fix(autoresearch): guarded before_agent_start against undefined event.systemPrompt

The handler called event.systemPrompt.join("\n\n") on a value that
upstream system prompt resolution can leave unset, crashing the
extension with 'event.systemPrompt is undefined'. Coerce defensively
so the autoresearch block still renders.

Fixes #3665

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed autoresearch's `before_agent_start` handler crashing when `event.systemPrompt` was undefined. The handler now coerces a missing system prompt to an empty string so the autoresearch block still renders. ([#3665](https://github.com/can1357/oh-my-pi/issues/3665))
+
 ## [16.2.2] - 2026-06-27
 
 ### Added

packages/coding-agent/src/autoresearch/index.ts

@@ -320,6 +320,11 @@ export const createAutoresearchExtension: ExtensionFactory = api => {
 		runtime.lastRunDuration = pendingRun?.durationSeconds ?? runtime.lastRunDuration;
 		runtime.lastRunAsi = pendingRun?.parsedAsi ?? runtime.lastRunAsi;
 		const state = runtime.state;
+		// `event.systemPrompt` is typed `string[]`, but upstream code paths can leave
+		// it unset (issue #3665). Coerce defensively so the autoresearch block still
+		// renders — the model just loses the upstream prefix for this turn, which is
+		// strictly better than crashing the handler.
+		const basePrompt = Array.isArray(event.systemPrompt) ? event.systemPrompt.join("\n\n") : "";
 		const currentSegmentResults = currentResults(state.results, state.currentSegment);
 		const baselineMetric = findBaselineMetric(state.results, state.currentSegment);
 		const baselineRunNumber = findBaselineRunNumber(state.results, state.currentSegment);
@@ -358,7 +363,7 @@ export const createAutoresearchExtension: ExtensionFactory = api => {
 			return {
 				systemPrompt: [
 					prompt.render(setupPromptTemplate, {
-						base_system_prompt: event.systemPrompt.join("\n\n"),
+						base_system_prompt: basePrompt,
 						has_goal: goal.trim().length > 0,
 						goal,
 						working_dir: ctx.cwd,
@@ -373,7 +378,7 @@ export const createAutoresearchExtension: ExtensionFactory = api => {
 		return {
 			systemPrompt: [
 				prompt.render(promptTemplate, {
-					base_system_prompt: event.systemPrompt.join("\n\n"),
+					base_system_prompt: basePrompt,
 					has_goal: goal.trim().length > 0,
 					goal,
 					working_dir: ctx.cwd,

