Make compression threshold editable in the UI. (#12317)

# Conflicts:
#	packages/cli/src/config/config.test.ts
#	packages/core/src/services/chatCompressionService.test.ts
#	packages/core/src/services/chatCompressionService.ts

docs/get-started/configuration-v1.md

@@ -473,21 +473,6 @@ a few things you can try in order of recommendation:
     "loadMemoryFromIncludeDirectories": true
     ```
 
-- **`chatCompression`** (object):
-  - **Description:** Controls the settings for chat history compression, both
-    automatic and when manually invoked through the /compress command.
-  - **Properties:**
-    - **`contextPercentageThreshold`** (number): A value between 0 and 1 that
-      specifies the token threshold for compression as a percentage of the
-      model's total token limit. For example, a value of `0.6` will trigger
-      compression when the chat history exceeds 60% of the token limit.
-  - **Example:**
-    ```json
-    "chatCompression": {
-      "contextPercentageThreshold": 0.6
-    }
-    ```
-
 - **`showLineNumbers`** (boolean):
   - **Description:** Controls whether line numbers are displayed in code blocks
     in the CLI output.

docs/get-started/configuration.md

@@ -244,13 +244,13 @@ their corresponding top-level category object in your `settings.json` file.
     example `{"run_shell_command": {"tokenBudget": 2000}}`
   - **Default:** `undefined`
 
-- **`model.chatCompression.contextPercentageThreshold`** (number):
+- **`model.compressionThreshold`** (number):
   - **Description:** Sets the threshold for chat history compression as a
-    percentage of the model's total token limit. This is a value between 0 and 1
+    fraction of the model's total token limit. This is a value between 0 and 1
     that applies to both automatic compression and the manual `/compress`
     command. For example, a value of `0.6` will trigger compression when the
     chat history exceeds 60% of the token limit.
-  - **Default:** `0.7`
+  - **Default:** `0.2`
 
 - **`model.skipNextSpeakerCheck`** (boolean):
   - **Description:** Skip the next speaker check.

packages/cli/src/config/config.test.ts

@@ -1572,9 +1572,13 @@ describe('loadCliConfig with includeDirectories', () => {
   });
 });
 
+<<<<<<< HEAD
 describe('loadCliConfig chatCompression', () => {
   const originalArgv = process.argv;
 
+=======
+describe('loadCliConfig compressionThreshold', () => {
+>>>>>>> 3332703f (Make compression threshold editable in the UI. (#12317))
   beforeEach(() => {
     vi.resetAllMocks();
     vi.mocked(os.homedir).mockReturnValue('/mock/home/user');
@@ -1587,28 +1591,36 @@ describe('loadCliConfig chatCompression', () => {
     vi.restoreAllMocks();
   });
 
-  it('should pass chatCompression settings to the core config', async () => {
+  it('should pass settings to the core config', async () => {
     process.argv = ['node', 'script.js'];
     const argv = await parseArguments({} as Settings);
     const settings: Settings = {
       model: {
-        chatCompression: {
+        compressionThreshold: 0.5,
-          contextPercentageThreshold: 0.5,
-        },
       },
     };
+<<<<<<< HEAD
     const config = await loadCliConfig(settings, [], 'test-session', argv);
     expect(config.getChatCompression()).toEqual({
       contextPercentageThreshold: 0.5,
     });
+=======
+    const config = await loadCliConfig(settings, 'test-session', argv);
+    expect(config.getCompressionThreshold()).toBe(0.5);
+>>>>>>> 3332703f (Make compression threshold editable in the UI. (#12317))
   });
 
