result(result_with_defaults)
+
+    # Even with defaults, should use aliases in output
+    assert "first" in structured_content_defaults
+    assert "second" in structured_content_defaults
+    assert "field_first" not in structured_content_defaults
+    assert "field_second" not in structured_content_defaults
+    assert structured_content_defaults["first"] is None
+    assert structured_content_defaults["second"] is None