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fix: open the modern listen stream whenever the filter is non-empty (#1920)

Browse source 
On the modern (2026-07-28) era every server→client notification rides the
`subscriptions/listen` stream, but the Inspector only ever opened that stream
from a resource-subscribe click. A tools-only server advertising
`tools.listChanged` therefore had no path to an open stream, so
`notifications/tools/list_changed` could never arrive — even though
`buildSubscriptionFilter()` already modelled the opt-in.

The trigger now matches the filter it builds: the stream opens (and stays open,
and reconnects) whenever the built filter carries anything — subscribed URIs or
an enabled list-change opt-in the server advertises. That mirrors the SDK's own
`ClientOptions.listChanged` auto-open, which opens on a non-empty config ∩
capability intersection. `connect()` opens it once the handlers are registered;
a failure there is handed to the reconnect machinery rather than failing the
connect.

`ResourceSubscriptionStreamState.active` deliberately keeps its narrower
meaning — "at least one URI is subscribed" — because it drives the Subscriptions
section's badge, which has nothing to report for a listChanged-only stream. That
also preserves the invariant the rest of the file is written against: an empty
subscribed set is never announced alongside an `active` stream.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HDdo1rNRVQnVRRdsqSrvG3
This commit is contained in:
cliffhall 2026-08-10 23:25:33 -04:00
commit 0325d81493
2 changed files with 321 additions and 28 deletions
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4
clients/web/src/components/groups/ResourceControls/ResourceControls.tsx
View file

@ -52,7 +52,9 @@ export interface ResourceControlsProps {
* Modern-era `subscriptions/listen` stream state (#1630). When `active`
* (modern era with at least one subscription) the Subscriptions section shows
* a stream-status badge in its panel and a status dot in its header. Legacy
* connections pass `active: false` (or omit it) and see neither.
* connections pass `active: false` (or omit it) and see neither — and so does
* a stream open purely for list-change notifications, which this section has
* nothing to say about (#1920).
*/
subscriptionStreamState?: ResourceSubscriptionStreamState;
/** Negotiated protocol era; gates the modern subscription stream chrome. */

218
clients/web/src/test/integration/mcp/inspectorClient-subscriptions-era.test.ts
View file