-  it('should have undefined chatCompression if not in settings', async () => {
+  it('should have undefined compressionThreshold if not in settings', async () => {
     process.argv = ['node', 'script.js'];
     const argv = await parseArguments({} as Settings);
     const settings: Settings = {};
+<<<<<<< HEAD
     const config = await loadCliConfig(settings, [], 'test-session', argv);
     expect(config.getChatCompression()).toBeUndefined();
+=======
+    const config = await loadCliConfig(settings, 'test-session', argv);
+    expect(config.getCompressionThreshold()).toBeUndefined();
+>>>>>>> 3332703f (Make compression threshold editable in the UI. (#12317))
   });
 });
 

packages/cli/src/config/config.ts

@@ -610,7 +610,7 @@ export async function loadCliConfig(
     noBrowser: !!process.env['NO_BROWSER'],
     summarizeToolOutput: settings.model?.summarizeToolOutput,
     ideMode,
-    chatCompression: settings.model?.chatCompression,
+    compressionThreshold: settings.model?.compressionThreshold,
     folderTrust,
     interactive,
     trustedFolder,

packages/cli/src/config/settings.test.ts

@@ -1051,15 +1051,15 @@ describe('Settings Loading and Merging', () => {
       });
     });
 
-    it('should merge chatCompression settings, with workspace taking precedence', () => {
+    it('should merge compressionThreshold settings, with workspace taking precedence', () => {
       (mockFsExistsSync as Mock).mockReturnValue(true);
       const userSettingsContent = {
         general: {},
-        model: { chatCompression: { contextPercentageThreshold: 0.5 } },
+        model: { compressionThreshold: 0.5 },
       };
       const workspaceSettingsContent = {
         general: {},
-        model: { chatCompression: { contextPercentageThreshold: 0.8 } },
+        model: { compressionThreshold: 0.8 },
       };
 
       (fs.readFileSync as Mock).mockImplementation(
@@ -1074,15 +1074,11 @@ describe('Settings Loading and Merging', () => {
 
       const settings = loadSettings(MOCK_WORKSPACE_DIR);
 
-      expect(settings.user.settings.model?.chatCompression).toEqual({
+      expect(settings.user.settings.model?.compressionThreshold).toEqual(0.5);
-        contextPercentageThreshold: 0.5,
+      expect(settings.workspace.settings.model?.compressionThreshold).toEqual(
-      });
+        0.8,
-      expect(settings.workspace.settings.model?.chatCompression).toEqual({
+      );
-        contextPercentageThreshold: 0.8,
+      expect(settings.merged.model?.compressionThreshold).toEqual(0.8);
-      });
-      expect(settings.merged.model?.chatCompression).toEqual({
-        contextPercentageThreshold: 0.8,
-      });
     });
 
     it('should merge output format settings, with workspace taking precedence', () => {
@@ -1109,13 +1105,13 @@ describe('Settings Loading and Merging', () => {
       expect(settings.merged.output?.format).toBe('json');
     });
 
-    it('should handle chatCompression when only in user settings', () => {
+    it('should handle compressionThreshold when only in user settings', () => {
       (mockFsExistsSync as Mock).mockImplementation(
         (p: fs.PathLike) => p === USER_SETTINGS_PATH,
       );
       const userSettingsContent = {
         general: {},
-        model: { chatCompression: { contextPercentageThreshold: 0.5 } },
+        model: { compressionThreshold: 0.5 },
       };
       (fs.readFileSync as Mock).mockImplementation(
         (p: fs.PathOrFileDescriptor) => {
@@ -1126,9 +1122,7 @@ describe('Settings Loading and Merging', () => {
       );
 
       const settings = loadSettings(MOCK_WORKSPACE_DIR);
-      expect(settings.merged.model?.chatCompression).toEqual({
+      expect(settings.merged.model?.compressionThreshold).toEqual(0.5);
-        contextPercentageThreshold: 0.5,
-      });
     });
 
     it('should have model as undefined if not in any settings file', () => {
@@ -1138,39 +1132,15 @@ describe('Settings Loading and Merging', () => {
       expect(settings.merged.model).toBeUndefined();
     });
 
