diff --git a/.changeset/web-session-subscription-cap.md b/.changeset/web-session-subscription-cap.md
new file mode 100644
index 0000000000..bf293e41ae
--- /dev/null
+++ b/.changeset/web-session-subscription-cap.md
@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Fix the web UI becoming sluggish after opening many sessions.
diff --git a/apps/kimi-web/src/composables/client/useWorkspaceState.ts b/apps/kimi-web/src/composables/client/useWorkspaceState.ts
index 033fe1f68f..2bfce976ef 100644
--- a/apps/kimi-web/src/composables/client/useWorkspaceState.ts
+++ b/apps/kimi-web/src/composables/client/useWorkspaceState.ts
@@ -118,7 +118,7 @@ export interface UseWorkspaceStateDeps {
   nextOptimisticMsgId: () => string;
   getEventConn: () => KimiEventConnection | null;
   syncSessionFromSnapshot: (sessionId: string) => Promise<SyncSessionResult>;
-  subscribeToSessionEvents: (sessionId: string) => void;
+  reopenSession: (sessionId: string) => Promise<SyncSessionResult>;
   hasLoadedMessages: (sessionId: string) => boolean;
   refreshSessionStatus: (sessionId: string) => Promise<void>;
   persistSessionProfile: (patch: PersistSessionProfilePatch) => void;
@@ -166,7 +166,7 @@ export function useWorkspaceState(rawState: ExtendedState, deps: UseWorkspaceSta
     nextOptimisticMsgId,
     getEventConn,
     syncSessionFromSnapshot,
-    subscribeToSessionEvents,
+    reopenSession,
     hasLoadedMessages,
     refreshSessionStatus,
     persistSessionProfile,
@@ -959,9 +959,12 @@ export function useWorkspaceState(rawState: ExtendedState, deps: UseWorkspaceSta
         const result = await syncSessionFromSnapshot(sessionId);
         if (result === 'not-found') return;
       } else {
-        // Re-open: resume from the tracked cursor; the daemon replays any
-        // missed durable events (or answers resync_required → snapshot).
-        subscribeToSessionEvents(sessionId);
+        // Re-open: if the session was evicted from the subscription cap since
+        // last open, reopenSession rebuilds it from a snapshot (the kept cursor
+        // may have skipped per-session events); otherwise it re-subscribes from
+        // the tracked cursor and the daemon replays any missed durable events.
+        const result = await reopenSession(sessionId);
+        if (result === 'not-found') return;
       }
 
       // Refresh sidecars AFTER the snapshot settles so status/usage updates
diff --git a/apps/kimi-web/src/composables/useKimiWebClient.ts b/apps/kimi-web/src/composables/useKimiWebClient.ts
index 5791758c49..2bf6428f32 100644
--- a/apps/kimi-web/src/composables/useKimiWebClient.ts
+++ b/apps/kimi-web/src/composables/useKimiWebClient.ts
@@ -503,6 +503,7 @@ function forgetSession(sessionId: string): void {
   // buffered event for this id would otherwise be reduced and recreate the very
   // per-session maps we are about to delete.
   eventConn?.unsubscribe(sessionId);
+  dropWsSubscription(sessionId);
   // Drain the streaming-event batcher too. unsubscribe() stops future server
   // frames, but events already queued for the next animation frame would
   // otherwise survive and be reduced AFTER the maps below are cleared —
@@ -1158,7 +1159,9 @@ async function syncSessionFromSnapshot(sessionId: string): Promise<SyncSessionRe
       // before live deltas (aligned by wire offset) start appending to it.
       eventConn.seedSnapshot(sessionId, snap);
       eventConn.subscribe(sessionId, { seq: snap.asOfSeq, epoch: snap.epoch });
+      retainWsSubscription(sessionId);
     }
+    sessionsWithStaleCursor.delete(sessionId);
     void pullSessionWarnings(sessionId);
     return 'ok';
   } catch (err) {
@@ -1181,6 +1184,65 @@ function hasLoadedMessages(sessionId: string): boolean {
   return Object.prototype.hasOwnProperty.call(rawState.messagesBySession, sessionId);
 }
 
