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## Motivation and Context The documented Rails controller pattern builds a fresh transport per request and renders `body.first`, but a modern client's `subscriptions/listen` made `handle_request` return the streaming `Proc` body, so exactly the documented configuration answered every notification-stream request with a 500 - quietly, since tool calls and listings kept working. Nothing gated the route: `serves_subscriptions_listen?` was hardcoded `true` and consulted only by `Server#discover` for capability stripping, and the modern path deliberately ignores `stateless:`. `StreamableHTTPTransport.new` now accepts `serve_subscriptions_listen:` (default `true`, behavior unchanged). When `false`, the transport skips the listen interception so the method falls through the dispatcher as unimplemented - the spec's 404 with JSON-RPC `-32601`, in an Array body the controller pattern can render - and `serves_subscriptions_listen?` reflects the setting, so `Server#discover` stops advertising the `listChanged`/`subscribe` capability flags through its existing wiring and the advertisement agrees with the actual behavior. The documented controller example passes the flag and explains why; refusing implicitly in `stateless:` mode was rejected because a stateless transport mounted as a long-lived Rack app serves listen streams correctly (listen delivery deliberately precedes the stateless notification guard). On the default streaming side, the listen body is now a small streaming-body object rather than a bare `Proc`: it still responds to `call` and deliberately not to `each` (Rack would otherwise classify it as enumerable and break the streaming), while `first` raises an error naming `serve_subscriptions_listen: false` and pointing at the transports documentation, so a host that buffers the body the documented way fails with guidance instead of a bare `NoMethodError`. Fixes #531. ## How Has This Been Tested? With the issue's reproduction script (default: a streaming body whose `body.first` raises the guidance; with the flag: 404, `application/json`, and an Array body whose first element is the `-32601` error), new transport tests covering the refusal shape, the capability stripping on `server/discover`, and the guidance raise with the body's streaming classification, the full suite, RuboCop, and the conformance suite, all green; the docs build passes every internal link and anchor check. ## Breaking Changes None. The keyword defaults to the current behavior; refusal is opt-in.
76 lines
2.8 KiB
Markdown
76 lines
2.8 KiB
Markdown
---
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layout: default
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title: Subscriptions
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nav_order: 12
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redirect_from:
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- /server/notification-subscriptions/
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---
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# Subscriptions
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`subscriptions/listen` is the long-lived notification subscription stream of MCP 2026-07-28 (SEP-2575),
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replacing the legacy HTTP GET listening stream.
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## Subscribing
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The client opts in via the `notifications` filter (`toolsListChanged` / `promptsListChanged` / `resourcesListChanged` / `resourceSubscriptions`),
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the server acknowledges the honored subset with `notifications/subscriptions/acknowledged` as the first stream message,
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and every delivered notification carries the correlating `io.modelcontextprotocol/subscriptionId` in `_meta`.
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The honored subset follows the capabilities the server declares (`listChanged` and `subscribe` flags).
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The client opens the stream with a `subscriptions/listen` request:
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```json
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{
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"jsonrpc": "2.0",
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"id": "listen-1",
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"method": "subscriptions/listen",
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"params": {
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"notifications": { "toolsListChanged": true, "resourceSubscriptions": ["file:///a.txt"] },
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"_meta": {
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"io.modelcontextprotocol/protocolVersion": "2026-07-28",
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"io.modelcontextprotocol/clientInfo": { "name": "my_client", "version": "1.0.0" },
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"io.modelcontextprotocol/clientCapabilities": {}
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}
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}
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}
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```
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The first SSE event on the stream is the acknowledgement:
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```json
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{
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"jsonrpc": "2.0",
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"method": "notifications/subscriptions/acknowledged",
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"params": {
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"notifications": { "toolsListChanged": true, "resourceSubscriptions": ["file:///a.txt"] },
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"_meta": { "io.modelcontextprotocol/subscriptionId": "listen-1" }
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}
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}
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```
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## Transport Support
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The stream is served on the Streamable HTTP modern path; stdio answers `-32601`.
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A host that cannot hold an SSE response open (such as the [Rails controller pattern](/server/transports/#rails-controller),
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which buffers the body) declines the method with `serve_subscriptions_listen: false`, answering `-32601` and dropping
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the `listChanged`/`subscribe` flags from discovery.
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## Limits and Keepalives
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Concurrent streams are capped by `max_listen_subscriptions:` (default 1000; pass `nil` to remove the cap), and each stream receives an SSE keepalive
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comment frame every `listen_keepalive_interval:` seconds (default 15) so a dropped connection frees its slot; pass `listen_keepalive_interval: nil`
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when an upstream proxy already keeps the stream alive.
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```ruby
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transport = MCP::Server::Transports::StreamableHTTPTransport.new(
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server,
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max_listen_subscriptions: 500,
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listen_keepalive_interval: 30,
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)
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```
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A stream stays open until the client closes the connection; on graceful shutdown via `transport.close`,
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each open stream receives its `subscriptions/listen` response before closing.
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See [Notifications](/server/notifications/) for the notification types themselves and their session scoping.
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