-    it('should ignore chatCompression if contextPercentageThreshold is invalid', () => {
+    it('should use user compressionThreshold if workspace does not define it', () => {
-      const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
-      (mockFsExistsSync as Mock).mockImplementation(
-        (p: fs.PathLike) => p === USER_SETTINGS_PATH,
-      );
-      const userSettingsContent = {
-        general: {},
-        model: { chatCompression: { contextPercentageThreshold: 1.5 } },
-      };
-      (fs.readFileSync as Mock).mockImplementation(
-        (p: fs.PathOrFileDescriptor) => {
-          if (p === USER_SETTINGS_PATH)
-            return JSON.stringify(userSettingsContent);
-          return '{}';
-        },
-      );
-
-      const settings = loadSettings(MOCK_WORKSPACE_DIR);
-      expect(settings.merged.model?.chatCompression).toEqual({
-        contextPercentageThreshold: 1.5,
-      });
-      warnSpy.mockRestore();
-    });
-
-    it('should deep merge chatCompression settings', () => {
       (mockFsExistsSync as Mock).mockReturnValue(true);
       const userSettingsContent = {
         general: {},
-        model: { chatCompression: { contextPercentageThreshold: 0.5 } },
+        model: { compressionThreshold: 0.5 },
       };
       const workspaceSettingsContent = {
         general: {},
-        model: { chatCompression: {} },
+        model: {},
       };
 
       (fs.readFileSync as Mock).mockImplementation(
@@ -1185,9 +1155,7 @@ describe('Settings Loading and Merging', () => {
 
       const settings = loadSettings(MOCK_WORKSPACE_DIR);
 
-      expect(settings.merged.model?.chatCompression).toEqual({
+      expect(settings.merged.model?.compressionThreshold).toEqual(0.5);
-        contextPercentageThreshold: 0.5,
-      });
     });
 
     it('should merge includeDirectories from all scopes', () => {
@@ -1972,9 +1940,6 @@ describe('Settings Loading and Merging', () => {
         },
         model: {
           name: 'gemini-pro',
-          chatCompression: {
-            contextPercentageThreshold: 0.5,
-          },
         },
         mcpServers: {
           'server-1': {
@@ -1993,9 +1958,6 @@ describe('Settings Loading and Merging', () => {
           myTheme: {},
         },
         model: 'gemini-pro',
-        chatCompression: {
-          contextPercentageThreshold: 0.5,
-        },
         mcpServers: {
           'server-1': {
             command: 'node server.js',
@@ -2035,9 +1997,6 @@ describe('Settings Loading and Merging', () => {
         },
         model: {
           name: 'gemini-pro',
-          chatCompression: {
-            contextPercentageThreshold: 0.8,
-          },
         },
         context: {
           fileName: 'CONTEXT.md',
@@ -2077,9 +2036,6 @@ describe('Settings Loading and Merging', () => {
         theme: 'dark',
         usageStatisticsEnabled: false,
         model: 'gemini-pro',
-        chatCompression: {
-          contextPercentageThreshold: 0.8,
-        },
         contextFileName: 'CONTEXT.md',
         includeDirectories: ['/src'],
         sandbox: true,

packages/cli/src/config/settings.ts

@@ -64,7 +64,7 @@ const MIGRATION_MAP: Record<string, string> = {
   autoAccept: 'tools.autoAccept',
   autoConfigureMaxOldSpaceSize: 'advanced.autoConfigureMemory',
   bugCommand: 'advanced.bugCommand',
-  chatCompression: 'model.chatCompression',
+  chatCompression: 'model.compressionThreshold',
   checkpointing: 'general.checkpointing',
   coreTools: 'tools.core',
   contextFileName: 'context.fileName',

packages/cli/src/config/settingsSchema.ts

@@ -14,7 +14,6 @@ import type {
   BugCommandSettings,
   TelemetrySettings,
   AuthType,
-  ChatCompressionSettings,
 } from '@google/gemini-cli-core';
 import {
   DEFAULT_TRUNCATE_TOOL_OUTPUT_LINES,
@@ -578,14 +577,15 @@ const SETTINGS_SCHEMA = {
         description: 'Settings for summarizing tool output.',
         showInDialog: false,
       },
-      chatCompression: {
+      compressionThreshold: {
-        type: 'object',
+        type: 'number',
-        label: 'Chat Compression',
+        label: 'Compression Threshold',
         category: 'Model',
         requiresRestart: false,
-        default: undefined as ChatCompressionSettings | undefined,
+        default: 0.2 as number,
-        description: 'Chat compression settings.',
+        description:
-        showInDialog: false,
+          'The fraction of context usage at which to trigger context compression (e.g. 0.2, 0.3).',
+        showInDialog: true,
       },
       skipNextSpeakerCheck: {
         type: 'boolean',

