server/discover probe classifier discards HTTP 401/403, and pin mode drops the cause

[cliffhall]

Affects: @modelcontextprotocol/client@2.0.0-beta.5

Two problems in the server/discover negotiation probe path mean a client using versionNegotiation auto or { pin } never starts authorization against an OAuth-protected server. The same server authorizes fine on the legacy era.

1. classifyHttpError ignores the HTTP status

It parses only a JSON-RPC error body from the probe response; outcome.status is never read:

function classifyHttpError(outcome, context) {
	const rpcError = parseJsonRpcErrorBody(outcome.body);
	if (rpcError !== void 0) return classifyRpcError(rpcError, context);
	return { kind: "legacy" };
}

A 401 or 403 on the probe therefore verdicts "no modern evidence" — indistinguishable from a 2025-era server — regardless of body shape. A server that answers 401 on server/discover is misread as legacy.

2. Pin mode rethrows without a cause

On that verdict, negotiateEra under kind: "pin" throws (index.mjs:2670 in the beta.5 build):

if (negotiation.kind === "pin") throw new SdkError(SdkErrorCode.EraNegotiationFailed, `Version negotiation failed: the server did not offer pinned protocol version ${negotiation.version} via server/discover (no fallback in pin mode)`);

No third argument, so the original SdkHttpError — status, body, everything — is discarded. The caller has no way to recover the 401, not even by walking the chain. Observed on a live server: the rejection is an SdkError/ERA_NEGOTIATION_FAILED with data undefined and no cause.

Related

normalizeReply maps a send rejection to auth-required only for SdkHttpError and UnauthorizedError. Any other typed auth error a consumer's transport raises falls through to network-error and is wrapped by classifyNetworkError as ERA_NEGOTIATION_FAILED with the original at data.cause. That one is recoverable, but only if the consumer knows to look there — and note it is data.cause, not cause, since SdkError's third parameter is data.

Impact and repro

Connecting to an OAuth-protected HTTP server with versionNegotiation: { mode: "auto" } or { mode: { pin: "2026-07-28" } } surfaces a dead-end negotiation failure instead of starting the OAuth flow. Reproduced against a live dual-era server whose bearer middleware guards the modern leg, so the probe itself is answered 401.

Full write-up, including a table of what connect() rejects with per era on both a direct and an intercepting transport: modelcontextprotocol/inspector#1805.

Asks

1. Treat HTTP 401/403 on the probe as auth-required rather than as absence of modern evidence.
2. Preserve the original error as cause when pin mode rethrows, so a status is recoverable even if (1) is declined.

MCP Inspector is working around this client-side (modelcontextprotocol/inspector#1806) by forcing auth-challenge interception on the probing eras, so the 401 becomes a typed error that survives the probe as data.cause. That workaround can be dropped once (1) lands.