The MCP subscriptions listen stream and notification delivery

Verified 2026-09-05 · 15 claims · sources tier 1

Also known as Last-Event-ID, subscriptions/listen, toolsListChanged.

The Model Context Protocol defines a subscriptions listen stream that a client opens to receive server notifications 9. Delivery requirements and transport handling across these streams are specified in the 2026-07-28 revision 13​12.

Notification delivery and filter negotiation

In the 2026-07-28 revision, a server that declared the list changed capability SHOULD send the tool list changed notification to clients that have opened a subscriptions listen stream with the tools list changed filter set to true 13. Delivery of a tool list changed notification to a subscribed client is defined as a SHOULD rather than a MUST 7. WarmTransfer's reading of the sources is that a client re-issuing the tools list request after a notification is optional rather than required, as the Tools page depicts it inside an optional fragment of its message flow diagram 6.

When negotiating subscription filters, a filter type the server does not support is omitted from the acknowledgment rather than raising an error, and the client-side check that would catch the omission is a SHOULD 8. Furthermore, the specification does not state what becomes of a notifiable event that occurs while no client holds a matching subscription; neither the subscriptions page, the streamable transport page, the tools page, nor the changelog addresses retention, queueing, or replay 14.

Transport behaviors

Transport requirements differ between streamable HTTP and standard input and output streams 10​12. For the standard input and output transport, a client MUST demultiplex incoming notifications by subscription identifier 12.

Under streamable HTTP, resumable server-sent event streams via the Last-Event-ID header are not supported in the 2026-07-28 revision 10. In addition, client reconnection after a dropped stream is graded MAY, and the specification publishes no backoff schedule, no retry cap, and no way to learn what was missed 11.

See also

See also Tool schema signature monitoring and drift handling. See also Model Context Protocol transports compared. See also Webex published MCP servers for external AI clients.

Applicability

Across the claims in this article, the evidence covers Model Context Protocol Project, verified as of 2026-09-05. The deployment model is not covered by any claim in this article. The claims in this article specifically apply to Model Context Protocol specification revision 2026-07-28 9​13​7​14​8​10​11​12.

What remains uncertain

The specification does not state what becomes of an event that occurs while no client holds a matching subscription, as retention, queueing, and replay are not addressed by any claim in this article 14. Whether any Webex MCP server implements subscriptions is not covered by any claim in this article. Whether a client can discover supported filters before subscribing is not covered by any claim in this article. The Discovery page new in this revision is not covered by any claim in this article. The specification Resources page is not covered by any claim in this article. The full list of subscribable event types is not covered by any claim in this article. How subscriptions differ between stdio and streamable HTTP beyond client demultiplexing, lack of Last-Event-ID support, and optional reconnection is not covered by any claim in this article. Whether the corpus extractor drops diagrams entirely is not covered by any claim in this article.

WarmTransfer. (2026, September 5). The MCP subscriptions listen stream and notification delivery. WarmTransfer. https://warmtransfer.net/knowledge/mcp-subscriptions-listen

@misc{warmtransfer-mcp-subscriptions-listen,
  title  = {The MCP subscriptions listen stream and notification delivery},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/mcp-subscriptions-listen},
  note   = {Verified 2026-09-05}
}