packages/core/src/config/config.ts

@@ -96,10 +96,6 @@ export interface BugCommandSettings {
   urlTemplate: string;
 }
 
-export interface ChatCompressionSettings {
-  contextPercentageThreshold?: number;
-}
-
 export interface SummarizeToolOutputSettings {
   tokenBudget?: number;
 }
@@ -261,7 +257,7 @@ export interface ConfigParameters {
   folderTrust?: boolean;
   ideMode?: boolean;
   loadMemoryFromIncludeDirectories?: boolean;
-  chatCompression?: ChatCompressionSettings;
+  compressionThreshold?: number;
   interactive?: boolean;
   trustedFolder?: boolean;
   useRipgrep?: boolean;
@@ -354,7 +350,7 @@ export class Config {
     | undefined;
   private readonly experimentalZedIntegration: boolean = false;
   private readonly loadMemoryFromIncludeDirectories: boolean = false;
-  private readonly chatCompression: ChatCompressionSettings | undefined;
+  private readonly compressionThreshold: number | undefined;
   private readonly interactive: boolean;
   private readonly ptyInfo: string;
   private readonly trustedFolder: boolean | undefined;
@@ -453,7 +449,7 @@ export class Config {
     this.ideMode = params.ideMode ?? false;
     this.loadMemoryFromIncludeDirectories =
       params.loadMemoryFromIncludeDirectories ?? false;
-    this.chatCompression = params.chatCompression;
+    this.compressionThreshold = params.compressionThreshold;
     this.interactive = params.interactive ?? false;
     this.ptyInfo = params.ptyInfo ?? 'child_process';
     this.trustedFolder = params.trustedFolder;
@@ -977,8 +973,8 @@ export class Config {
     this.fileSystemService = fileSystemService;
   }
 
-  getChatCompression(): ChatCompressionSettings | undefined {
+  getCompressionThreshold(): number | undefined {
-    return this.chatCompression;
+    return this.compressionThreshold;
   }
 
