diff --git a/electron/desktop-logger.cjs b/electron/desktop-logger.cjs
new file mode 100644
index 0000000..f15de16
--- /dev/null
+++ b/electron/desktop-logger.cjs
@@ -0,0 +1,116 @@
+/** Bounded desktop diagnostics. Error paths must never write to a failed pipe. */
+const fs = require("node:fs");
+const path = require("node:path");
+
+const MAX_LOG_BYTES = 5 * 1024 * 1024;
+
+function createDesktopLogger({ directory, stdout, stderr, filesystem = fs, maxFileBytes = MAX_LOG_BYTES }) {
+	let initialized = false;
+	let stdoutEnabled = Boolean(stdout);
+	let writing = false;
+	const location = () => path.join(directory(), "main.log");
+
+	// Read only the retained tail, even when an older release left a huge file.
+	function trim(file) {
+		let handle;
+		try {
+			handle = filesystem.openSync(file, "r+");
+			const size = filesystem.fstatSync(handle).size;
+			if (size <= maxFileBytes) return;
+			const tail = Buffer.alloc(maxFileBytes);
+			let read = 0;
+			while (read < tail.length) {
+				const count = filesystem.readSync(handle, tail, read, tail.length - read, size - tail.length + read);
+				if (!count) break;
+				read += count;
+			}
+			filesystem.writeFileSync(handle, tail.subarray(0, read));
+			filesystem.ftruncateSync(handle, read);
+		} catch (error) {
+			if (error.code !== "ENOENT") throw error;
+		} finally {
+			if (handle !== undefined) filesystem.closeSync(handle);
+		}
+	}
+
+	function initialize() {
+		try {
+			filesystem.mkdirSync(directory(), { recursive: true });
+			trim(location());
+			trim(`${location()}.1`);
+			initialized = true;
+		} catch {
+			// Diagnostics are best effort; never report a logger failure via itself.
+		}
+	}
+
+	function record(line) {
+		const prefix = Buffer.from(`[${new Date().toISOString()}] `);
+		const body = Buffer.from(String(line));
+		if (prefix.length + body.length + 1 <= maxFileBytes) return Buffer.concat([prefix, body, Buffer.from("\n")]);
+		const marker = Buffer.from("[truncated] ");
+		const tail = body.subarray(body.length - (maxFileBytes - prefix.length - marker.length - 1));
+		// A retained suffix can start inside a UTF-8 character.
+		let start = 0;
+		while (start < tail.length && (tail[start] & 0xc0) === 0x80) start++;
+		return Buffer.concat([prefix, marker, tail.subarray(start), Buffer.from("\n")]);
+	}
+
+	function append(entry) {
+		try {
+			if (!initialized) initialize();
+			if (!initialized) return;
+			const file = location();
+			let size = 0;
+			try { size = filesystem.statSync(file).size; } catch (error) { if (error.code !== "ENOENT") throw error; }
+			if (size + entry.length > maxFileBytes) {
+				filesystem.rmSync(`${file}.1`, { force: true });
+				filesystem.renameSync(file, `${file}.1`);
+			}
+			filesystem.appendFileSync(file, entry);
+		} catch {
+			// Includes ENOSPC and rotation failures. Do not fall back to a pipe.
+		}
+	}
+
+	function log(line, { fileOnly = false } = {}) {
+		if (writing) return;
+		writing = true;
+		try {
+			const entry = record(line);
+			append(entry);
+			if (!fileOnly && stdoutEnabled && !stdout.destroyed && !stdout.writableEnded) {
+				try {
+					// A false return means backpressure: skip mirroring until drained.
+					if (!stdout.write(entry, (error) => { if (error) disableStdout(error); })) stdoutEnabled = false;
+				} catch (error) {
+					disableStdout(error);
+				}
+			}
+		} catch {
+			// Formatting must not escape into the uncaught-exception handler either.
+		} finally {
+			writing = false;
+		}
+	}
+
+	let stdoutFailed = false;
+	function disableStdout(error) {
+		stdoutEnabled = false;
+		if (stdoutFailed) return;
+		stdoutFailed = true;
+		append(record(`stdout disabled: ${error?.code ?? error?.message ?? "stream error"}`));
+	}
+	stdout?.on("error", disableStdout);
+	stdout?.on("drain", () => { if (!stdoutFailed) stdoutEnabled = true; });
+	// Console / dependency writes to stderr can fail asynchronously as well.
