Generate tool outputSchema in serialization mode

outputSchema was built with pydantic's default validation mode, while
structuredContent is dumped with by_alias=True. Serialization aliases
and computed fields showed up in the content but not in the schema, so
clients validating the result rejected it.

Pass mode="serialization" to json_schema so the two agree.

Author: guptaishaan

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

@@ -127,7 +127,9 @@ class FuncMetadata(BaseModel):
     def model_post_init(self, context: Any, /) -> None:
         if self.output_model is not None and self.output_schema is None:
             # StrictJsonSchema raises instead of warning, so an unserializable return type fails construction.
-            schema = self._output_adapter(self.output_model).json_schema(schema_generator=StrictJsonSchema)
+            adapter = self._output_adapter(self.output_model)
+            # Serialization mode, so the schema describes the aliased/computed fields structured_content carries.
+            schema = adapter.json_schema(schema_generator=StrictJsonSchema, mode="serialization")
             self.output_schema = _inline_root_ref(schema)
 
     def _output_adapter(self, output_model: Any) -> TypeAdapter[Any]:

tests/server/mcpserver/test_func_metadata.py

@@ -11,7 +11,7 @@
 import pytest
 from dirty_equals import IsPartialDict
 from mcp_types import CallToolResult, ContentBlock, EmbeddedResource, InputRequiredResult, TextContent
-from pydantic import BaseModel, Field, ValidationError
+from pydantic import BaseModel, Field, ValidationError, computed_field
 from typing_extensions import NotRequired, ReadOnly, Required
 
 from mcp import MCPDeprecationWarning
@@ -1520,3 +1520,23 @@ def fn() -> StepA | StepB: ...  # pragma: no branch
 
     meta = func_metadata(fn)
     assert meta.output_schema is None
+
+
+def test_output_schema_matches_structured_content_for_serialization_aliases_and_computed_fields():
+    class Out(BaseModel):
+        user_id: int = Field(serialization_alias="userId")
+
+        @computed_field
+        @property
+        def doubled(self) -> int:
+            return self.user_id * 2
+
+    def tool() -> Out:  # pragma: no cover
+        return Out(user_id=3)
+
+    meta = func_metadata(tool)
+    assert meta.output_schema is not None
+    result = meta.convert_result(Out(user_id=3))
+    assert result.structured_content == {"userId": 3, "doubled": 6}
+    assert set(meta.output_schema["properties"]) == {"userId", "doubled"}
+    assert set(meta.output_schema["required"]) == {"userId", "doubled"}