   isInteractiveShellEnabled(): boolean {

packages/core/src/services/chatCompressionService.test.ts

@@ -0,0 +1,294 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import {
+  ChatCompressionService,
+  findCompressSplitPoint,
+} from './chatCompressionService.js';
+import type { Content, GenerateContentResponse } from '@google/genai';
+import { CompressionStatus } from '../core/turn.js';
+import { tokenLimit } from '../core/tokenLimits.js';
+import type { GeminiChat } from '../core/geminiChat.js';
+import type { Config } from '../config/config.js';
+import { getInitialChatHistory } from '../utils/environmentContext.js';
+import type { ContentGenerator } from '../core/contentGenerator.js';
+
+vi.mock('../core/tokenLimits.js');
+vi.mock('../telemetry/loggers.js');
+vi.mock('../utils/environmentContext.js');
+
+describe('findCompressSplitPoint', () => {
+  it('should throw an error for non-positive numbers', () => {
+    expect(() => findCompressSplitPoint([], 0)).toThrow(
+      'Fraction must be between 0 and 1',
+    );
+  });
+
+  it('should throw an error for a fraction greater than or equal to 1', () => {
+    expect(() => findCompressSplitPoint([], 1)).toThrow(
+      'Fraction must be between 0 and 1',
+    );
+  });
+
+  it('should handle an empty history', () => {
+    expect(findCompressSplitPoint([], 0.5)).toBe(0);
+  });
+
+  it('should handle a fraction in the middle', () => {
+    const history: Content[] = [
+      { role: 'user', parts: [{ text: 'This is the first message.' }] }, // JSON length: 66 (19%)
+      { role: 'model', parts: [{ text: 'This is the second message.' }] }, // JSON length: 68 (40%)
+      { role: 'user', parts: [{ text: 'This is the third message.' }] }, // JSON length: 66 (60%)
+      { role: 'model', parts: [{ text: 'This is the fourth message.' }] }, // JSON length: 68 (80%)
+      { role: 'user', parts: [{ text: 'This is the fifth message.' }] }, // JSON length: 65 (100%)
+    ];
+    expect(findCompressSplitPoint(history, 0.5)).toBe(4);
+  });
+
+  it('should handle a fraction of last index', () => {
+    const history: Content[] = [
+      { role: 'user', parts: [{ text: 'This is the first message.' }] }, // JSON length: 66 (19%)
+      { role: 'model', parts: [{ text: 'This is the second message.' }] }, // JSON length: 68 (40%)
+      { role: 'user', parts: [{ text: 'This is the third message.' }] }, // JSON length: 66 (60%)
+      { role: 'model', parts: [{ text: 'This is the fourth message.' }] }, // JSON length: 68 (80%)
+      { role: 'user', parts: [{ text: 'This is the fifth message.' }] }, // JSON length: 65 (100%)
+    ];
+    expect(findCompressSplitPoint(history, 0.9)).toBe(4);
+  });
+
+  it('should handle a fraction of after last index', () => {
+    const history: Content[] = [
+      { role: 'user', parts: [{ text: 'This is the first message.' }] }, // JSON length: 66 (24%)
+      { role: 'model', parts: [{ text: 'This is the second message.' }] }, // JSON length: 68 (50%)
+      { role: 'user', parts: [{ text: 'This is the third message.' }] }, // JSON length: 66 (74%)
+      { role: 'model', parts: [{ text: 'This is the fourth message.' }] }, // JSON length: 68 (100%)
+    ];
+    expect(findCompressSplitPoint(history, 0.8)).toBe(4);
+  });
+
+  it('should return earlier splitpoint if no valid ones are after threshold', () => {
