[Sampling/Elicitation] Stateless servers silently failing on session-requiring features

[KKonstantinov]

Describe the bug
Streamable HTTP is silently failing when trying to execute a feature that is only supported if sessions are enabled.

Guilty code here:

typescript-sdk/src/server/streamableHttp.ts

Lines 665 to 685 in a1608a6

	if (requestId === undefined) {
	// For standalone SSE streams, we can only send requests and notifications
	if (isJSONRPCResponse(message) || isJSONRPCError(message)) {
	throw new Error("Cannot send a response on a standalone SSE stream unless resuming a previous client request");
	}
	const standaloneSse = this._streamMapping.get(this._standaloneSseStreamId)
	if (standaloneSse === undefined) {
	// The spec says the server MAY send messages on the stream, so it's ok to discard if no stream
	return;
	}
	// Generate and store event ID if event store is provided
	let eventId: string | undefined;
	if (this._eventStore) {
	// Stores the event and gets the generated event ID
	eventId = await this._eventStore.storeEvent(this._standaloneSseStreamId, message);
	}
	// Send the message to the standalone SSE stream
	this.writeSSEEvent(standaloneSse, message, eventId);
	return;

It exits at the following:

        // The spec says the server MAY send messages on the stream, so it's ok to discard if no stream
        return;

It does not help that none of the readme docs do not state explicitly that these features require session implementation (which brings other complexities like event store, etc. especially when horizontally scaled).

To Reproduce
Steps to reproduce the behavior:

1. Run a stateless MCP server
2. Attempt sampling or elicitation as per README.md - Sampling section
3. Observe a timeout happening with no clear indication or a specified error.

Expected behavior

1. Clear error being sent back.
2. Logging being done.
3. Not silently returning.

[KKonstantinov]

Happy to raise a PR fixing this and also updating the README to state the fact that sessions are required.

I further think on the README, a feature matrix could be presented that describes the different set-ups & the capabilities (or lack thereof) available in that set up.

[jinweili1]

Met same problem, and could we support those two features in stateless scenario?

[hjerbii]

I have the same issue!