+// ---------------------------------------------------------------------------
+// WS subscription cap (LRU eviction)
+// ---------------------------------------------------------------------------
+//
+// Every opened session subscribes to its WS event stream, and the socket keeps
+// subscriptions across reconnects (re-sending them in `client_hello`). Without
+// a cap, a user who has opened hundreds of sessions stays subscribed to all of
+// them: every background session's status/meta/usage event then flows through
+// the reducer and dirties the sidebar computeds — the root cause of "the UI
+// gets sluggish once I have a lot of sessions".
+//
+// Keep only the most-recently-opened sessions subscribed (MRU order, index 0 =
+// newest). The active session is always retained.
+//
+// Eviction drops the live WS subscription but keeps the session's cursor so a
+// quick re-open can resume cheaply. However, a cursor kept across an eviction
+// can go stale: some session events (`event.session.status_changed`,
+// `session.meta.updated`, ...) are broadcast to EVERY connection (see
+// `isGlobalSessionEvent` on the server) and still advance `lastSeqBySession`
+// for an unsubscribed session. If a session emits per-session durable events
+// while evicted and then a global event, the cursor jumps past the missed
+// events. Evicted sessions are therefore tracked in `sessionsWithStaleCursor`;
+// when one is re-opened we rebuild from a snapshot (see `reopenSession`) rather
+// than resume from a cursor that may have skipped events.
+const MAX_WS_SUBSCRIPTIONS = 4;
+const wsSubscriptionOrder: string[] = [];
+const sessionsWithStaleCursor = new Set<string>();
+
+function retainWsSubscription(sessionId: string): void {
+  const idx = wsSubscriptionOrder.indexOf(sessionId);
+  if (idx !== -1) wsSubscriptionOrder.splice(idx, 1);
+  wsSubscriptionOrder.unshift(sessionId);
+  // Evict the oldest entries past the cap, skipping the active session. The
+  // active session is NOT guaranteed to sit at the front: first-time opens only
+  // retain after an awaited snapshot, so rapid clicks can complete out of order
+  // and leave the active session at the tail. Skipping it (rather than breaking
+  // when the tail is active) keeps the cap effective.
+  while (wsSubscriptionOrder.length > MAX_WS_SUBSCRIPTIONS) {
+    let victimIdx = -1;
+    for (let i = wsSubscriptionOrder.length - 1; i >= 0; i--) {
+      if (wsSubscriptionOrder[i] !== rawState.activeSessionId) {
+        victimIdx = i;
+        break;
+      }
+    }
+    if (victimIdx === -1) break;
+    const [victim] = wsSubscriptionOrder.splice(victimIdx, 1);
+    if (victim === undefined) break;
+    eventConn?.unsubscribe(victim);
+    sessionsWithStaleCursor.add(victim);
+  }
+}
+
+function dropWsSubscription(sessionId: string): void {
+  const idx = wsSubscriptionOrder.indexOf(sessionId);
+  if (idx !== -1) wsSubscriptionOrder.splice(idx, 1);
+  sessionsWithStaleCursor.delete(sessionId);
+}
+
 function subscribeToSessionEvents(sessionId: string): void {
   connectEventsIfNeeded();
   if (eventConn) {
@@ -1192,7 +1254,27 @@ function subscribeToSessionEvents(sessionId: string): void {
     const seq = rawState.lastSeqBySession[sessionId] ?? 0;
     const epoch = epochBySession[sessionId];
     eventConn.subscribe(sessionId, { seq, epoch });
+    retainWsSubscription(sessionId);
+  }
+}
+
+/** Re-open an already-loaded session. If it was evicted from the subscription
+ *  cap since it was last open, rebuild from a snapshot: resuming from the kept
+ *  cursor would skip the per-session events that arrived while unsubscribed,
+ *  and replaying from seq 0 would make the projector regenerate message ids and
+ *  duplicate the already-loaded transcript. Otherwise just re-subscribe from the
+ *  tracked cursor.
+ *
+ * The stale marker is only read here; `syncSessionFromSnapshot` clears it once
+ * the snapshot succeeds. If the snapshot fails transiently the marker stays, so
+ * the next re-open retries the snapshot instead of falling back to a cursor
+ * that may have skipped events while unsubscribed. */
+async function reopenSession(sessionId: string): Promise<SyncSessionResult> {
+  if (sessionsWithStaleCursor.has(sessionId)) {
+    return syncSessionFromSnapshot(sessionId);
   }
+  subscribeToSessionEvents(sessionId);
+  return 'ok';
 }
 
 // ---------------------------------------------------------------------------
@@ -2120,7 +2202,7 @@ const workspaceState = useWorkspaceState(rawState, {
   nextOptimisticMsgId,
   getEventConn: () => eventConn,
   syncSessionFromSnapshot,
-  subscribeToSessionEvents,
+  reopenSession,
   hasLoadedMessages,
   refreshSessionStatus,
   persistSessionProfile,