+    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: [{ functionCall: { name: 'foo', args: {} } }] },
+    ];
+    // Can't return 4 because the previous item has a function call.
+    expect(findCompressSplitPoint(history, 0.99)).toBe(2);
+  });
+
+  it('should handle a history with only one item', () => {
+    const historyWithEmptyParts: Content[] = [
+      { role: 'user', parts: [{ text: 'Message 1' }] },
+    ];
+    expect(findCompressSplitPoint(historyWithEmptyParts, 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', mimeType: 'text/plain' } }],
+      },
+      { role: 'user', parts: [{ text: 'Message 2' }] },
+    ];
+    expect(findCompressSplitPoint(historyWithEmptyParts, 0.5)).toBe(2);
+  });
+});
+
+describe('ChatCompressionService', () => {
+  let service: ChatCompressionService;
+  let mockChat: GeminiChat;
+  let mockConfig: Config;
+  const mockModel = 'gemini-pro';
+  const mockPromptId = 'test-prompt-id';
+
+  beforeEach(() => {
+    service = new ChatCompressionService();
+    mockChat = {
+      getHistory: vi.fn(),
+      getLastPromptTokenCount: vi.fn().mockReturnValue(500),
+    } as unknown as GeminiChat;
+    mockConfig = {
+      getCompressionThreshold: vi.fn(),
+      getContentGenerator: vi.fn(),
+    } as unknown as Config;
+
+    vi.mocked(tokenLimit).mockReturnValue(1000);
+    vi.mocked(getInitialChatHistory).mockImplementation(
+      async (_config, extraHistory) => extraHistory || [],
+    );
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('should return NOOP if history is empty', async () => {
+    vi.mocked(mockChat.getHistory).mockReturnValue([]);
+    const result = await service.compress(
+      mockChat,
+      mockPromptId,
+      false,
+      mockModel,
+      mockConfig,
+      false,
+    );
+    expect(result.info.compressionStatus).toBe(CompressionStatus.NOOP);
+    expect(result.newHistory).toBeNull();
+  });
+
+  it('should return NOOP if previously failed and not forced', async () => {
+    vi.mocked(mockChat.getHistory).mockReturnValue([
+      { role: 'user', parts: [{ text: 'hi' }] },
+    ]);
+    const result = await service.compress(
+      mockChat,
+      mockPromptId,
+      false,
+      mockModel,
+      mockConfig,
+      true,
+    );
+    expect(result.info.compressionStatus).toBe(CompressionStatus.NOOP);
+    expect(result.newHistory).toBeNull();
+  });
+
+  it('should return NOOP if under token threshold and not forced', async () => {
+    vi.mocked(mockChat.getHistory).mockReturnValue([
+      { role: 'user', parts: [{ text: 'hi' }] },
+    ]);
+    vi.mocked(mockChat.getLastPromptTokenCount).mockReturnValue(600);
+    vi.mocked(tokenLimit).mockReturnValue(1000);
+    // Threshold is 0.7 * 1000 = 700. 600 < 700, so NOOP.
+
+    const result = await service.compress(
+      mockChat,
+      mockPromptId,
+      false,
+      mockModel,
+      mockConfig,
+      false,
+    );
+    expect(result.info.compressionStatus).toBe(CompressionStatus.NOOP);
+    expect(result.newHistory).toBeNull();
+  });
+
+  it('should compress if over token threshold', async () => {
+    const history: Content[] = [
+      { role: 'user', parts: [{ text: 'msg1' }] },
+      { role: 'model', parts: [{ text: 'msg2' }] },
+      { role: 'user', parts: [{ text: 'msg3' }] },
+      { role: 'model', parts: [{ text: 'msg4' }] },
+    ];
+    vi.mocked(mockChat.getHistory).mockReturnValue(history);