@ -8,6 +8,7 @@ import {
type TestServerHttp,
createTestServerInfo,
createNumberedResources,
createEchoTool,
} from "@modelcontextprotocol/inspector-test-server";
import type { ServerConfig } from "@modelcontextprotocol/inspector-test-server";
import type { MessageEntry } from "@inspector/core/mcp/types.js";
@ -23,6 +24,13 @@ describe("resource subscriptions era fork (#1630)", () => {
let server: TestServerHttp | null = null;
const RESOURCE_URI = "test://resource_0";
/**
* Every list-change opt-in off. The listen filter is config ∩ capability and
* the SDK server advertises `listChanged` for every list it registers, so this
* is what leaves the filter empty when nothing is subscribed — i.e. the
* pre-#1920 world these lifecycle tests were written against.
*/
const NO_LIST_CHANGED = { tools: false, resources: false, prompts: false };
const RESOURCE_URI_2 = "test://resource_1";
afterEach(async () => {
@ -59,15 +67,28 @@ describe("resource subscriptions era fork (#1630)", () => {
return started;
}
/**
* @param listChangedNotifications the Inspector-side opt-ins. The SDK server
* advertises `listChanged` for every list it registers, so this — not the
* server config — is how a test gets an *empty* listen filter: the filter is
* config ∩ capability, and with every opt-in off only a subscribed URI can
* open the stream (#1920).
*/
async function connect(
url: string,
era: "legacy" | "modern",
listChangedNotifications?: {
tools?: boolean;
resources?: boolean;
prompts?: boolean;
},
): Promise<{ connected: InspectorClient; messages: MessageEntry[] }> {
const connected = new InspectorClient(
{ type: "streamable-http", url },
{
environment: { transport: createTransportNode },
versionNegotiation: eraToVersionNegotiation(era),
...(listChangedNotifications ? { listChangedNotifications } : {}),
},
);
const messages: MessageEntry[] = [];
@ -125,15 +146,43 @@ describe("resource subscriptions era fork (#1630)", () => {
});
it("closes the stream when the last subscription is removed", async () => {
// Nothing else in the filter (no list-change opt-ins), so the last URI
// leaving empties it and the stream closes (#1920).
const started = await startServer({});
const { connected } = await connect(started.url, "modern");
const { connected, messages } = await connect(
started.url,
"modern",
NO_LIST_CHANGED,
);
await connected.subscribeToResource(RESOURCE_URI);
expect(connected.getResourceSubscriptionStreamState().active).toBe(true);
messages.length = 0;
await connected.unsubscribeFromResource(RESOURCE_URI);
const streamState = connected.getResourceSubscriptionStreamState();
expect(streamState.active).toBe(false);
expect(connected.getSubscribedResources()).toEqual([]);
// No re-listen: there is nothing left to listen for.
expect(methodsSent(messages)).not.toContain("subscriptions/listen");
});
it("keeps the stream open past the last subscription when a listChanged opt-in remains", async () => {
// The other half of the above: with an advertised listChanged the filter
// is still non-empty, so the last unsubscribe re-lists rather than
// closing — and the state goes inactive because the *Subscriptions*
// section has nothing to report, not because the stream is gone (#1920).
const started = await startServer({});
const { connected, messages } = await connect(started.url, "modern");
await connected.subscribeToResource(RESOURCE_URI);
messages.length = 0;
await connected.unsubscribeFromResource(RESOURCE_URI);
expect(connected.getSubscribedResources()).toEqual([]);
expect(connected.getResourceSubscriptionStreamState().active).toBe(false);
const filter = lastListenFilter(messages);
expect(filter?.resourcesListChanged).toBe(true);
expect(filter?.resourceSubscriptions).toBeUndefined();
});
it("re-lists (stream stays open) when one of several URIs is removed", async () => {
@ -197,6 +246,145 @@ describe("resource subscriptions era fork (#1630)", () => {
});
});
// The stream used to be reachable only by subscribing to a resource, so a
// server with no resources could never open it — and its `tools.listChanged`
// notifications, which ride the same stream, could never arrive (#1920).
describe("modern era: opening the stream for listChanged alone (#1920)", () => {
async function startToolsOnlyServer(
listChanged: ServerConfig["listChanged"],
): Promise<TestServerHttp> {
const started = createTestServerHttp({
serverInfo: createTestServerInfo("tools-only-test", "1.0.0"),
tools: [createEchoTool()],
listChanged,
modern: {},
});
await started.start();
server = started;
return started;
}
it("opens the stream on connect against a tools-only server", async () => {
const started = await startToolsOnlyServer({ tools: true });
const { connected, messages } = await connect(started.url, "modern");
expect(connected.getCapabilities()?.resources).toBeUndefined();
expect(methodsSent(messages)).toContain("subscriptions/listen");
const filter = lastListenFilter(messages);
expect(filter?.toolsListChanged).toBe(true);
// Nothing is subscribed, so the filter carries no URIs at all.
expect(filter?.resourceSubscriptions).toBeUndefined();
// …and the Subscriptions section still has nothing to report.
expect(connected.getSubscribedResources()).toEqual([]);
expect(connected.getResourceSubscriptionStreamState().active).toBe(false);
});
it("leaves the stream closed when the opt-in is disabled in config", async () => {
// The filter is config ∩ capability, so turning the handler off in the
// Inspector's own options is enough to keep the stream closed.
