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

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

packages/core/src/core/geminiChat.ts

@@ -691,9 +691,13 @@ export class GeminiChat {
     const history = curated
       ? extractCuratedHistory(this.history)
       : this.history;
-    // Deep copy the history to avoid mutating the history outside of the
-    // chat session.
-    return structuredClone(history);
+    // Return a shallow copy of the array to prevent callers from mutating
+    // the internal history array (push/pop/splice). Content objects are
+    // shared references — callers MUST NOT mutate them in place.
+    // This replaces a prior structuredClone() which deep-copied the entire
+    // conversation on every call, causing O(n) memory pressure per turn
+    // that compounded into OOM crashes in long-running sessions.
+    return [...history];
   }
 
   /**

packages/core/src/core/turn.ts

@@ -241,6 +241,7 @@ export class Turn {
   readonly pendingToolCalls: ToolCallRequestInfo[] = [];
   private debugResponses: GenerateContentResponse[] = [];
   private pendingCitations = new Set<string>();
+  private cachedResponseText: string | undefined = undefined;
   finishReason: FinishReason | undefined = undefined;
 
   constructor(
@@ -432,11 +433,15 @@ export class Turn {
   /**
    * Get the concatenated response text from all responses in this turn.
    * This extracts and joins all text content from the model's responses.
+   * The result is cached since this is called multiple times per turn.
    */
   getResponseText(): string {
-    return this.debugResponses
-      .map((response) => getResponseText(response))
-      .filter((text): text is string => text !== null)
-      .join(' ');
+    if (this.cachedResponseText === undefined) {
+      this.cachedResponseText = this.debugResponses
+        .map((response) => getResponseText(response))
+        .filter((text): text is string => text !== null)
+        .join(' ');
+    }
+    return this.cachedResponseText;
   }
 }

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.',