+    vi.mocked(mockChat.getLastPromptTokenCount).mockReturnValue(800);
+    vi.mocked(tokenLimit).mockReturnValue(1000);
+    const mockGenerateContent = vi.fn().mockResolvedValue({
+      candidates: [
+        {
+          content: {
+            parts: [{ text: 'Summary' }],
+          },
+        },
+      ],
+    } as unknown as GenerateContentResponse);
+    vi.mocked(mockConfig.getContentGenerator).mockReturnValue({
+      generateContent: mockGenerateContent,
+    } as unknown as ContentGenerator);
+
+    const result = await service.compress(
+      mockChat,
+      mockPromptId,
+      false,
+      mockModel,
+      mockConfig,
+      false,
+    );
+
+    expect(result.info.compressionStatus).toBe(CompressionStatus.COMPRESSED);
+    expect(result.newHistory).not.toBeNull();
+    expect(result.newHistory![0].parts![0].text).toBe('Summary');
+    expect(mockGenerateContent).toHaveBeenCalled();
+  });
+
+  it('should force compress even if under threshold', async () => {
+    const history: Content[] = [
+      { role: 'user', parts: [{ text: 'msg1' }] },
+      { role: 'model', parts: [{ text: 'msg2' }] },
+      { role: 'user', parts: [{ text: 'msg3' }] },
+      { role: 'model', parts: [{ text: 'msg4' }] },
+    ];
+    vi.mocked(mockChat.getHistory).mockReturnValue(history);
+    vi.mocked(mockChat.getLastPromptTokenCount).mockReturnValue(100);
+    vi.mocked(tokenLimit).mockReturnValue(1000);
+
+    const mockGenerateContent = vi.fn().mockResolvedValue({
+      candidates: [
+        {
+          content: {
+            parts: [{ text: 'Summary' }],
+          },
+        },
+      ],
+    } as unknown as GenerateContentResponse);
+    vi.mocked(mockConfig.getContentGenerator).mockReturnValue({
+      generateContent: mockGenerateContent,
+    } as unknown as ContentGenerator);
+
+    const result = await service.compress(
+      mockChat,
+      mockPromptId,
+      true, // forced
+      mockModel,
+      mockConfig,
+      false,
+    );
+
+    expect(result.info.compressionStatus).toBe(CompressionStatus.COMPRESSED);
+    expect(result.newHistory).not.toBeNull();
+  });
+
+  it('should return FAILED if new token count is inflated', async () => {
+    const history: Content[] = [
+      { role: 'user', parts: [{ text: 'msg1' }] },
+      { role: 'model', parts: [{ text: 'msg2' }] },
+    ];
+    vi.mocked(mockChat.getHistory).mockReturnValue(history);
+    vi.mocked(mockChat.getLastPromptTokenCount).mockReturnValue(10);
+    vi.mocked(tokenLimit).mockReturnValue(1000);
+
+    const longSummary = 'a'.repeat(1000); // Long summary to inflate token count
+    const mockGenerateContent = vi.fn().mockResolvedValue({
+      candidates: [
+        {
+          content: {
+            parts: [{ text: longSummary }],
+          },
+        },
+      ],
+    } as unknown as GenerateContentResponse);
+    vi.mocked(mockConfig.getContentGenerator).mockReturnValue({
+      generateContent: mockGenerateContent,
+    } as unknown as ContentGenerator);
+
+    const result = await service.compress(
+      mockChat,
+      mockPromptId,
+      true,
+      mockModel,
+      mockConfig,
+      false,
+    );
+
+    expect(result.info.compressionStatus).toBe(
+      CompressionStatus.COMPRESSION_FAILED_INFLATED_TOKEN_COUNT,
+    );
+    expect(result.newHistory).toBeNull();
+  });
+});

