The bug in InMemoryEventStore break SSE stream resumability

[y0geshdev]

Describe the bug
The standalong sse stream Id is coded to _GET_stream and the current InMemoryEventStore generate event Ids in the format -
${streamId}-${Date.now()}_${Math.random().toString(36).substring(2, 10)}

The replayEventsAfter(...) is called when client try to re-connect and resume the stream which further calls getStreamIdFromEventId(...) -

    // Extract the stream ID from the event ID
    const streamId = this.getStreamIdFromEventId(lastEventId);
    if (!streamId) {
      return '';
    }

The issue is that getStreamIdFromEventId(...) will always return an empty string as the streamId due to the current eventId format and the code will return from this if check.

To Reproduce
Steps to reproduce the behavior:

1. Create a tool with at least 2 elicitation requests.
2. Call the tool.
3. Try to reconnect with SSE stream - nothing will be returned.

Expected behavior
MCP client should be able to re-connect to SSE stream.

Fix
The fix could be to use a different delimiter than underscore when combining streamId with rest of the eventId.
Used dash("-") in below sample fix -

  /**
   * Generates a unique event ID for a given stream ID
   */
  private generateEventId(streamId: string): string {
--->    return `${streamId}**-**${Date.now()}_${Math.random().toString(36).substring(2, 10)}`;
  }

  /**
   * Extracts the stream ID from an event ID
   */
  private getStreamIdFromEventId(eventId: string): string {
--->    const parts = eventId.split('**-**');
    return parts.length > 0 ? parts[0] : '';
  }

[ochafik]

Hi @y0geshdev , thanks for filing this!

Which version / revision of the SDK are you using? In the current codebase the format & split should be both using underscores, see https://github.com/modelcontextprotocol/typescript-sdk/blame/7146eede0b9cc6b87a94f8361e3bd884b59b6508/src/examples/shared/inMemoryEventStore.ts#L15C11-L15C26

Happy to reopen if i missed something :-)

[y0geshdev]

@ochafik I checked out the latest version (1.20.0) and tried resuming stream using last-event-id and it doesn't work.

The issue is that streamId for resumable standalone SSE stream is _GET_stream. Due to this, InMemoryEventStore.generateEventId(...) will return an eventId of format _GET_stream_.... Now when I sent this format eventId as last-event-id in header to resume the stream, InMemoryEventStore.getStreamIdFromEventId(...) will try to extract the streamId from eventId and will always return '' (empty string) -

    // InMemoryEventStore.ts
    /**
     * Extracts the stream ID from an event ID
     */
    private getStreamIdFromEventId(eventId: string): string {  // the eventId for resumable stream is like - _GET_stream_...
        const parts = eventId.split('_');  // parts = ['', GET, stream, ...]
        return parts.length > 0 ? parts[0] : '';  // this will always return '' (empty string)
    }

The fix is to use a different delimited than _ (as this is being used in StreamableHTTPServerTransport_standaloneSseStreamId ) to append and extract streamId in InMemoryEventStore from eventId.

To repro the issue, try to resume the SSE event stream.
Can we re-open this issue and fix this please?

[mcp-claude]

SSE stream resumability is broken when using InMemoryEventStore with the standalone SSE stream (_GET_stream). replayEventsAfter always returns early with '' because extracting the stream ID from the event ID via split('_')[0] yields '' when the stream ID itself starts with _.

Workaround: use a custom EventStore implementation that stores the streamId alongside the event ID and looks it up directly instead of parsing it out of the event ID string.

Fix: in getStreamIdFromEventId, look up this.events.get(eventId)?.streamId directly — no parsing needed, since the map already stores the streamId.

