[v2] Legacy-era cancel of a resumed request: notifications/cancelled inherits the request's resumptionToken, so the transport swallows the cancellation and opens a rogue resumed GET #2646

What happened?

On a legacy-era (2025-11-25) Streamable HTTP connection, cancelling a request whose SSE stream has already resumed at least once (so the request carries a resumption token) never sends the cancellation to the server — instead the client opens a rogue resumed GET for the request it just cancelled.

Two halves, one on each side of the transport boundary:

1. Protocol's cancel path forwards the original request's resumption options onto the cancellation notification. In packages/core-internal/src/shared/protocol.ts, the cancel closure (non-streamCloseCancels branch, ~line 1454) sends:

   this._transport?.send(
       this._envelopeOutbound({ jsonrpc: '2.0', method: 'notifications/cancelled', params: { requestId: messageId, reason: String(reason) } }),
       { relatedRequestId, resumptionToken, onresumptiontoken }
   );

   resumptionToken here is the ORIGINAL request's token (from the caller's RequestOptions), not anything belonging to the notifications/cancelled message.

2. StreamableHTTPClientTransport._send's resume short-circuit treats any send carrying a resumptionToken as a stream resume. Its first branch is if (resumptionToken) { this._startOrAuthSse({ resumptionToken, ... }); return; } — the message body is never POSTed.

Net effect on the legacy era whenever the original request had resumed (its Last-Event-ID is live) and then times out or is aborted:

• The notifications/cancelled POST — the only spec cancellation signal on that era — is silently swallowed; the server keeps computing.
• A GET with the old Last-Event-ID opens for a request the client has already settled; if the server later completes it, the replayed response surfaces for an unknown message id.

The modern era is unaffected (streamCloseCancels aborts the per-request stream instead of POSTing). Timeout-triggered cancels of never-resumed requests are also unaffected (resumptionToken undefined).

Repro path

1. Legacy-era Streamable HTTP session; send a long-running request with onresumptiontoken capturing the token, server primes the SSE stream with an ID-bearing event and drops it so the client resumes (its internal resumptionToken is now set), request continues on the resumed stream.
2. Let the request time out (or abort its signal).
3. Observe the wire: no notifications/cancelled POST; instead a GET with Last-Event-ID for the cancelled request.

Two fix options

• (a) protocol.ts (preferred): don't forward resumptionToken/onresumptiontoken on the cancellation send — they describe the original request's stream, not the notification. relatedRequestId can stay. One-line change, wire-visible only in that the cancellation actually goes out.
• (b) transport-side: scope _send's resume short-circuit to messages that are requests (never notifications/responses), so a notification always POSTs regardless of stray resumption options. More defensive but leaves the odd protocol-side coupling in place.

Related context: the resume short-circuit path was touched by #2644 (observer threading), which is where this was noticed; the fix itself is deliberately not part of that PR since option (a) lives in core-internal.

SDK version

Read from main @ cc4b416 (packages/core-internal/src/shared/protocol.ts, packages/client/src/client/streamableHttp.ts); the same structure ships in @modelcontextprotocol/[email protected] / core-internal 2.0.0.

Area

Client / Transports, core-internal

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