2 files changed, 118 insertions, 8 deletions

diff --git a/packages/core/src/core/client.test.ts b/packages/core/src/core/client.test.ts
index dc3b8455..9d3791fd 100644
--- a/packages/core/src/core/client.test.ts
+++ b/packages/core/src/core/client.test.ts
@@ -8,11 +8,12 @@ import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
 
 import {
   Chat,
+  Content,
   EmbedContentResponse,
   GenerateContentResponse,
   GoogleGenAI,
 } from '@google/genai';
-import { GeminiClient } from './client.js';
+import { findIndexAfterFraction, GeminiClient } from './client.js';
 import { AuthType, ContentGenerator } from './contentGenerator.js';
 import { GeminiChat } from './geminiChat.js';
 import { Config } from '../config/config.js';
@@ -65,6 +66,54 @@ vi.mock('../telemetry/index.js', () => ({
   logApiError: vi.fn(),
 }));
 
+describe('findIndexAfterFraction', () => {
+  const history: Content[] = [
+    { role: 'user', parts: [{ text: 'This is the first message.' }] },
+    { role: 'model', parts: [{ text: 'This is the second message.' }] },
+    { role: 'user', parts: [{ text: 'This is the third message.' }] },
+    { role: 'model', parts: [{ text: 'This is the fourth message.' }] },
+    { role: 'user', parts: [{ text: 'This is the fifth message.' }] },
+  ];
+
+  it('should throw an error for non-positive numbers', () => {
+    expect(() => findIndexAfterFraction(history, 0)).toThrow(
+      'Fraction must be between 0 and 1',
+    );
+  });
+
+  it('should throw an error for a fraction greater than or equal to 1', () => {
+    expect(() => findIndexAfterFraction(history, 1)).toThrow(
+      'Fraction must be between 0 and 1',
+    );
+  });
+
+  it('should handle a fraction in the middle', () => {
+    // Total length is 257. 257 * 0.5 = 128.5
+    // 0: 53
+    // 1: 53 + 54 = 107
+    // 2: 107 + 53 = 160
+    // 160 >= 128.5, so index is 2
+    expect(findIndexAfterFraction(history, 0.5)).toBe(2);
+  });
+
+  it('should handle an empty history', () => {
+    expect(findIndexAfterFraction([], 0.5)).toBe(0);
+  });
+
+  it('should handle a history with only one item', () => {
+    expect(findIndexAfterFraction(history.slice(0, 1), 0.5)).toBe(0);
+  });
+
+  it('should handle history with weird parts', () => {
+    const historyWithEmptyParts: Content[] = [
+      { role: 'user', parts: [{ text: 'Message 1' }] },
+      { role: 'model', parts: [{ fileData: { fileUri: 'derp' } }] },
+      { role: 'user', parts: [{ text: 'Message 2' }] },
+    ];
+    expect(findIndexAfterFraction(historyWithEmptyParts, 0.5)).toBe(1);
+  });
+});
+
 describe('Gemini Client (client.ts)', () => {
   let client: GeminiClient;
   beforeEach(async () => {
@@ -384,6 +433,7 @@ describe('Gemini Client (client.ts)', () => {
             { role: 'user', parts: [{ text: '...history...' }] },
           ]),
         addHistory: vi.fn(),
+        setHistory: vi.fn(),
         sendMessage: mockSendMessage,
       };
       client['chat'] = mockChat as GeminiChat;
@@ -735,6 +785,7 @@ describe('Gemini Client (client.ts)', () => {
 
       const mockChat: Partial<GeminiChat> = {
         getHistory: vi.fn().mockReturnValue(mockChatHistory),
+        setHistory: vi.fn(),
         sendMessage: mockSendMessage,
       };
 
diff --git a/packages/core/src/core/client.ts b/packages/core/src/core/client.ts
index 69ed0dff..6cfcd407 100644
--- a/packages/core/src/core/client.ts
+++ b/packages/core/src/core/client.ts
@@ -45,6 +45,39 @@ function isThinkingSupported(model: string) {
   return false;
 }
 
