Notification 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" }
  }
}

Transport Support

The stream is served on the Streamable HTTP modern path; stdio answers -32601.

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.