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

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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
1 changed files with 321 additions and 28 deletions
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116
core/mcp/inspectorClient.ts
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@ -479,7 +479,10 @@ export class InspectorClient extends InspectorClientEventTarget {
// `McpSubscription` whose filter's `resourceSubscriptions` mirrors
// `subscribedResources`; mutating the set re-lists (close old, open new), and
// an unexpected `"remote"` close re-lists (reconnect-by-re-listen — the stream
// is not resumable). `null` when no URI is subscribed or on the legacy era.
// is not resumable). `null` on the legacy era, and on the modern era whenever
// the filter is empty — which is *not* the same as "no URI subscribed": the
// stream also carries the list-change opt-ins, so a tools-only server opens it
// with no subscriptions at all (#1920).
private modernSubscription: McpSubscription | null = null;
// Monotonic guard so a stale re-list/reconnect (whose `listen()` or `closed`
// resolves after a newer refresh already started) can detect it lost the race
@ -2056,6 +2059,14 @@ export class InspectorClient extends InspectorClientEventTarget {
// a progress notification handler so the Protocol's _onprogress stays; timeout reset
// and routing work, and we inject the caller's progressToken into dispatched events.
}
// Modern era: the handlers registered above only fire for notifications
// that reach us, and on this era every server→client notification rides
// the `subscriptions/listen` stream. Open it now when the filter says
// there is something to listen for — otherwise a list-change opt-in on a
// server with no resources would have no way in, since a subscribe click
// was the only thing that ever opened the stream (#1920).
await this.openModernListenStreamOnConnect();
} catch (error) {
if (!isConnectAuthRecoveryError(error)) {
this.status = "error";
@ -4893,9 +4904,13 @@ export class InspectorClient extends InspectorClientEventTarget {
* opted-in notification type (SEP §7.4).
*/
private buildSubscriptionFilter(): SubscriptionFilter {
const filter: SubscriptionFilter = {
resourceSubscriptions: Array.from(this.subscribedResources),
};
const filter: SubscriptionFilter = {};
// Omitted rather than sent empty: a listChanged-only stream (#1920) is not
// subscribing to any resource, and `[]` would say it asked for none of a set
// it is participating in.
if (this.subscribedResources.size > 0) {
filter.resourceSubscriptions = Array.from(this.subscribedResources);
}
if (
this.listChangedNotifications.tools &&
this.capabilities?.tools?.listChanged
@ -4917,6 +4932,69 @@ export class InspectorClient extends InspectorClientEventTarget {
return filter;
}
/**
* Whether the modern listen stream should be open: the built filter carries
* something to listen for (#1920). Before #1920 this was "at least one URI is
* subscribed", which made the stream unreachable on a server with no resources
* — a tools-only server advertising `tools.listChanged` had no way to open it,
* so `notifications/tools/list_changed` could never arrive. The filter already
* modelled the list-change opt-ins; only the trigger was narrower than the
* filter it built. This matches the SDK's own `ClientOptions.listChanged`
* auto-open, which opens whenever the effective (config ∩ capability)
* intersection is non-empty.
*
* Note this is deliberately *not* the same predicate as the stream state's
* `active` — see `modernStreamActive()`.
*/
private wantsModernStream(): boolean {
const filter = this.buildSubscriptionFilter();
return (
(filter.resourceSubscriptions?.length ?? 0) > 0 ||
filter.toolsListChanged === true ||
filter.resourcesListChanged === true ||
filter.promptsListChanged === true
);
}
/**
* Whether the stream state reports `active` — i.e. whether the *Subscriptions*
* UI has a stream to describe. That is a narrower question than
* `wantsModernStream()`: `ResourceSubscriptionStreamState` drives the
* Subscriptions section's badge, so a stream open purely for list-change
* notifications (no subscribed URI) has nothing to report there and stays
* `active: false` (#1920). Keeping the two apart also preserves the invariant
* the rest of this file is written against — an empty subscribed set is never
* announced alongside an `active` stream.
*/
private modernStreamActive(): boolean {
return this.subscribedResources.size > 0;
}
/**
* Open the modern listen stream at the end of a successful connect, when the
* filter is non-empty (#1920). Only the list-change opt-ins can make it
* non-empty here — the subscribed set is emptied by `resetSessionState()` on
* the way in — so this is exactly the "server advertises a listChanged the
* Inspector wants" case that had no trigger before.
*
* A failure is not allowed to fail the connect: the handshake succeeded and
* every request-scoped feature works without this stream. It is reported the
* way a lost stream is — hand it to the reconnect machinery, which retries
* with backoff and settles on `"ended"` past the cap.
