Feat: Implement Anchored Iterative Summarization in adk-js context compactors

core/src/common.ts

@@ -106,6 +106,8 @@ export {
   type CodeExecutionResult,
   type File,
 } from './code_executors/code_execution_utils.js';
+export {AnchoredContextCompactor} from './context/anchored_context_compactor.js';
+export type {AnchoredContextCompactorOptions} from './context/anchored_context_compactor.js';
 export type {BaseContextCompactor} from './context/base_context_compactor.js';
 export type {BaseSummarizer} from './context/summarizers/base_summarizer.js';
 export {LlmSummarizer} from './context/summarizers/llm_summarizer.js';
@@ -116,7 +118,7 @@ export {TrajectoryThoughtPruningCompactor} from './context/trajectory_thought_pr
 export type {TrajectoryThoughtPruningCompactorOptions} from './context/trajectory_thought_pruning_compactor.js';
 export {TruncatingContextCompactor} from './context/truncating_context_compactor.js';
 export type {TruncatingContextCompactorOptions} from './context/truncating_context_compactor.js';
-export {isCompactedEvent} from './events/compacted_event.js';
+export {isCompactedEvent, isScratchpadEvent} from './events/compacted_event.js';
 export type {CompactedEvent} from './events/compacted_event.js';
 export {
   createEvent,

core/src/context/anchored_context_compactor.ts

@@ -0,0 +1,165 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {InvocationContext} from '../agents/invocation_context.js';
+import {CompactedEvent, isScratchpadEvent} from '../events/compacted_event.js';
+import {
+  Event,
+  getEventTokens,
+  getFunctionCalls,
+  getFunctionResponses,
+} from '../events/event.js';
+import {BaseContextCompactor} from './base_context_compactor.js';
+import {BaseSummarizer} from './summarizers/base_summarizer.js';
+
+export interface AnchoredContextCompactorOptions {
+  /** The maximum number of tokens to retain in the session history before compaction. */
+  tokenThreshold: number;
+  /**
+   * The minimum number of raw events to keep at the end of the session.
+   * Compaction will not affect these tail events (unless needed for tool splits).
+   */
+  eventRetentionSize: number;
+  /** The summarizer used to create the compacted event content. */
+  summarizer: BaseSummarizer;
+}
+
+/**
+ * A context compactor that maintains a single persistent 'Scratchpad' or
+ * 'State Tracker' event at the top of the context history.
+ *
+ * When compaction is triggered, it merges new raw events into the existing
+ * Scratchpad event and discards them from the active history view.
+ */
+export class AnchoredContextCompactor implements BaseContextCompactor {
+  private readonly tokenThreshold: number;
+  private readonly eventRetentionSize: number;
+  private readonly summarizer: BaseSummarizer;
+
+  constructor(options: AnchoredContextCompactorOptions) {
+    this.tokenThreshold = options.tokenThreshold;
+    this.eventRetentionSize = options.eventRetentionSize;
+    this.summarizer = options.summarizer;
+  }
+
+  private getActiveEvents(events: Event[]): Event[] {
+    let latestScratchpad: CompactedEvent | undefined = undefined;
+
+    for (let i = events.length - 1; i >= 0; i--) {
+      const e = events[i];
+      if (isScratchpadEvent(e)) {
+        latestScratchpad = e;
+        break;
+      }
+    }
+
+    if (!latestScratchpad) {
+      return events;
+    }
+
+    const activeRawEvents = events.filter(
+      (e) => e.timestamp > latestScratchpad!.endTime && !isScratchpadEvent(e),
+    );
+
+    return [latestScratchpad, ...activeRawEvents];
+  }
+
+  shouldCompact(
+    invocationContext: InvocationContext,
+  ): boolean | Promise<boolean> {
+    const events = invocationContext.session.events;
+    const activeEvents = this.getActiveEvents(events);
+    const hasScratchpad =
+      activeEvents.length > 0 && isScratchpadEvent(activeEvents[0]);
+    const rawEvents = hasScratchpad ? activeEvents.slice(1) : activeEvents;
+
+    if (rawEvents.length <= this.eventRetentionSize) {
+      return false;
+    }
+
+    const totalTokens = activeEvents.reduce(
+      (sum, event) => sum + getEventTokens(event),
+      0,
+    );
+
+    return totalTokens > this.tokenThreshold;
+  }
+
+  async compact(invocationContext: InvocationContext): Promise<void> {
+    const events = invocationContext.session.events;
+    const activeEvents = this.getActiveEvents(events);
+    const hasScratchpad =
+      activeEvents.length > 0 && isScratchpadEvent(activeEvents[0]);
+    const rawEvents = hasScratchpad ? activeEvents.slice(1) : activeEvents;
+
+    if (rawEvents.length <= this.eventRetentionSize) {
+      return;
+    }
+
+    // Determine the baseline index to retain from the active raw events.
+    let retainStartIndex = Math.max(
+      0,
+      rawEvents.length - this.eventRetentionSize,
+    );
+
+    // Prevent splitting between a tool call and its response.
+    while (retainStartIndex > 0) {
+      const eventToRetain = rawEvents[retainStartIndex];
+      const previousEvent = rawEvents[retainStartIndex - 1];
+
+      if (
+        getFunctionResponses(eventToRetain).length > 0 &&
+        getFunctionCalls(previousEvent).length > 0
+      ) {
+        retainStartIndex--;
+      } else {
+        // No conflict, safe to split here.
+        break;
+      }
+    }
+
+    if (retainStartIndex === 0) {
+      // Cannot compact if we have to retain everything
+      return;
+    }
+
+    // Extract raw events to compact.
+    const rawEventsToCompact = rawEvents.slice(0, retainStartIndex);
+
+    let scratchpadEvent: CompactedEvent;
+
+    if (hasScratchpad) {
+      const existingScratchpad = activeEvents[0] as CompactedEvent;
+      scratchpadEvent = await this.summarizer.summarize([
+        existingScratchpad,
+        ...rawEventsToCompact,
+      ]);
+    } else {
+      scratchpadEvent = await this.summarizer.summarize(rawEventsToCompact);
+    }
+
+    // Ensure the event is marked as scratchpad and has system author.
+    const updatedScratchpad = {
+      ...scratchpadEvent,
+      isScratchpad: true,
+      author: 'system',
+    } as CompactedEvent;
+
+    // Reconstruct the events list: inactive events + new scratchpad + active retained events
+    const inactiveEvents = events.slice(0, events.indexOf(activeEvents[0]));
+    const retainedRawEvents = rawEvents.slice(retainStartIndex);
+
+    const newEventsList = [
+      ...inactiveEvents,
+      updatedScratchpad,
+      ...retainedRawEvents,
+    ];
+
+    // Mutate the original session events array.
+    events.length = 0;
+    events.push(...newEventsList);
+  }
+}

