fix(mcp): answer an unknown resource or prompt with a standard error code - #107
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…code ReadResourceAsync and GetPromptAsync threw a code-less McpException for an unknown URI or name, which the SDK surfaces as InternalError (-32603) — a broken server, as far as a host can tell, rather than a missing resource. SEP-2164 standardises the codes so hosts can tell "gone" from "broken". The codes follow the SDK 2.2.0 handlers Repl replaces on the mcp serve path, so a Repl server answers exactly as one built on the SDK alone would. An unknown prompt is InvalidParams (-32602) on every revision. An unknown resource URI is the legacy ResourceNotFound (-32002) before 2026-07-28 and InvalidParams from that revision on — McpServerImpl selects between the two by negotiated version, and McpErrorCode's own documentation says new code should prefer InvalidParams. Issue #75 predates that revision and asked for ResourceNotFound unconditionally, which would answer a modern client with a code its revision no longer uses; the era comes from the same per-request signal (IsSessionlessRequest) every other revision-dependent behaviour in the handler reads. TDD: all four tests red first with InternalError, exactly as the issue observed. Hard-coding ResourceNotFound turns the modern-revision test red, and only that one. Refs #75
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Closes #75.
Previously, an unknown resource URI or prompt name surfaced as
InternalError(-32603). To a host, that looks like a broken server, not a missing resource. Per SEP-2164, both now carry a standard code:2025-11-252026-07-28resources/readwith an unknown URIResourceNotFound(-32002)InvalidParams(-32602)prompts/getwith an unknown nameInvalidParams(-32602)InvalidParams(-32602)This deliberately differs from the issue text. #75 asked for
ResourceNotFoundon every revision, but it was written before 2026-07-28. From that revision on, an unresolvable URI is reported withInvalidParams. The SDK's own handler (McpServerImpl) selects between the two codes by negotiated version, andMcpErrorCode's docs say new code should preferInvalidParams. Repl now answers exactly as a server built on the SDK alone would. The revision comes from the per-request signal the handler already uses for every other revision-dependent behaviour.docs/mcp-conformance.mdgains a row in What differs by revision and a SEP-2164 entry.Out of scope, and unchanged: an unknown tool name still returns a tool result with
isError, whereas the SDK's own handler throwsInvalidParams. #75 names only resources and prompts. I can file it separately if we want parity there too.Test plan
Given_McpProtocolErrorCodes, with the two test names from the issue plus one per revision. All four were red first withInternalError, as the issue observed.ResourceNotFoundturns the modern-revision test red, and only that test.docs/mcp-conformance.md.