packages/coding-agent/test/autoresearch-before-agent-start.test.ts

@@ -0,0 +1,135 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "bun:test";
+import { createAutoresearchExtension } from "@oh-my-pi/pi-coding-agent/autoresearch";
+import { closeAllAutoresearchStorages } from "@oh-my-pi/pi-coding-agent/autoresearch/storage";
+import type {
+	BeforeAgentStartEvent,
+	BeforeAgentStartEventResult,
+	ExtensionAPI,
+	ExtensionContext,
+	ExtensionHandler,
+	SessionStartEvent,
+} from "@oh-my-pi/pi-coding-agent/extensibility/extensions";
+import * as git from "@oh-my-pi/pi-coding-agent/utils/git";
+import { TempDir } from "@oh-my-pi/pi-utils";
+
+// Reproduces issue #3665: when the upstream system prompt resolution leaves
+// `event.systemPrompt` unset, the autoresearch handler must still render its
+// own block instead of crashing with `event.systemPrompt.join is not a function`.
+
+interface CapturedHandlers {
+	session_start?: ExtensionHandler<SessionStartEvent>;
+	before_agent_start?: ExtensionHandler<BeforeAgentStartEvent, BeforeAgentStartEventResult>;
+}
+
+function buildHarness(): { handlers: CapturedHandlers; activeTools: string[] } {
+	const handlers: CapturedHandlers = {};
+	const activeTools: string[] = [];
+	const api = {
+		appendEntry(): void {},
+		exec: async () => ({ code: 0, stderr: "", stdout: "" }),
+		on(event: string, handler: ExtensionHandler<unknown, unknown>): void {
+			(handlers as Record<string, ExtensionHandler<unknown, unknown>>)[event] = handler;
+		},
+		registerCommand(): void {},
+		registerShortcut(): void {},
+		registerTool(): void {},
+		getActiveTools: (): string[] => [...activeTools],
+		setActiveTools: async (names: string[]): Promise<void> => {
+			activeTools.splice(0, activeTools.length, ...names);
+		},
+		sendUserMessage(): void {},
+		sendMessage(): void {},
+	} as unknown as ExtensionAPI;
+	createAutoresearchExtension(api);
+	return { handlers, activeTools };
+}
+
+function makeCtx(cwd: string): ExtensionContext {
+	return {
+		cwd,
+		hasUI: false,
+		hasPendingMessages: () => false,
+		sessionManager: {
+			getSessionId: () => "session-bas-test",
+			getBranch: () => [
+				{
+					type: "custom",
+					customType: "autoresearch-control",
+					id: "ctrl-1",
+					parentId: null,
+					timestamp: new Date(0).toISOString(),
+					data: { mode: "on", goal: "speed up the thing" },
+				},
+			],
+		},
+	} as unknown as ExtensionContext;
+}
+
+describe("autoresearch before_agent_start handler", () => {
+	let dbDir: TempDir;
+	let cwdDir: TempDir;
+
+	beforeEach(() => {
+		dbDir = TempDir.createSync("@pi-autoresearch-bas-test-");
+		process.env.OMP_AUTORESEARCH_DB_DIR = dbDir.path();
+		cwdDir = TempDir.createSync("@pi-autoresearch-bas-cwd-");
+		vi.spyOn(git.branch, "current").mockResolvedValue("autoresearch/test");
+		vi.spyOn(git.repo, "root").mockResolvedValue(cwdDir.path());
+	});
+
+	afterEach(() => {
+		delete process.env.OMP_AUTORESEARCH_DB_DIR;
+		closeAllAutoresearchStorages();
+		cwdDir.removeSync();
+		dbDir.removeSync();
+		vi.restoreAllMocks();
+	});
+
+	it("renders an autoresearch block when event.systemPrompt is undefined (issue #3665)", async () => {
+		const { handlers } = buildHarness();
+		if (!handlers.session_start || !handlers.before_agent_start) {
+			throw new Error("Autoresearch extension should register both session_start and before_agent_start");
+		}
+
+		const ctx = makeCtx(cwdDir.path());
+		await handlers.session_start({ type: "session_start" } as SessionStartEvent, ctx);
+
+		// Crash repro: upstream leaves event.systemPrompt unset; handler must
+		// not throw, and the rendered block must still contain the autoresearch
+		// header so the model gets its mode-specific instructions.
+		const event = {
+			type: "before_agent_start",
+			prompt: "kick off",
+			images: undefined,
+			systemPrompt: undefined,
+		} as unknown as BeforeAgentStartEvent;
+
+		const result = (await handlers.before_agent_start(event, ctx)) as BeforeAgentStartEventResult;
+		expect(result).toBeDefined();
+		expect(Array.isArray(result.systemPrompt)).toBe(true);
+		const blocks = result.systemPrompt as string[];
+		expect(blocks).toHaveLength(1);
+		expect(blocks[0]).toContain("Autoresearch Mode");
+	});
+
+	it("joins event.systemPrompt blocks into the rendered base prompt", async () => {
+		const { handlers } = buildHarness();
+		if (!handlers.session_start || !handlers.before_agent_start) {
+			throw new Error("Autoresearch extension should register both session_start and before_agent_start");
+		}
+
+		const ctx = makeCtx(cwdDir.path());
+		await handlers.session_start({ type: "session_start" } as SessionStartEvent, ctx);
+
+		const event: BeforeAgentStartEvent = {
+			type: "before_agent_start",
+			prompt: "kick off",
+			systemPrompt: ["alpha block", "beta block"],
+		};
+
+		const result = (await handlers.before_agent_start(event, ctx)) as BeforeAgentStartEventResult;
+		expect(result).toBeDefined();
+		const rendered = (result.systemPrompt as string[])[0];
+		expect(rendered.startsWith("alpha block\n\nbeta block")).toBe(true);
+	});
+});