+	let stderrFailed = false;
+	stderr?.on("error", (error) => {
+		if (stderrFailed) return;
+		stderrFailed = true;
+		log(`stderr disabled: ${error?.code ?? error?.message ?? "stream error"}`, { fileOnly: true });
+	});
+	return { log, initialize };
+}
+
+module.exports = { createDesktopLogger, MAX_LOG_BYTES };
diff --git a/electron/main.cjs b/electron/main.cjs
index 4f53c96..e27ec1d 100644
--- a/electron/main.cjs
+++ b/electron/main.cjs
@@ -37,6 +37,7 @@ const fs = require("node:fs");
 const http = require("node:http");
 const net = require("node:net");
 const path = require("node:path");
+const { createDesktopLogger } = require("./desktop-logger.cjs");
 
 // Mirrors SIDEBAR_AUTOCLOSE in components/sidebar.tsx — keep in sync (the
 // renderer owns the number; this is the IPC widen target).
@@ -143,13 +144,20 @@ let subscriptionChild = null;
 let desktopAPISecret = null;
 let reopenWindow = null;
 const trustedAppWindows = new Set();
+const desktopLogger = createDesktopLogger({
+	directory: () => path.join(app.getPath("userData"), "logs"),
+	stdout: process.stdout,
+	stderr: process.stderr,
+	filesystem: fs,
+});
 
 // A GUI shell must never die to a stray exception (EPIPE dialogs, a race
 // in an event handler): the window + child server are the product, and
 // anything main-side is recoverable by logging it. Log-and-continue.
 process.on("uncaughtException", (err) => {
 	try {
-		log(`uncaught: ${err?.stack ?? err}`);
+		// File only: writing to stdout here used to turn EPIPE into a tight loop.
+		desktopLogger.log(`uncaught: ${err?.stack ?? err}`, { fileOnly: true });
 	} catch {
 		/* nothing left to write to */
 	}
@@ -177,25 +185,7 @@ function freePort() {
  *  still win over the file inside the child (its documented rule). */
 
 function log(line) {
-	const stamp = new Date().toISOString();
-	// The stdout write is guarded (EPIPE hardening, 2026-10-10): a packaged
-	// app launched with a consumer-owned pipe (a launcher that exits, a
-	// piped run) must not die on its next log line when the pipe breaks.
-	// The file below stays the durable channel.
-	try {
-		console.log(`[lh ${stamp}] ${line}`);
-	} catch {
-		/* stdout gone — userData/logs/main.log still carries it */
-	}
-	// Belt and braces: also append to userData/logs so a misbehaving child
-	// can be diagnosed after the window is gone. Best effort only.
-	try {
-		const dir = path.join(app.getPath("userData"), "logs");
-		fs.mkdirSync(dir, { recursive: true });
-		fs.appendFileSync(path.join(dir, "main.log"), `[${stamp}] ${line}\n`);
-	} catch {
-		/* best effort */
-	}
+	desktopLogger.log(line);
 }
 
 function startServer(port) {
@@ -656,6 +646,7 @@ async function main() {
 	process.on("SIGHUP", killServer);
 
 	await app.whenReady();
+	desktopLogger.initialize();
 
 	// Dev mode: point at a running `next dev` server (LH_DEV_URL) — no
 	// spawn, no packaging, full HMR. The rest of this shell still applies
@@ -771,4 +762,4 @@ async function main() {
 		checkForUpdates();
 		setInterval(() => checkForUpdates(), UPDATE_CHECK_INTERVAL_MS);
 	}
-}
\ No newline at end of file
+}
diff --git a/tests/desktop-logger.test.ts b/tests/desktop-logger.test.ts
new file mode 100644
index 0000000..896bbce
--- /dev/null
+++ b/tests/desktop-logger.test.ts
@@ -0,0 +1,196 @@
+import assert from "node:assert/strict";
+import { spawn } from "node:child_process";
+import { EventEmitter, once } from "node:events";
+import fs from "node:fs";
+import { createRequire } from "node:module";
+import os from "node:os";
+import path from "node:path";
+import { describe, it } from "node:test";
+import type { TestContext } from "node:test";
+import { fileURLToPath } from "node:url";
+import { createContext, runInContext } from "node:vm";
+
+const require = createRequire(import.meta.url);
+const { createDesktopLogger, MAX_LOG_BYTES } = require("../electron/desktop-logger.cjs");
+const mainFile = fileURLToPath(new URL("../electron/main.cjs", import.meta.url));
+const mainRequire = createRequire(mainFile);
+
+function directory(t: TestContext) {
+	const dir = fs.mkdtempSync(path.join(os.tmpdir(), "lh-desktop-logger-"));
