Add public subscriptions/listen server handler (SEP-2575) - #1775
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July 29, 2026 14:18
Adds an opt-in McpServerHandlers.SubscriptionsListenHandler and a WithSubscriptionsListenHandler builder method so server authors can own the long-lived subscriptions/listen stream for custom subscription kinds, application-driven notifications, and stateless Streamable HTTP delivery over the held-open POST response. Fixes modelcontextprotocol#1662 Copilot-Session: 80653343-5dcb-43cb-89b0-8ac8e572c4f7
The client dispatches incoming messages concurrently, so cross-notification arrival order is not observable at the handler level. Assert only that the acknowledgement and streamed notification each arrive tagged with the subscription id, and prove fan-out suppression via the synchronous empty-fan-out plus a completed-and-empty channel check. Copilot-Session: 80653343-5dcb-43cb-89b0-8ac8e572c4f7
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Fixes #1662
Adds an opt-in, typed
subscriptions/listenserver handler so server authors can own the long-lived subscription stream directly.What
McpServerHandlers.SubscriptionsListenHandlerhandler slot.WithSubscriptionsListenHandler(...)builder method.When set, the handler is a full replacement for the built-in
subscriptions/listenhandling: it owns the stream, sends the acknowledgement, tags notifications with the subscription id, and receives no automatic*/list_changedfan-out. It runs through the same request pipeline as other typed handlers, so it can stream notifications over the request response stream even under stateless Streamable HTTP, where the held-open POST is the only solicited server-to-client channel. Protocol-version gating (2026-07-28+) is enforced by the SDK. No server capabilities are auto-advertised, so authors declare only what their handler delivers.Tests
Acknowledgement-first ordering and subscription-id tagging, replacement suppressing automatic list-changed fan-out, pre-2026-07-28 rejection, cancellation cleanup, and stateless streaming over the held-open POST. Full core suite passes.