MCPServer answers unknown prompt name or missing required argument with code 0 / -32603 instead of -32602

Finding: According to an external report (not reproduced by KFA), the Python MCP SDK's MCPServer returns the wrong JSON-RPC error code for prompts/get when the prompt name is unregistered or a required argument is missing. The reporter observed this on mcp 2.3.0 from PyPI and on main@91941ed, using Python 3.13.16 (https://github.com/modelcontextprotocol/python-sdk/issues/3669). Reported behavior: with protocol 2025-11-25 the client receives code 0 with messages such as 'Unknown prompt: does_not_exist' and 'Missing required arguments: {'name'}'. With protocol 2026-07-28 both cases return -32603 'Internal server error', so the client is not told which name was wrong. The reporter says the spec's prompts error-handling section calls for -32602 (Invalid params) in both cases. We did not verify that claim. The reporter also says the TypeScript SDK throws InvalidParams for an unknown prompt. Reported cause: the reporter states that MCPServer raises a plain ValueError for both cases. No maintainer has confirmed this or the intended behavior; the thread has no comments. Contrast case: a prompt function that itself raises (RuntimeError) returns code 0 on 2025-11-25 and -32603 on 2026-07-28. The reporter calls this a real internal error, so -32603 there is not part of the complaint. Context from the issue: the repo's interaction suite reportedly already lists the unknown-name and missing-required-args cases as divergences for 2025-11-25, and the 2026-07-28 arm is excluded. docs/servers/prompts.md reportedly documents the missing-argument case as -32603, so a fix would change documented behavior. The reporter links related items #3659 and #3342 and mentions older PR #1872 for the unknown-name case, which reportedly conflicts with main. Status: no workaround, fix commit, or release is cited in the source. The reporter offers to submit a PR if outside PRs are accepted. Diagnostic use: if you see code 0 or -32603 from get_prompt for an unknown name or missing argument on these versions, it matches this report. Treat it as an unconfirmed upstream issue; client code should not assume -32602 until a fix is released. Limitation: this is a single external report with a repro script; KFA did not run it, and no maintainer response exists yet.