fix(coding-agent): prevent Windows taskkill spawn crashes

closes #6596

packages/agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed Windows `NodeExecutionEnv` aborts crashing when `taskkill.exe` is unavailable on `PATH` ([#6596](https://github.com/earendil-works/pi/issues/6596)).
+
 ## [0.84.3] - 2026-08-24
 
 ### Fixed

packages/agent/src/harness/env/nodejs.ts

@@ -253,11 +253,17 @@ function getShellEnv(
 function killProcessTree(pid: number): void {
 	if (process.platform === "win32") {
 		try {
-			spawn("taskkill", ["/F", "/T", "/PID", String(pid)], {
-				stdio: "ignore",
-				detached: true,
-				windowsHide: true,
-			});
+			const child = spawn(
+				join(process.env.SystemRoot ?? "C:\\Windows", "System32", "taskkill.exe"),
+				["/F", "/T", "/PID", String(pid)],
+				{
+					stdio: "ignore",
+					detached: true,
+					windowsHide: true,
+				},
+			);
+			// A failed spawn emits "error" asynchronously; consume it to avoid crashing Node.
+			child.once("error", () => {});
 		} catch {
 			// Ignore errors.
 		}

packages/agent/test/harness/nodejs-env.test.ts

@@ -496,6 +496,48 @@ describe("NodeExecutionEnv", () => {
 		if (!result.ok) expect(result.error).toMatchObject({ code: "aborted" });
 	});
 
+	it.skipIf(process.platform === "win32")("ignores asynchronous taskkill spawn errors during abort", async () => {
+		const root = createTempDir();
+		const pidFile = join(root, "shell.pid");
+		const controller = new AbortController();
+		const env = new NodeExecutionEnv({ cwd: root, shellPath: "/bin/bash" });
+		const platformDescriptor = Object.getOwnPropertyDescriptor(process, "platform");
+		const previousSystemRoot = process.env.SystemRoot;
+		process.env.SystemRoot = "/definitely/missing/windows";
+		Object.defineProperty(process, "platform", { configurable: true, value: "win32" });
+
+		let pid: number | undefined;
+		try {
+			const execution = env.exec(`echo $$ > ${toBashSingleQuotedArg(pidFile)}; exec sleep 60`, {
+				abortSignal: controller.signal,
+			});
+			for (let attempt = 0; attempt < 100 && !existsSync(pidFile); attempt++) {
+				await new Promise((resolve) => setTimeout(resolve, 10));
+			}
+			expect(existsSync(pidFile)).toBe(true);
+
+			controller.abort();
+			await new Promise((resolve) => setTimeout(resolve, 0));
+			pid = Number.parseInt(readFileSync(pidFile, "utf8"), 10);
+			process.kill(pid, "SIGKILL");
+
+			const result = await execution;
+			expect(result).toMatchObject({ ok: false, error: { code: "aborted" } });
+		} finally {
+			if (pid === undefined && existsSync(pidFile)) {
+				pid = Number.parseInt(readFileSync(pidFile, "utf8"), 10);
+			}
+			if (pid !== undefined && Number.isFinite(pid)) {
+				try {
+					process.kill(pid, "SIGKILL");
+				} catch {}
+			}
+			if (previousSystemRoot === undefined) delete process.env.SystemRoot;
+			else process.env.SystemRoot = previousSystemRoot;
+			if (platformDescriptor) Object.defineProperty(process, "platform", platformDescriptor);
+		}
+	});
+
 	it("captures large shell output to a full output file through the execution env", async () => {
 		const root = createTempDir();
 		const env = new NodeExecutionEnv({ cwd: root });

packages/coding-agent/CHANGELOG.md

@@ -8,6 +8,7 @@
 
 ### Fixed
 
+- Fixed Windows shell aborts crashing Pi when `taskkill.exe` is unavailable on `PATH` ([#6596](https://github.com/earendil-works/pi/issues/6596)).
 - Fixed extension messages sent with `triggerTurn: false` while the agent is running being inserted between a tool call and its result, which made providers that validate message order reject the replayed history. They are now appended once the turn's tool results are in ([#8537](https://github.com/earendil-works/pi/issues/8537)).
 
 ## [0.84.3] - 2026-08-24

packages/coding-agent/src/utils/shell.ts

@@ -1,5 +1,5 @@
 import { existsSync } from "node:fs";
-import { delimiter } from "node:path";
+import { delimiter, join } from "node:path";
 import { spawn, spawnSync } from "child_process";
 import { getBinDir } from "../config.ts";
 
@@ -215,15 +215,21 @@ export function killTrackedDetachedChildren(): void {
  */
 export function killProcessTree(pid: number): void {
 	if (process.platform === "win32") {
-		// Use taskkill on Windows to kill process tree
+		// Use the trusted System32 executable so cleanup does not depend on PATH.
 		try {
-			spawn("taskkill", ["/F", "/T", "/PID", String(pid)], {
-				stdio: "ignore",
-				detached: true,
-				windowsHide: true,
-			});
+			const child = spawn(
+				join(process.env.SystemRoot ?? "C:\\Windows", "System32", "taskkill.exe"),
+				["/F", "/T", "/PID", String(pid)],
+				{
+					stdio: "ignore",
+					detached: true,
+					windowsHide: true,
+				},
+			);
+			// A failed spawn emits "error" asynchronously; consume it to avoid crashing Node.
+			child.once("error", () => {});
 		} catch {
-			// Ignore errors if taskkill fails
+			// Ignore errors if taskkill fails.
 		}
 	} else {
 		// Use SIGKILL on Unix/Linux/Mac

packages/coding-agent/test/suite/regressions/6596-taskkill-enoent.test.ts

@@ -0,0 +1,52 @@
+import type { ChildProcess } from "node:child_process";
+import { EventEmitter } from "node:events";
+import { join } from "node:path";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+const { spawnMock } = vi.hoisted(() => ({ spawnMock: vi.fn() }));
+
+vi.mock("child_process", async (importOriginal) => {
+	const actual = await importOriginal<typeof import("child_process")>();
+	return { ...actual, spawn: spawnMock };
+});
+
+import { killProcessTree } from "../../../src/utils/shell.ts";
+
+function withWindowsPlatform(test: () => void): void {
+	const platformDescriptor = Object.getOwnPropertyDescriptor(process, "platform");
+	try {
+		Object.defineProperty(process, "platform", { configurable: true, value: "win32" });
+		test();
+	} finally {
+		if (platformDescriptor) Object.defineProperty(process, "platform", platformDescriptor);
+	}
+}
+
+afterEach(() => {
+	spawnMock.mockReset();
+});
+
+describe("issue #6596 taskkill spawn failures", () => {
+	it("uses System32 taskkill and consumes its asynchronous spawn error", () => {
+		const child = new EventEmitter() as ChildProcess;
+		const previousSystemRoot = process.env.SystemRoot;
+		process.env.SystemRoot = "C:\\CustomWindows";
+		spawnMock.mockReturnValue(child);
+
+		try {
+			withWindowsPlatform(() => {
+				killProcessTree(1234);
+			});
+		} finally {
+			if (previousSystemRoot === undefined) delete process.env.SystemRoot;
+			else process.env.SystemRoot = previousSystemRoot;
+		}
+
+		expect(spawnMock).toHaveBeenCalledWith(
+			join("C:\\CustomWindows", "System32", "taskkill.exe"),
+			["/F", "/T", "/PID", "1234"],
+			{ detached: true, stdio: "ignore", windowsHide: true },
+		);
+		expect(() => child.emit("error", new Error("spawn taskkill ENOENT"))).not.toThrow();
+	});
+});