[v2] StreamableHTTPClientTransport: auth awaits in _send are un-abortable — requestSignal cannot cancel token()/onUnauthorized/metadata-discovery parks

What happened?

StreamableHTTPClientTransport threads TransportSendOptions.requestSignal through its fetches and SSE reconnect chain, but none of the auth awaits on the send path can see it, so a request that is "cancelled" by its signal can stay parked indefinitely inside the transport:

1. _commonHeaders() (packages/client/src/client/streamableHttp.ts, await this._authProvider?.token()) runs before the per-request signal is combined into a fetch signal. A token() that hangs (wedged token refresh, keychain prompt, slow OAuth broker) parks send() forever; aborting requestSignal has no effect because nothing between the caller and the pending token() observes it.
2. onUnauthorized (both the POST _send path and the GET _startOrAuthSse path): await this._authProvider.onUnauthorized({ response, serverUrl, fetchFn }) receives no AbortSignal, so a 401-triggered recovery flow cannot be cancelled.
3. The SDK's own OAuth flow behind the adapter — auth(...) / _stepUpAuthorize(...) and their protected-resource/authorization-server metadata discovery fetches — receive fetchFn but no signal, so a slow/black-holed discovery endpoint parks the send with no way to abort.

Consequences: Protocol.request()'s timeout still rejects the caller's promise (the response-handler timer is independent), but the underlying send remains parked and un-collectable, and paths that suspend on the send itself — notably Client.listen(), whose abort teardown relies on requestSignal reaching the transport — cannot be torn down. This is the streamable-HTTP sibling of the stdio 'drain' park (#2552) and one of the ways the listen() escape in #2641 stays wedged.

What did you expect?

Aborting TransportSendOptions.requestSignal should settle send() promptly no matter which phase it is in — header/token acquisition, 401 recovery, step-up authorization, or metadata discovery — the same way it already aborts the fetch and the SSE resume chain.

Code to reproduce

Observed on the published package (no server needed — the park happens before any fetch):

// node repro.mjs — @modelcontextprotocol/client@2.0.0
import { StreamableHTTPClientTransport } from "@modelcontextprotocol/client";

const transport = new StreamableHTTPClientTransport(new URL("http://127.0.0.1:9/mcp"), {
  authProvider: { token: () => new Promise(() => {}) }, // hung token()
});
await transport.start();

const ac = new AbortController();
const send = transport
  .send({ jsonrpc: "2.0", id: 1, method: "tools/list", params: {} }, { requestSignal: ac.signal })
  .then(
    () => "resolved (unexpected)",
    (e) => `rejected: ${e?.name ?? e}`,
  );
setTimeout(() => ac.abort(new Error("caller abort")), 100);

console.log(await Promise.race([
  send,
  new Promise((r) => setTimeout(() => r("send() still pending 2s after requestSignal abort — un-abortable park"), 2000)),
]));

Output on 2.0.0 (same code paths on main @ cc4b416):

send() still pending 2s after requestSignal abort — un-abortable park

The onUnauthorized / metadata-discovery variants park the same way, just later in the send (after a 401 instead of before the first fetch).

Suggested fix

Thread the request's abort signal through the auth chain as optional parameters so existing providers keep working unchanged:

• Combine requestSignal with the transport signal before header acquisition, and race token() against it (or pass { signal } to token() as an optional argument in the AuthProvider contract).
• Add signal?: AbortSignal to the onUnauthorized callback options and to the internal auth(...) / step-up / metadata-discovery fetch plumbing, defaulting to undefined.
• On abort, reject the send with the signal's reason (marked as an intentional abort so onerror stays suppressed, matching the existing fetch-abort discipline).

SDK version

@modelcontextprotocol/client@2.0.0 (repro above); same code on main @ cc4b416.

Area

Client / Transports

Generated by Claude Code

Start in packages/client/src/client/streamableHttp.ts, tracing _commonHeaders(), the POST _send path, and the GET _startOrAuthSse path. Then follow the authProvider token/onUnauthorized calls into auth(), _stepUpAuthorize(), and metadata discovery. Done means requestSignal aborts send() promptly during each listed auth phase, while existing fetch and SSE abort behavior remains intact; use the provided hung-token reproduction to verify.