fix(coding-agent): handle async fs.watch error events in theme watcher

closes #2791

packages/coding-agent/CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Fixed
 
 - RpcClient now forwards subprocess stderr to parent process in real-time ([#2805](https://github.com/badlogic/pi-mono/issues/2805))
+- Theme file watcher now handles async `fs.watch` error events instead of crashing the process ([#2791](https://github.com/badlogic/pi-mono/issues/2791))
 
 ## [0.65.0] - 2026-04-03
 

packages/coding-agent/src/modes/interactive/theme/theme.ts

@@ -798,6 +798,14 @@ function startThemeWatcher(): void {
 			}
 			scheduleReload();
 		});
+		themeWatcher.on("error", () => {
+			try {
+				themeWatcher?.close();
+			} catch {
+				/* ignore */
+			}
+			themeWatcher = undefined;
+		});
 	} catch (_error) {
 		// Ignore errors starting watcher
 	}

packages/coding-agent/test/suite/regressions/2791-fswatch-error-crash.test.ts

@@ -0,0 +1,106 @@
+import { execFileSync } from "node:child_process";
+import { mkdirSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+
+/**
+ * Regression test for https://github.com/badlogic/pi-mono/issues/2791
+ *
+ * fs.watch() returns an FSWatcher (EventEmitter). If the watcher emits an
+ * 'error' event after creation and no error handler is attached, Node.js
+ * treats it as an uncaught exception and terminates the process.
+ *
+ * We test this by spawning a child process that:
+ * 1. Sets up a custom theme with the watcher enabled
+ * 2. Finds the FSWatcher via process._getActiveHandles()
+ * 3. Emits a synthetic 'error' event on it
+ * 4. If the watcher has no error handler -> crash (exit != 0) -> bug present
+ * 5. If the watcher has an error handler -> clean exit (exit 0) -> bug fixed
+ */
+describe("issue #2791 fs.watch error event crashes process", () => {
+	let tempRoot: string;
+
+	beforeEach(() => {
+		tempRoot = mkdtempSync(join(tmpdir(), "pi-2791-"));
+		const agentDir = join(tempRoot, "agent");
+		const themesDir = join(agentDir, "themes");
+		mkdirSync(themesDir, { recursive: true });
+
+		// Copy dark.json as "custom-test" theme
+		const darkThemePath = join(__dirname, "../../../src/modes/interactive/theme/dark.json");
+		const darkTheme = JSON.parse(readFileSync(darkThemePath, "utf-8"));
+		darkTheme.name = "custom-test";
+		writeFileSync(join(themesDir, "custom-test.json"), JSON.stringify(darkTheme, null, 2));
+	});
+
+	afterEach(() => {
+		rmSync(tempRoot, { recursive: true, force: true });
+	});
+
+	it("process should survive an error event on the theme FSWatcher", () => {
+		const themeModulePath = join(__dirname, "../../../src/modes/interactive/theme/theme.js").replace(/\\/g, "/");
+		const agentDir = join(tempRoot, "agent").replace(/\\/g, "/");
+
+		// Script that sets up the watcher and emits a synthetic error on it.
+		// If no .on('error') handler is attached, EventEmitter.emit('error')
+		// throws, which either crashes the process or gets caught by our try/catch.
+		const scriptPath = join(tempRoot, "test-watcher-error.mts");
+		writeFileSync(
+			scriptPath,
+			`
+import { setTheme, stopThemeWatcher } from "${themeModulePath}";
+
+process.env.PI_CODING_AGENT_DIR = "${agentDir}";
+
+setTheme("custom-test", true);
+
+// Find the FSWatcher among active handles
+const handles = (process as any)._getActiveHandles();
+const fsWatcher = handles.find((h: any) => h.constructor?.name === "FSWatcher");
+
+if (!fsWatcher) {
+	process.stderr.write("no FSWatcher found among active handles\\n");
+	process.exit(2);
+}
+
+const errorListenerCount = fsWatcher.listenerCount("error");
+if (errorListenerCount === 0) {
+	process.stderr.write("BUG: FSWatcher has no error handler (issue #2791)\\n");
+}
+
+// Emitting 'error' on an EventEmitter with no error listener throws.
+// This simulates an async OS error (e.g. ReadDirectoryChangesW invalidation).
+try {
+	fsWatcher.emit("error", new Error("simulated OS watcher failure"));
+} catch {
+	process.stderr.write("error event was unhandled and threw\\n");
+	process.exit(1);
+}
+
+stopThemeWatcher();
+process.exit(0);
+`,
+		);
+
+		let _stdout = "";
+		let stderr = "";
+		let exitCode: number;
+		try {
+			_stdout = execFileSync("npx", ["tsx", scriptPath], {
+				timeout: 10000,
+				encoding: "utf-8",
+				env: { ...process.env, PI_CODING_AGENT_DIR: agentDir },
+				stdio: ["pipe", "pipe", "pipe"],
+			});
+			exitCode = 0;
+		} catch (err: unknown) {
+			const e = err as { status: number; stdout: string; stderr: string };
+			_stdout = e.stdout ?? "";
+			stderr = e.stderr ?? "";
+			exitCode = e.status ?? 1;
+		}
+
+		expect(exitCode, `Child crashed (exit ${exitCode}). stderr: ${stderr.trim()}`).toBe(0);
+	});
+});