+	t.after(() => fs.rmSync(dir, { recursive: true, force: true }));
+	return dir;
+}
+
+function retained(dir: string, cap: number) {
+	const names = fs.readdirSync(dir).sort();
+	assert.ok(names.length <= 2);
+	for (const name of names) {
+		assert.ok(["main.log", "main.log.1"].includes(name));
+		assert.ok(fs.statSync(path.join(dir, name)).size <= cap, `${name} exceeds the byte cap`);
+	}
+	return ["main.log.1", "main.log"].filter(name => names.includes(name)).map(name => fs.readFileSync(path.join(dir, name), "utf8")).join("");
+}
+
+class Output extends EventEmitter {
+	writes = 0;
+	write() { this.writes++; return true; }
+}
+
+describe("bounded desktop logs", () => {
+	it("rotates repeated floods within two 5 MiB files and keeps the latest entries", (t) => {
+		const dir = directory(t);
+		assert.equal(MAX_LOG_BYTES, 5 * 1024 * 1024);
+		const logger = createDesktopLogger({ directory: () => dir });
+		for (let i = 0; i < 35; i++) logger.log(`${i} ${"x".repeat(1024 * 1024)}`);
+		logger.log("most recent diagnostic");
+		const text = retained(dir, MAX_LOG_BYTES);
+		assert.match(text, /most recent diagnostic\n$/);
+		assert.doesNotMatch(text, /\] 0 /);
+	});
+
+	it("trims both existing files on startup using a bounded tail read", (t) => {
+		const dir = directory(t);
+		for (const name of ["main.log", "main.log.1"]) {
+			const file = path.join(dir, name);
+			// Sparse legacy file: reproduces a huge prior log without allocating it.
+			const fd = fs.openSync(file, "w");
+			fs.writeSync(fd, Buffer.from(`recent ${name}\n`), 0, Buffer.byteLength(`recent ${name}\n`), 200 * 1024 ** 3);
+			fs.closeSync(fd);
+		}
+		let largestRead = 0;
+		const filesystem = { ...fs, readSync: (...args: Parameters<typeof fs.readSync>) => {
+			largestRead = Math.max(largestRead, (args[1] as Buffer).byteLength);
+			return fs.readSync(...args);
+		} };
+		const logger = createDesktopLogger({ directory: () => dir, filesystem });
+		logger.initialize();
+		assert.ok(largestRead <= MAX_LOG_BYTES);
+		assert.match(retained(dir, MAX_LOG_BYTES), /recent main.log\n$/);
+		assert.match(fs.readFileSync(path.join(dir, "main.log.1"), "utf8"), /recent main.log.1\n$/);
+		logger.log("after restart");
+		createDesktopLogger({ directory: () => dir }).log("after second restart");
+		assert.match(retained(dir, MAX_LOG_BYTES), /after second restart\n$/);
+	});
+
+	it("caps one oversized Unicode entry by bytes, retaining valid UTF-8 and its suffix", (t) => {
+		const dir = directory(t);
+		const logger = createDesktopLogger({ directory: () => dir, maxFileBytes: 256 });
+		logger.log(`${"🙂".repeat(1000)}latest`);
+		const text = retained(dir, 256);
+		assert.match(text, /\[truncated\]/);
+		assert.match(text, /latest\n$/);
+		assert.ok(!text.includes("�"));
+	});
+
+	it("disables stdout permanently on asynchronous errors, with one file-only diagnostic", (t) => {
+		const dir = directory(t);
+		const stdout = new Output();
+		const stderr = new Output();
+		const logger = createDesktopLogger({ directory: () => dir, stdout, stderr });
+		logger.log("before failure");
+		const error = Object.assign(new Error("broken pipe"), { code: "EPIPE" });
+		stdout.emit("error", error);
+		stdout.emit("drain");
+		stdout.emit("error", error);
+		stderr.emit("error", error);
+		stderr.emit("error", error);
+		logger.log("after failure");
+		assert.equal(stdout.writes, 1);
+		const text = retained(dir, MAX_LOG_BYTES);
+		assert.equal(text.match(/stdout disabled/g)?.length, 1);
+		assert.equal(text.match(/stderr disabled/g)?.length, 1);
+		assert.match(text, /after failure/);
+	});
+
+	it("handles synchronous write failures and bounds buffering under backpressure", (t) => {
+		const dir = directory(t);
+		const stdout = new Output();
+		stdout.write = () => { stdout.writes++; throw Object.assign(new Error("closed"), { code: "EPIPE" }); };
+		const logger = createDesktopLogger({ directory: () => dir, stdout });
+		assert.doesNotThrow(() => { logger.log("first"); logger.log("second"); });
+		assert.equal(stdout.writes, 1);
+		const slow = new Output();
+		slow.write = () => { slow.writes++; return false; };
