60 lines
2.8 KiB
Markdown
60 lines
2.8 KiB
Markdown
# subscriptions
|
|
|
|
Server-originated change notifications on the 2026-07-28 protocol. A client
|
|
opens one `subscriptions/listen` request whose response **is** the stream; the
|
|
server publishes with `ctx.notify_resource_updated(uri)` /
|
|
`ctx.notify_tools_changed()` and the SDK does the wire work (ack-first,
|
|
per-stream filtering, subscription-id tagging). Replaces the handshake-era
|
|
`resources/subscribe` + standalone-GET notification path.
|
|
|
|
The client edits a note it did not subscribe to (silence), edits the one it
|
|
did (a tagged `notifications/resources/updated`), registers a tool at runtime
|
|
(`notifications/tools/list_changed`, then re-lists and calls it), and finally
|
|
stops listening - cancelling the parked request releases the local task, and
|
|
closing the connection ends the stream server-side.
|
|
|
|
## Run it
|
|
|
|
```bash
|
|
# HTTP — the client self-hosts the server on a free port, runs, then tears it
|
|
# down (subscriptions/listen is 2026-era only)
|
|
uv run python -m stories.subscriptions.client --http
|
|
# same, against the lowlevel-API server variant
|
|
uv run python -m stories.subscriptions.client --http --server server_lowlevel
|
|
```
|
|
|
|
## What to look at
|
|
|
|
- `client.py` — stream frames arrive as ordinary server notifications via the
|
|
constructor-only `message_handler=`. There is no client-side listen API yet,
|
|
so opening the stream drops to the `client.session` escape hatch; the request
|
|
parks for the stream's lifetime. Cancelling it releases the local task; over
|
|
HTTP the server-side stream ends when the connection closes. Every frame's
|
|
`_meta["io.modelcontextprotocol/subscriptionId"]` is the listen request's
|
|
JSON-RPC id.
|
|
- `server.py` — publishing is one `await ctx.notify_*()` line per change; the
|
|
filter, the tagging, and the ack ordering are the SDK's job. Publishing with
|
|
no subscribers is a no-op.
|
|
- `server_lowlevel.py` — the same machinery held by hand: an
|
|
`InMemorySubscriptionBus`, handlers that `await bus.publish(...)`, and
|
|
`ListenHandler(bus)` passed as `on_subscriptions_listen=`. A multi-replica
|
|
deployment swaps the bus for one backed by its own pub/sub
|
|
(`MCPServer(subscriptions=...)` on the high-level server).
|
|
|
|
## Caveats
|
|
|
|
- 2026-era only: on a 2025 connection the method does not exist (clients there
|
|
use `resources/subscribe` and unsolicited notifications instead), so the
|
|
story pins the modern era and has no legacy leg.
|
|
- No replay: events published while no stream is open are not queued. The
|
|
contract after a dropped stream is re-listen and re-fetch.
|
|
|
|
## Spec
|
|
|
|
[Subscriptions — basic utilities](https://modelcontextprotocol.io/specification/draft/basic/utilities/subscriptions)
|
|
|
|
## See also
|
|
|
|
`streaming/` (request-scoped notifications), `events/` (the events extension
|
|
on top of this channel, deferred), and `docs/advanced/subscriptions.md` (the
|
|
narrative version).
|