const started = await startToolsOnlyServer({ tools: true });
const connected = new InspectorClient(
{ type: "streamable-http", url: started.url },
{
environment: { transport: createTransportNode },
versionNegotiation: eraToVersionNegotiation("modern"),
listChangedNotifications: { tools: false },
},
);
const messages: MessageEntry[] = [];
connected.addEventListener("message", (event) => {
messages.push(event.detail);
});
await connected.connect();
client = connected;
expect(methodsSent(messages)).not.toContain("subscriptions/listen");
});
it("opens no stream on the legacy era", async () => {
// Legacy has no `subscriptions/listen` at all — list-change notifications
// arrive on the session's own channel.
const started = createTestServerHttp({
serverInfo: createTestServerInfo("tools-only-legacy-test", "1.0.0"),
tools: [createEchoTool()],
listChanged: { tools: true },
});
await started.start();
server = started;
const { connected, messages } = await connect(started.url, "legacy");
expect(connected.getProtocolEra()).toBe("legacy");
expect(methodsSent(messages)).not.toContain("subscriptions/listen");
expect(connected.getResourceSubscriptionStreamState().active).toBe(false);
});
it("reconnects a dropped listChanged-only stream (no subscriptions to keep it alive)", async () => {
const started = await startToolsOnlyServer({ tools: true });
const { connected } = await connect(started.url, "modern");
const int = connected as unknown as {
modernSubscription: McpSubscription | null;
modernListenGeneration: number;
onModernSubscriptionClosed(
subscription: McpSubscription,
reason: "local" | "graceful" | "remote",
generation: number,
): void;
};
const dropped = int.modernSubscription;
expect(dropped).not.toBeNull();
if (!dropped) return;
int.onModernSubscriptionClosed(
dropped,
"remote",
int.modernListenGeneration,
);
// Reconnect-by-re-listen runs on the filter, not on the subscribed set —
// which is empty here, and used to be the reason to give up.
expect(connected.getResourceSubscriptionStreamState().status).toBe(
"reconnecting",
);
await vi.waitFor(() => {
expect(int.modernSubscription).not.toBeNull();
});
expect(connected.getResourceSubscriptionStreamState().status).toBe(
"acknowledged",
);
});
it("connects anyway when the connect-time listen fails, and retries", async () => {
// The handshake succeeded; every request-scoped feature works without the
// stream, so the failure is handed to the reconnect machinery instead of
// failing `connect()`.
const started = await startToolsOnlyServer({ tools: true });
const connected = new InspectorClient(
{ type: "streamable-http", url: started.url },
{
environment: { transport: createTransportNode },
versionNegotiation: eraToVersionNegotiation("modern"),
},
);
// Shadow the private refresh on the instance so the connect-time open
// fails without a live client to reach into (there is none until
// `connect()` builds one).
(
connected as unknown as {
refreshModernSubscription: () => Promise<void>;
}
).refreshModernSubscription = () =>
Promise.reject(new Error("listen boom"));
await expect(connected.connect()).resolves.toBeUndefined();
client = connected;
expect(connected.getStatus()).toBe("connected");
expect(connected.getResourceSubscriptionStreamState().status).toBe(
"reconnecting",
);
});
});
describe("legacy era", () => {
it("subscribes via resources/subscribe with no listen stream", async () => {
const started = await startServer(undefined);
@ -355,8 +543,17 @@ describe("resource subscriptions era fork (#1630)", () => {
// Reachable without any close() failure: a rejecting `listen()` does it,
// which is what a user subscribing while the reconnect timer re-lists
// (i.e. exactly when the server is flaky) can produce.
//
// No list-change opt-ins, so connect leaves the stream closed (#1920) and
// the first subscribe's `listen()` is reached synchronously — with a
// stream to tear down first, the swap below would land after this call
// already read `client.listen`.
const started = await startServer({});
const { connected } = await connect(started.url, "modern");
const { connected } = await connect(
started.url,
"modern",
NO_LIST_CHANGED,
);
const int = internals(connected);
const real = int.client.listen;
@ -504,8 +701,15 @@ describe("resource subscriptions era fork (#1630)", () => {
});
it("reflects the subscription optimistically as 'connecting' before the ack", async () => {
// No list-change opt-ins → no connect-time stream, so the subscribe's
// `listen()` is the first one and this test's held ack is the one it waits
// on (#1920).
const started = await startServer({});
const { connected } = await connect(started.url, "modern");
const { connected } = await connect(
started.url,
"modern",
NO_LIST_CHANGED,
);
const int = internals(connected);
// Hold the listen ack so we can observe the pre-ack (optimistic) state.
@ -632,8 +836,14 @@ describe("resource subscriptions era fork (#1630)", () => {
});
it("does not reconnect when the subscription set empties before the timer fires", async () => {
// No list-change opt-ins, so emptying the set empties the *filter* —
// which is what the timer's bail now tests (#1920).
const started = await startServer({});
const { connected } = await connect(started.url, "modern");
const { connected } = await connect(
started.url,
"modern",
NO_LIST_CHANGED,
);
await connected.subscribeToResource(RESOURCE_URI);
const int = internals(connected);
const fake = await installFakeSubscription(int);