From 39fe31d2d31301163efdeee173eb5a5aa874dea5 Mon Sep 17 00:00:00 2001
From: mkorwel <matt.korwel@gmail.com>
Date: Wed, 11 Feb 2026 22:31:43 -0600
Subject: [PATCH] agent harnness

---
 integration-tests/agent_harness_e2e.test.ts | 44 +++++++++++++++++++++
 packages/core/src/agents/harness.ts         | 13 +++++-
 packages/core/src/core/client.test.ts       |  1 +
 packages/core/src/core/client.ts            |  1 +
 4 files changed, 57 insertions(+), 2 deletions(-)
 create mode 100644 integration-tests/agent_harness_e2e.test.ts

diff --git a/integration-tests/agent_harness_e2e.test.ts b/integration-tests/agent_harness_e2e.test.ts
new file mode 100644
index 0000000000..ea37006acc
--- /dev/null
+++ b/integration-tests/agent_harness_e2e.test.ts
@@ -0,0 +1,44 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { TestRig } from './test-helper.js';
+
+describe('Agent Harness E2E', () => {
+  let rig: TestRig;
+
+  beforeEach(() => {
+    rig = new TestRig();
+  });
+
+  afterEach(async () => await rig.cleanup());
+
+  it('should execute a simple prompt using the agent harness', async () => {
+    await rig.setup('agent-harness-simple');
+
+    // Run with the harness enabled via env var
+    // Turn 1
+    const result1 = await rig.run({
+      args: ['chat', 'My name is GeminiUser'],
+      env: {
+        ...process.env,
+        GEMINI_ENABLE_AGENT_HARNESS: 'true',
+      },
+    });
+    expect(result1).toBeDefined();
+
+    // Turn 2
+    const result2 = await rig.run({
+      args: ['chat', 'What is my name?', '--resume', 'latest'],
+      env: {
+        ...process.env,
+        GEMINI_ENABLE_AGENT_HARNESS: 'true',
+      },
+    });
+
+    expect(result2).toContain('GeminiUser');
+  }, 30000);
+});
diff --git a/packages/core/src/agents/harness.ts b/packages/core/src/agents/harness.ts
index 3967bdf5bd..0645586e58 100644
--- a/packages/core/src/agents/harness.ts
+++ b/packages/core/src/agents/harness.ts
@@ -4,7 +4,11 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import { type Part, type FunctionDeclaration } from '@google/genai';
+import {
+  type Part,
+  type FunctionDeclaration,
+  type Content,
+} from '@google/genai';
 import { type Config } from '../config/config.js';
 import { GeminiChat } from '../core/geminiChat.js';
 import {
@@ -45,6 +49,8 @@ export interface AgentHarnessOptions {
   inputs?: AgentInputs;
   /** If provided, this prompt_id will be used as a prefix. */
   parentPromptId?: string;
+  /** Existing chat history to initialize with (e.g. for main agent turns). */
+  history?: Content[];
 }
 
 /**
@@ -60,6 +66,7 @@ export class AgentHarness {
   private readonly compressionService: ChatCompressionService;
   private readonly toolOutputMaskingService: ToolOutputMaskingService;
   private readonly toolRegistry: ToolRegistry;
+  private readonly initialHistory?: Content[];
 
   private chat?: GeminiChat;
   private currentSequenceModel: string | null = null;
@@ -68,6 +75,7 @@ export class AgentHarness {
   constructor(options: AgentHarnessOptions) {
     this.config = options.config;
     this.behavior = options.behavior;
+    this.initialHistory = options.history;
 
     this.loopDetector = new LoopDetectionService(this.config);
     this.compressionService = new ChatCompressionService();
@@ -89,7 +97,8 @@ export class AgentHarness {
 
   private async createChat(): Promise<GeminiChat> {
     const systemInstruction = await this.behavior.getSystemInstruction();
-    const history = await this.behavior.getInitialHistory();
+    const history =
+      this.initialHistory ?? (await this.behavior.getInitialHistory());
     const tools = this.prepareToolsList();
 
     return new GeminiChat(
diff --git a/packages/core/src/core/client.test.ts b/packages/core/src/core/client.test.ts
index 900abac591..a3849a52de 100644
--- a/packages/core/src/core/client.test.ts
+++ b/packages/core/src/core/client.test.ts
@@ -243,6 +243,7 @@ describe('Gemini Client (client.ts)', () => {
       getShowModelInfoInChat: vi.fn().mockReturnValue(false),
       getContinueOnFailedApiCall: vi.fn(),
       getProjectRoot: vi.fn().mockReturnValue('/test/project/root'),
+      isAgentHarnessEnabled: vi.fn().mockReturnValue(false),
       storage: {
         getProjectTempDir: vi.fn().mockReturnValue('/test/temp'),
       },
diff --git a/packages/core/src/core/client.ts b/packages/core/src/core/client.ts
index 257ad2fb3e..bdec2e4311 100644
--- a/packages/core/src/core/client.ts
+++ b/packages/core/src/core/client.ts
@@ -812,6 +812,7 @@ export class GeminiClient {
       if (!this.harness || this.lastPromptId !== prompt_id) {
         this.harness = AgentFactory.createHarness(this.config, undefined, {
           parentPromptId: prompt_id,
+          history: this.getChat().getHistory(),
         });
         this.lastPromptId = prompt_id;
       }