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The Subscriptions page documented the acknowledgement-first contract but not what delivery means:
the stream carries only changes that happen after the acknowledgement, nothing is queued while
no stream is connected, and a dropped stream is gone together with anything it missed.
The Python SDK's subscriptions page states the same contract ("It is not a replay log").
Spelling it out gives client implementers the fetch-after-subscribing guidance and
makes the acknowledgement-window behavior explicit.
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Subscriptions
subscriptions/listen is the long-lived notification subscription stream of MCP 2026-07-28 (SEP-2575),
replacing the legacy HTTP GET listening stream.
Subscribing
The client opts in via the notifications filter (toolsListChanged / promptsListChanged / resourcesListChanged / resourceSubscriptions),
the server acknowledges the honored subset with notifications/subscriptions/acknowledged as the first stream message,
and every delivered notification carries the correlating io.modelcontextprotocol/subscriptionId in _meta.
The honored subset follows the capabilities the server declares (listChanged and subscribe flags).
The client opens the stream with a subscriptions/listen request:
{
"jsonrpc": "2.0",
"id": "listen-1",
"method": "subscriptions/listen",
"params": {
"notifications": { "toolsListChanged": true, "resourceSubscriptions": ["file:///a.txt"] },
"_meta": {
"io.modelcontextprotocol/protocolVersion": "2026-07-28",
"io.modelcontextprotocol/clientInfo": { "name": "my_client", "version": "1.0.0" },
"io.modelcontextprotocol/clientCapabilities": {}
}
}
}
The first SSE event on the stream is the acknowledgement:
{
"jsonrpc": "2.0",
"method": "notifications/subscriptions/acknowledged",
"params": {
"notifications": { "toolsListChanged": true, "resourceSubscriptions": ["file:///a.txt"] },
"_meta": { "io.modelcontextprotocol/subscriptionId": "listen-1" }
}
}
Delivery Semantics
A stream delivers only the changes that happen after its acknowledgement; it is not a replay log. Nothing is queued while no stream is connected, and a dropped stream is gone together with anything it missed. Clients fetch the current state after subscribing and treat each notification as a cue to refetch, rather than as a payload.
Transport Support
The stream is served on the Streamable HTTP modern path; stdio answers -32601.
A host that cannot hold an SSE response open (such as the Rails controller pattern,
which buffers the body) declines the method with serve_subscriptions_listen: false, answering -32601 and dropping
the listChanged/subscribe flags from discovery.
Limits and Keepalives
Concurrent streams are capped by max_listen_subscriptions: (default 1000; pass nil to remove the cap), and each stream receives an SSE keepalive
comment frame every listen_keepalive_interval: seconds (default 15) so a dropped connection frees its slot; pass listen_keepalive_interval: nil
when an upstream proxy already keeps the stream alive.
transport = MCP::Server::Transports::StreamableHTTPTransport.new(
server,
max_listen_subscriptions: 500,
listen_keepalive_interval: 30,
)
A stream stays open until the client closes the connection; on graceful shutdown via transport.close,
each open stream receives its subscriptions/listen response before closing.
See Notifications for the notification types themselves and their session scoping.