perf(core): fix OOM crash in long-running sessions (#19608)

Co-authored-by: Jacob Richman <jacob314@gmail.com>

packages/core/src/utils/events.ts

@@ -232,6 +232,7 @@ type EventBacklogItem = {
 
 export class CoreEventEmitter extends EventEmitter<CoreEvents> {
   private _eventBacklog: EventBacklogItem[] = [];
+  private _backlogHead = 0;
   private static readonly MAX_BACKLOG_SIZE = 10000;
 
   constructor() {
@@ -243,8 +244,17 @@ export class CoreEventEmitter extends EventEmitter<CoreEvents> {
     ...args: CoreEvents[K]
   ): void {
     if (this.listenerCount(event) === 0) {
-      if (this._eventBacklog.length >= CoreEventEmitter.MAX_BACKLOG_SIZE) {
-        this._eventBacklog.shift();
+      const backlogSize = this._eventBacklog.length - this._backlogHead;
+      if (backlogSize >= CoreEventEmitter.MAX_BACKLOG_SIZE) {
+        // Evict oldest entry. Use a head pointer instead of shift() to avoid
+        // O(n) array reindexing on every eviction at capacity.
+        (this._eventBacklog as unknown[])[this._backlogHead] = undefined;
+        this._backlogHead++;
+        // Compact once dead entries exceed half capacity to bound memory
+        if (this._backlogHead >= CoreEventEmitter.MAX_BACKLOG_SIZE / 2) {
+          this._eventBacklog = this._eventBacklog.slice(this._backlogHead);
+          this._backlogHead = 0;
+        }
       }
       // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
       this._eventBacklog.push({ event, args } as EventBacklogItem);
@@ -391,9 +401,13 @@ export class CoreEventEmitter extends EventEmitter<CoreEvents> {
    * subscribes.
    */
   drainBacklogs(): void {
-    const backlog = [...this._eventBacklog];
-    this._eventBacklog.length = 0; // Clear in-place
-    for (const item of backlog) {
+    const backlog = this._eventBacklog;
+    const head = this._backlogHead;
+    this._eventBacklog = [];
+    this._backlogHead = 0;
+    for (let i = head; i < backlog.length; i++) {
+      const item = backlog[i];
+      if (item === undefined) continue;
       // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
       (this.emit as (event: keyof CoreEvents, ...args: unknown[]) => boolean)(
         item.event,

packages/core/src/utils/nextSpeakerChecker.ts

@@ -87,7 +87,6 @@ export async function checkNextSpeaker(
     lastComprehensiveMessage.parts &&
     lastComprehensiveMessage.parts.length === 0
   ) {
-    lastComprehensiveMessage.parts.push({ text: '' });
     return {
       reasoning:
         'The last message was a filler model message with no content (nothing for user to act on), model should speak next.',