From d74cd0de5c2efc8cea612bb00df6f6db037e565e Mon Sep 17 00:00:00 2001
From: can1357 <me@can.ac>
Date: Mon, 9 Mar 2026 14:18:21 +0100
Subject: [PATCH] fix handoff system prompt reset

---
 .../coding-agent/src/session/agent-session.ts |  1 +
 .../test/agent-session-handoff.test.ts        | 96 +++++++++++++++++++
 2 files changed, 97 insertions(+)

diff --git a/packages/coding-agent/src/session/agent-session.ts b/packages/coding-agent/src/session/agent-session.ts
index 009b923d3..8ad0527b6 100644
--- a/packages/coding-agent/src/session/agent-session.ts
+++ b/packages/coding-agent/src/session/agent-session.ts
@@ -3243,6 +3243,7 @@ export class AgentSession {
 			}
 			this.#promptInFlightCount++;
 			try {
+				this.agent.setSystemPrompt(this.#baseSystemPrompt);
 				await this.#promptAgentWithIdleRetry([
 					{
 						role: "developer",
diff --git a/packages/coding-agent/test/agent-session-handoff.test.ts b/packages/coding-agent/test/agent-session-handoff.test.ts
index 183c5ee88..f98b6886f 100644
--- a/packages/coding-agent/test/agent-session-handoff.test.ts
+++ b/packages/coding-agent/test/agent-session-handoff.test.ts
@@ -6,6 +6,7 @@ import { getBundledModel } from "@oh-my-pi/pi-ai/models";
 import { AssistantMessageEventStream } from "@oh-my-pi/pi-ai/utils/event-stream";
 import { ModelRegistry } from "@oh-my-pi/pi-coding-agent/config/model-registry";
 import { Settings } from "@oh-my-pi/pi-coding-agent/config/settings";
+import type { ExtensionRunner } from "@oh-my-pi/pi-coding-agent/extensibility/extensions";
 import { AgentSession, type AgentSessionEvent } from "@oh-my-pi/pi-coding-agent/session/agent-session";
 import { AuthStorage } from "@oh-my-pi/pi-coding-agent/session/auth-storage";
 import { SessionManager } from "@oh-my-pi/pi-coding-agent/session/session-manager";
@@ -433,6 +434,101 @@ describe("AgentSession handoff", () => {
 		});
 	});
 
+	it("resets to the base system prompt before generating a handoff", async () => {
+		const model = session.model;
+		if (!model) {
+			throw new Error("Expected model to be set");
+		}
+		await session.dispose();
+		sessionManager = SessionManager.create(tempDir.path());
+
+		const emitBeforeAgentStart = vi.fn().mockResolvedValueOnce({ systemPrompt: "Hook override" });
+		const extensionRunner = {
+			emitBeforeAgentStart,
+			emit: vi.fn().mockResolvedValue(undefined),
+		} as unknown as ExtensionRunner;
+
+		const observedSystemPrompts: string[] = [];
+		let streamCallCount = 0;
+		const agent = new Agent({
+			getApiKey: () => "test-key",
+			initialState: {
+				model,
+				systemPrompt: "Test",
+				tools: [],
+				messages: [],
+			},
+			streamFn: (_model, context) => {
+				observedSystemPrompts.push(context.systemPrompt ?? "");
+				streamCallCount++;
+				const stream = new MockAssistantStream();
+				queueMicrotask(() => {
+					const message: AssistantMessage = {
+						role: "assistant",
+						content: [
+							{
+								type: "text",
+								text: streamCallCount === 1 ? "normal response" : "## Goal\nContinue from here",
+							},
+						],
+						api: model.api,
+						provider: model.provider,
+						model: model.id,
+						stopReason: "stop",
+						usage: {
+							input: 0,
+							output: 0,
+							cacheRead: 0,
+							cacheWrite: 0,
+							totalTokens: 0,
+							cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+						},
+						timestamp: Date.now(),
+					};
+					stream.push({ type: "start", partial: message });
+					stream.push({ type: "done", reason: "stop", message });
+				});
+				return stream;
+			},
+		});
+
+		session = new AgentSession({
+			agent,
+			sessionManager,
+			settings: Settings.isolated({ "compaction.enabled": false }),
+			modelRegistry,
+			extensionRunner,
+		});
+		sessionManager.appendMessage({
+			role: "user",
+			content: [{ type: "text", text: "seed" }],
+			timestamp: Date.now() - 2,
+		});
+		sessionManager.appendMessage({
+			role: "assistant",
+			content: [{ type: "text", text: "seed response" }],
+			api: model.api,
+			provider: model.provider,
+			model: model.id,
+			stopReason: "stop",
+			usage: {
+				input: 16,
+				output: 8,
+				cacheRead: 0,
+				cacheWrite: 0,
+				totalTokens: 24,
+				cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+			},
+			timestamp: Date.now() - 1,
+		});
+
+		await session.prompt("hello from user");
+		await session.handoff();
+
+		expect(emitBeforeAgentStart).toHaveBeenCalledTimes(1);
+		expect(observedSystemPrompts).toEqual(["Hook override", "Test"]);
+	});
+
 	it("saves auto-handoff document to disk when enabled", async () => {
 		session.settings.set("compaction.handoffSaveToDisk", true);