Repro script (examples/server/src/inMemoryEventStore.ts logic, self-contained)

// repro.ts — run with: npx tsx repro.ts

type JSONRPCMessage = { jsonrpc: string; id?: number; method?: string };

class InMemoryEventStore {
    private events: Map<string, { streamId: string; message: JSONRPCMessage }> = new Map();

    private generateEventId(streamId: string): string {
        return `${streamId}_${Date.now()}_${Math.random().toString(36).slice(2, 10)}`;
    }

    private getStreamIdFromEventId(eventId: string): string {
        const parts = eventId.split('_');
        return parts.length > 0 ? parts[0]! : '';
    }

    async storeEvent(streamId: string, message: JSONRPCMessage): Promise<string> {
        const eventId = this.generateEventId(streamId);
        this.events.set(eventId, { streamId, message });
        return eventId;
    }

    async replayEventsAfter(
        lastEventId: string,
        { send }: { send: (eventId: string, message: JSONRPCMessage) => Promise<void> }
    ): Promise<string> {
        if (!lastEventId || !this.events.has(lastEventId)) return '';
        const streamId = this.getStreamIdFromEventId(lastEventId);
        if (!streamId) return '';   // <-- always hits here for _GET_stream
        let foundLastEvent = false;
        const sortedEvents = [...this.events.entries()].toSorted((a, b) => a[0].localeCompare(b[0]));
        for (const [eventId, { streamId: eventStreamId, message }] of sortedEvents) {
            if (eventStreamId !== streamId) continue;
            if (eventId === lastEventId) { foundLastEvent = true; continue; }
            if (foundLastEvent) await send(eventId, message);
        }
        return streamId;
    }
}

async function main() {
    const store = new InMemoryEventStore();
    const streamId = '_GET_stream'; // packages/server/src/server/streamableHttp.ts:234
    const eventId1 = await store.storeEvent(streamId, { jsonrpc: '2.0', id: 1, method: 'ping' });
    const eventId2 = await store.storeEvent(streamId, { jsonrpc: '2.0', id: 2, method: 'pong' });

    console.log('eventId1:', eventId1);
    const parts = eventId1.split('_');
    console.log('split result:', parts);
    console.log('parts[0]:', JSON.stringify(parts[0]));  // ""

    const replayed: JSONRPCMessage[] = [];
    const returned = await store.replayEventsAfter(eventId1, {
        send: async (_, msg) => { replayed.push(msg); }
    });
    console.log('returned streamId:', JSON.stringify(returned));  // ""
    console.log('events replayed:', replayed.length);             // 0
}
main();

Command and output

$ npx tsx repro.ts
streamId used: _GET_stream
eventId1: _GET_stream_1776385498370_e2n92y3y
eventId2: _GET_stream_1776385498370_tsuxnf6s

split("_") of eventId1: [ '', 'GET', 'stream', '1776385498370', 'e2n92y3y' ]
parts[0] (extracted streamId): ""
=> extracted streamId is empty string: true

replayEventsAfter result:
  returned streamId: ""
  events replayed: 0

[REPRODUCED] Bug confirmed: replayEventsAfter returns empty string and replays nothing
  Expected: 1 event replayed (msg2), returned streamId="_GET_stream"

Code path

• packages/server/src/server/streamableHttp.ts:234 — _standaloneSseStreamId = '_GET_stream'
• examples/server/src/inMemoryEventStore.ts:15 — generates _GET_stream_<ts>_<rand>
• examples/server/src/inMemoryEventStore.ts:22 — splits on _: ['', 'GET', 'stream', ...]
• examples/server/src/inMemoryEventStore.ts:23 — returns parts[0] = ''
• examples/server/src/inMemoryEventStore.ts:50-52 — if (!streamId) return '' fires, aborting replay

present on both main (b8886e7) and origin/v1.x (bf1e022).

suggested fix

// examples/server/src/inMemoryEventStore.ts
     private getStreamIdFromEventId(eventId: string): string {
-        const parts = eventId.split('_');
-        return parts.length > 0 ? parts[0]! : '';
+        return this.events.get(eventId)?.streamId ?? '';
     }

// test/integration/test/taskResumability.test.ts
-        const streamId = lastEventId.split('_')[0] ?? '';
+        const streamId = this.events.get(lastEventId)!.streamId;

test to verify: stores two events under _GET_stream, calls replayEventsAfter on the first event ID, and asserts the returned stream ID equals '_GET_stream' and exactly one event was replayed.

[ifandelse]

I would be willing to look into contributing to fix this issue, but checking here with the team first as to not waste anyone's time. Is this up for grabs?