*/
private async openModernListenStreamOnConnect(): Promise<void> {
if (!this.isModernEra() || !this.wantsModernStream()) return;
try {
await this.refreshModernSubscription();
} catch (error) {
this.logger.error(
{ error },
"Failed to open the modern subscriptions/listen stream on connect",
);
this.reconcileModernStreamStateAfterFailedRefresh();
}
}
/** Cancel a pending reconnect re-listen, if any (#1630). */
private clearModernReconnectTimer(): void {
if (this.modernReconnectTimer !== undefined) {
@ -4927,9 +5005,10 @@ export class InspectorClient extends InspectorClientEventTarget {
/**
* (Re-)establish the modern `subscriptions/listen` stream to match the current
* `subscribedResources` set (#1630). Because the stream is not resumable,
* every filter change re-lists: the existing stream is closed and a fresh
* `listen()` opened. With no subscribed URIs the stream is left closed.
* filter (#1630). Because the stream is not resumable, every filter change
* re-lists: the existing stream is closed and a fresh `listen()` opened. With
* an empty filter — no subscribed URIs *and* no enabled list-change opt-in the
* server advertises — the stream is left closed (#1920).
*
* `modernListenGeneration` guards against races — if a newer refresh starts
* while this one awaits its acknowledgement, the just-opened stream is
@ -4955,8 +5034,8 @@ export class InspectorClient extends InspectorClientEventTarget {
await closeSubscriptionBestEffort(previous);
}
// Nothing subscribed → keep the stream closed.
if (this.subscribedResources.size === 0) {
// Nothing to listen for → keep the stream closed.
if (!this.wantsModernStream()) {
this.setModernStreamState(INACTIVE_SUBSCRIPTION_STREAM_STATE);
return;
}
@ -4978,7 +5057,7 @@ export class InspectorClient extends InspectorClientEventTarget {
// starts fresh next time (#1630).
this.modernReconnectAttempts = 0;
this.setModernStreamState({
active: true,
active: this.modernStreamActive(),
status: "acknowledged",
honoredUris: subscription.honoredFilter.resourceSubscriptions ?? [],
});
@ -5024,7 +5103,7 @@ export class InspectorClient extends InspectorClientEventTarget {
const shouldReconnect =
reason === "remote" &&
!isTerminalStatus(this.status) &&
this.subscribedResources.size > 0;
this.wantsModernStream();
if (!shouldReconnect) {
// "stream gone but subscriptions remain" renders the same whether we gave
// up after failed reconnects or the server closed it gracefully: keep the
@ -5033,7 +5112,7 @@ export class InspectorClient extends InspectorClientEventTarget {
// until the next `connect()` calls `resetSubscriptionStream`, which moves
// them together — so "active with an empty set" is never observable.)
this.setModernStreamState({
active: this.subscribedResources.size > 0,
active: this.modernStreamActive(),
status: "ended",
honoredUris: [],
});
@ -5073,7 +5152,7 @@ export class InspectorClient extends InspectorClientEventTarget {
* its error either way — the retry is about the subscriptions, not the call.
*/
private reconcileModernStreamStateAfterFailedRefresh(): void {
if (this.subscribedResources.size === 0) {
if (!this.wantsModernStream()) {
this.setModernStreamState(INACTIVE_SUBSCRIPTION_STREAM_STATE);
return;
}
@ -5088,7 +5167,7 @@ export class InspectorClient extends InspectorClientEventTarget {
*/
private scheduleModernReconnect(): void {
this.setModernStreamState({
active: true,
active: this.modernStreamActive(),
status: "reconnecting",
honoredUris: [],
});
@ -5101,10 +5180,7 @@ export class InspectorClient extends InspectorClientEventTarget {
this.modernReconnectTimer = undefined;
// Disconnect/unsubscribe may have raced the timer — bail if the reconnect
// is no longer wanted.
if (
isTerminalStatus(this.status) ||
this.subscribedResources.size === 0
) {
if (isTerminalStatus(this.status) || !this.wantsModernStream()) {
return;
}
this.refreshModernSubscription(true).catch(() =>
@ -5123,10 +5199,10 @@ export class InspectorClient extends InspectorClientEventTarget {
if (
this.modernReconnectAttempts > MODERN_RECONNECT_MAX_ATTEMPTS ||
isTerminalStatus(this.status) ||
this.subscribedResources.size === 0
!this.wantsModernStream()
) {
this.setModernStreamState({
active: this.subscribedResources.size > 0,
active: this.modernStreamActive(),
status: "ended",
honoredUris: [],
});