packages/core/src/services/chatCompressionService.ts

@@ -0,0 +1,215 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type { Content } from '@google/genai';
+import type { Config } from '../config/config.js';
+import type { GeminiChat } from '../core/geminiChat.js';
+import { type ChatCompressionInfo, CompressionStatus } from '../core/turn.js';
+import { tokenLimit } from '../core/tokenLimits.js';
+import { getCompressionPrompt } from '../core/prompts.js';
+import { getResponseText } from '../utils/partUtils.js';
+import { logChatCompression } from '../telemetry/loggers.js';
+import { makeChatCompressionEvent } from '../telemetry/types.js';
+import { getInitialChatHistory } from '../utils/environmentContext.js';
+
+/**
+ * Default 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.
+ */
+export const DEFAULT_COMPRESSION_TOKEN_THRESHOLD = 0.2;
+
+/**
+ * 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.
+ */
+export const COMPRESSION_PRESERVE_THRESHOLD = 0.3;
+
+/**
+ * Returns the index of the oldest item to keep when compressing. May return
+ * contents.length which indicates that everything should be compressed.
+ *
+ * Exported for testing purposes.
+ */
+export function findCompressSplitPoint(
+  contents: Content[],
+  fraction: number,
+): number {
+  if (fraction <= 0 || fraction >= 1) {
+    throw new Error('Fraction must be between 0 and 1');
+  }
+
+  const charCounts = contents.map((content) => JSON.stringify(content).length);
+  const totalCharCount = charCounts.reduce((a, b) => a + b, 0);
+  const targetCharCount = totalCharCount * fraction;
+
+  let lastSplitPoint = 0; // 0 is always valid (compress nothing)
+  let cumulativeCharCount = 0;
+  for (let i = 0; i < contents.length; i++) {
+    const content = contents[i];
+    if (
+      content.role === 'user' &&
+      !content.parts?.some((part) => !!part.functionResponse)
+    ) {
+      if (cumulativeCharCount >= targetCharCount) {
+        return i;
+      }
+      lastSplitPoint = i;
+    }
+    cumulativeCharCount += charCounts[i];
+  }
+
+  // We found no split points after targetCharCount.
+  // Check if it's safe to compress everything.
+  const lastContent = contents[contents.length - 1];
+  if (
+    lastContent?.role === 'model' &&
+    !lastContent?.parts?.some((part) => part.functionCall)
+  ) {
+    return contents.length;
+  }
+
+  // Can't compress everything so just compress at last splitpoint.
+  return lastSplitPoint;
+}
+
+export class ChatCompressionService {
+  async compress(
+    chat: GeminiChat,
+    promptId: string,
+    force: boolean,
+    model: string,
+    config: Config,
+    hasFailedCompressionAttempt: boolean,
+  ): Promise<{ newHistory: Content[] | null; info: ChatCompressionInfo }> {
+    const curatedHistory = chat.getHistory(true);
+
+    // Regardless of `force`, don't do anything if the history is empty.
+    if (
+      curatedHistory.length === 0 ||
+      (hasFailedCompressionAttempt && !force)
+    ) {
+      return {
+        newHistory: null,
+        info: {
+          originalTokenCount: 0,
+          newTokenCount: 0,
+          compressionStatus: CompressionStatus.NOOP,
+        },
+      };
+    }
+
+    const originalTokenCount = chat.getLastPromptTokenCount();
+
+    // Don't compress if not forced and we are under the limit.
+    if (!force) {
+      const threshold =
+        config.getCompressionThreshold() ?? DEFAULT_COMPRESSION_TOKEN_THRESHOLD;
+      if (originalTokenCount < threshold * tokenLimit(model)) {
+        return {
+          newHistory: null,
+          info: {
+            originalTokenCount,
+            newTokenCount: originalTokenCount,
+            compressionStatus: CompressionStatus.NOOP,
+          },
+        };
+      }
+    }
+
+    const splitPoint = findCompressSplitPoint(
+      curatedHistory,
+      1 - COMPRESSION_PRESERVE_THRESHOLD,
+    );
+
+    const historyToCompress = curatedHistory.slice(0, splitPoint);
+    const historyToKeep = curatedHistory.slice(splitPoint);
+
+    if (historyToCompress.length === 0) {
+      return {
+        newHistory: null,
+        info: {
+          originalTokenCount,
+          newTokenCount: originalTokenCount,
+          compressionStatus: CompressionStatus.NOOP,
+        },
+      };
+    }
+
+    const summaryResponse = await config.getContentGenerator().generateContent(
+      {
+        model,
+        contents: [
+          ...historyToCompress,
+          {
+            role: 'user',
+            parts: [
+              {
+                text: 'First, reason in your scratchpad. Then, generate the <state_snapshot>.',
+              },
+            ],
+          },
+        ],
+        config: {
+          systemInstruction: { text: getCompressionPrompt() },
+        },
+      },
+      promptId,
+    );
+    const summary = getResponseText(summaryResponse) ?? '';
+
+    const extraHistory: Content[] = [
+      {
+        role: 'user',
+        parts: [{ text: summary }],
+      },
+      {
+        role: 'model',
+        parts: [{ text: 'Got it. Thanks for the additional context!' }],
+      },
+      ...historyToKeep,
+    ];
+
+    // Use a shared utility to construct the initial history for an accurate token count.
+    const fullNewHistory = await getInitialChatHistory(config, extraHistory);
+
+    // Estimate token count 1 token ≈ 4 characters
+    const newTokenCount = Math.floor(
+      fullNewHistory.reduce(
+        (total, content) => total + JSON.stringify(content).length,
+        0,
+      ) / 4,
+    );
+
+    logChatCompression(
+      config,
+      makeChatCompressionEvent({
+        tokens_before: originalTokenCount,
+        tokens_after: newTokenCount,
+      }),
+    );
+
+    if (newTokenCount > originalTokenCount) {
+      return {
+        newHistory: null,
+        info: {
+          originalTokenCount,
+          newTokenCount,
+          compressionStatus:
+            CompressionStatus.COMPRESSION_FAILED_INFLATED_TOKEN_COUNT,
+        },
+      };
+    } else {
+      return {
+        newHistory: extraHistory,
+        info: {
+          originalTokenCount,
+          newTokenCount,
+          compressionStatus: CompressionStatus.COMPRESSED,
+        },
+      };
+    }
+  }
+}