core/src/context/token_based_context_compactor.ts

@@ -6,7 +6,7 @@
 
 import {InvocationContext} from '../agents/invocation_context.js';
 import {CompactedEvent, isCompactedEvent} from '../events/compacted_event.js';
-import {Event, stringifyContent} from '../events/event.js';
+import {Event, getEventTokens} from '../events/event.js';
 import {BaseContextCompactor} from './base_context_compactor.js';
 import {BaseSummarizer} from './summarizers/base_summarizer.js';
 
@@ -142,15 +142,6 @@ export class TokenBasedContextCompactor implements BaseContextCompactor {
   }
 }
 
-function getEventTokens(event: Event): number {
-  if (event.usageMetadata?.promptTokenCount !== undefined) {
-    return event.usageMetadata.promptTokenCount;
-  }
-  // Estimate: 4 chars per token.
-  const contentStr = stringifyContent(event);
-  return Math.ceil(contentStr.length / 4);
-}
-
 function hasFunctionCall(event: Event): boolean {
   return !!event.content?.parts?.some(
     (part) => part.functionCall !== undefined,

core/src/events/compacted_event.ts

@@ -30,6 +30,11 @@ export interface CompactedEvent extends Event {
    * The summarized content of the compacted events.
    */
   compactedContent: string;
+
+  /**
+   * Identifies this compacted event as the persistent context scratchpad.
+   */
+  isScratchpad?: boolean;
 }
 
 /**
@@ -39,6 +44,17 @@ export function isCompactedEvent(event: Event): event is CompactedEvent {
   return 'isCompacted' in event && event.isCompacted === true;
 }
 
+/**
+ * Type guard to check if an event is a scratchpad CompactedEvent.
+ */
+export function isScratchpadEvent(
+  event: Event,
+): event is CompactedEvent & {isScratchpad: true} {
+  return (
+    isCompactedEvent(event) && (event as CompactedEvent).isScratchpad === true
+  );
+}
+
 export function createCompactedEvent(
   params: Partial<CompactedEvent> = {},
 ): CompactedEvent {
@@ -48,5 +64,6 @@ export function createCompactedEvent(
     startTime: params.startTime!,
     endTime: params.endTime!,
     compactedContent: params.compactedContent!,
+    isScratchpad: params.isScratchpad,
   };
 }

core/src/events/event.ts

@@ -166,6 +166,18 @@ export function stringifyContent(event: Event): string {
     .join('');
 }
 
+/**
+ * Estimates the number of tokens in the event based on usage metadata or characters.
+ */
+export function getEventTokens(event: Event): number {
+  if (event.usageMetadata?.promptTokenCount !== undefined) {
+    return event.usageMetadata.promptTokenCount;
+  }
+  // Estimate: 4 chars per token.
+  const contentStr = stringifyContent(event);
+  return Math.ceil(contentStr.length / 4);
+}
+
 /**
  * Returns whether the event contains any thought parts.
  */