+/**
+ * Returns the index of the content after the fraction of the total characters in the history.
+ *
+ * Exported for testing purposes.
+ */
+export function findIndexAfterFraction(
+  history: Content[],
+  fraction: number,
+): number {
+  if (fraction <= 0 || fraction >= 1) {
+    throw new Error('Fraction must be between 0 and 1');
+  }
+
+  const contentLengths = history.map(
+    (content) => JSON.stringify(content).length,
+  );
+
+  const totalCharacters = contentLengths.reduce(
+    (sum, length) => sum + length,
+    0,
+  );
+  const targetCharacters = totalCharacters * fraction;
+
+  let charactersSoFar = 0;
+  for (let i = 0; i < contentLengths.length; i++) {
+    charactersSoFar += contentLengths[i];
+    if (charactersSoFar >= targetCharacters) {
+      return i;
+    }
+  }
+  return contentLengths.length;
+}
+
 export class GeminiClient {
   private chat?: GeminiChat;
   private contentGenerator?: ContentGenerator;
@@ -54,7 +87,16 @@ export class GeminiClient {
     topP: 1,
   };
   private readonly MAX_TURNS = 100;
-  private readonly TOKEN_THRESHOLD_FOR_SUMMARIZATION = 0.7;
+  /**
+   * Threshold for compression token count as a fraction of the model's token limit.
+   * If the chat history exceeds this threshold, it will be compressed.
+   */
+  private readonly COMPRESSION_TOKEN_THRESHOLD = 0.7;
+  /**
+   * The fraction of the latest chat history to keep. A value of 0.3
+   * means that only the last 30% of the chat history will be kept after compression.
+   */
+  private readonly COMPRESSION_PRESERVE_THRESHOLD = 0.3;
 
   constructor(private config: Config) {
     if (config.getProxy()) {
@@ -90,11 +132,11 @@ export class GeminiClient {
     return this.chat;
   }
 
-  async getHistory(): Promise<Content[]> {
+  getHistory(): Content[] {
     return this.getChat().getHistory();
   }
 
-  async setHistory(history: Content[]): Promise<void> {
+  setHistory(history: Content[]) {
     this.getChat().setHistory(history);
   }
 
@@ -441,25 +483,41 @@ export class GeminiClient {
 
     const model = this.config.getModel();
 
-    let { totalTokens: originalTokenCount } =
+    const { totalTokens: originalTokenCount } =
       await this.getContentGenerator().countTokens({
         model,
         contents: curatedHistory,
       });
     if (originalTokenCount === undefined) {
       console.warn(`Could not determine token count for model ${model}.`);
-      originalTokenCount = 0;
+      return null;
     }
 
     // Don't compress if not forced and we are under the limit.
     if (
       !force &&
-      originalTokenCount <
-        this.TOKEN_THRESHOLD_FOR_SUMMARIZATION * tokenLimit(model)
+      originalTokenCount < this.COMPRESSION_TOKEN_THRESHOLD * tokenLimit(model)
     ) {
       return null;
     }
 
+    let compressBeforeIndex = findIndexAfterFraction(
+      curatedHistory,
+      1 - this.COMPRESSION_PRESERVE_THRESHOLD,
+    );
+    // Find the first user message after the index. This is the start of the next turn.
+    while (
+      compressBeforeIndex < curatedHistory.length &&
+      curatedHistory[compressBeforeIndex]?.role !== 'user'
+    ) {
+      compressBeforeIndex++;
+    }
+
+    const historyToCompress = curatedHistory.slice(0, compressBeforeIndex);
+    const historyToKeep = curatedHistory.slice(compressBeforeIndex);
+
+    this.getChat().setHistory(historyToCompress);
+
     const { text: summary } = await this.getChat().sendMessage({
       message: {
         text: 'First, reason in your scratchpad. Then, generate the <state_snapshot>.',
@@ -477,6 +535,7 @@ export class GeminiClient {
         role: 'model',
         parts: [{ text: 'Got it. Thanks for the additional context!' }],
       },
+      ...historyToKeep,
     ]);
 
     const { totalTokens: newTokenCount } =