+		const slowLogger = createDesktopLogger({ directory: () => dir, stdout: slow });
+		for (let i = 0; i < 100; i++) slowLogger.log(`slow ${i}`);
+		assert.equal(slow.writes, 1);
+		slow.emit("drain");
+		slowLogger.log("drained");
+		assert.equal(slow.writes, 2);
+	});
+
+	it("never propagates full-disk or rotation errors or mirrors file-only errors", (t) => {
+		const dir = directory(t);
+		const stdout = new Output();
+		const logger = createDesktopLogger({ directory: () => dir, stdout, filesystem: {
+			...fs, appendFileSync() { throw Object.assign(new Error("disk full"), { code: "ENOSPC" }); },
+		} });
+		assert.doesNotThrow(() => logger.log("uncaught: diagnostic", { fileOnly: true }));
+		assert.equal(stdout.writes, 0);
+		const file = path.join(dir, "main.log");
+		fs.writeFileSync(file, "old\n".repeat(60));
+		const blockedRotation = createDesktopLogger({ directory: () => dir, maxFileBytes: 256, filesystem: {
+			...fs, renameSync() { throw new Error("rotation denied"); },
+		} });
+		assert.doesNotThrow(() => blockedRotation.log("new".repeat(100)));
+		assert.equal(fs.statSync(file).size, 240, "a failed rotation must not append beyond the cap");
+	});
+});
+
+describe("desktop shell logging integration", () => {
+	it("records uncaught errors without touching stdout", (t) => {
+		const dir = directory(t);
+		const handlers = new Map<string, (error: Error) => void>();
+		const stdout = new Output();
+		const context = createContext({
+			require: (id: string) => id === "electron" ? { app: { setName() {}, getPath: () => dir, requestSingleInstanceLock: () => false, quit() {} } } : mainRequire(id),
+			process: { argv: [], env: {}, stdout, stderr: new Output(), on: (name: string, handler: (error: Error) => void) => handlers.set(name, handler) },
+			__dirname: path.dirname(mainFile),
+		});
+		runInContext(fs.readFileSync(mainFile, "utf8"), context, { filename: mainFile });
+		handlers.get("uncaughtException")!(new Error("original failure"));
+		assert.equal(stdout.writes, 0);
+		assert.match(retained(path.join(dir, "logs"), MAX_LOG_BYTES), /uncaught: Error: original failure/);
+	});
+
+	it("survives a real closed stdout pipe, records EPIPE once, and exits normally", async (t) => {
+		const dir = directory(t);
+		const source = `
+const fs = require('node:fs');
+const vm = require('node:vm');
+const mainFile = ${JSON.stringify(mainFile)};
+const mainRequire = require('node:module').createRequire(mainFile);
+const context = vm.createContext({
+  require: id => id === 'electron' ? {app: {setName() {}, getPath: () => ${JSON.stringify(dir)}, requestSingleInstanceLock: () => false, quit() {}}} : mainRequire(id),
+  process, console, __dirname: require('node:path').dirname(mainFile)
+});
+vm.runInContext(fs.readFileSync(mainFile, 'utf8'), context, {filename: mainFile});
+process.once('message', () => {
+  vm.runInContext('log("trigger broken pipe")', context);
+  setTimeout(() => {
+    vm.runInContext('log("still alive after broken pipe")', context);
+    process.disconnect();
+  }, 100);
+});
+process.send('ready');
+`;
+		const child = spawn(process.execPath, ["-e", source], { stdio: ["ignore", "pipe", "pipe", "ipc"] });
+		t.after(() => { if (child.exitCode === null) child.kill("SIGKILL"); });
+		const deadline = setTimeout(() => child.kill("SIGKILL"), 5000);
+		t.after(() => clearTimeout(deadline));
+		let stderr = "";
+		child.stderr!.on("data", chunk => { stderr = (stderr + chunk).slice(-4000); });
+		const exited = once(child, "exit");
+		await Promise.race([once(child, "message"), exited.then(() => { throw new Error(`Fixture exited before ready: ${stderr}`); })]);
+		child.stdout!.destroy();
+		child.send("start");
+		const [code, signal] = await exited;
+		assert.equal(signal, null, stderr);
+		assert.equal(code, 0, stderr);
+		const text = retained(path.join(dir, "logs"), MAX_LOG_BYTES);
+		assert.equal(text.match(/EPIPE/g)?.length, 1);
+		assert.doesNotMatch(text, /uncaught:/);
+		assert.match(text, /still alive after broken pipe/);
+		assert.ok(Buffer.byteLength(text) < 1024, "a closed pipe must not produce a log flood");
+	});
+});