diff --git a/src/core/zip.ts b/src/core/zip.ts
new file mode 100644
index 0000000..9c132f3
--- /dev/null
+++ b/src/core/zip.ts
@@ -0,0 +1,99 @@
+// A store-only ZIP writer (spec 002 §2): covers are JPEG/PNG/WebP and don't shrink, so no compression.
+
+export interface ZipEntry {
+  name: string;
+  bytes: Uint8Array;
+  modified: Date;
+}
+
+export class ZipError extends Error {
+  override name = "ZipError";
+}
+
+const CRC_TABLE = (() => {
+  const table = new Uint32Array(256);
+  for (let n = 0; n < 256; n++) {
+    let c = n;
+    for (let k = 0; k < 8; k++) c = c & 1 ? 0xedb88320 ^ (c >>> 1) : c >>> 1;
+    table[n] = c >>> 0;
+  }
+  return table;
+})();
+
+export function crc32(bytes: Uint8Array): number {
+  let c = 0xffffffff;
+  for (let i = 0; i < bytes.length; i++) c = CRC_TABLE[(c ^ bytes[i]!) & 0xff]! ^ (c >>> 8);
+  return (c ^ 0xffffffff) >>> 0;
+}
+
+/** MS-DOS time and date, from the UTC fields (ZIP has no time zone). */
+function dosTime(d: Date): { time: number; date: number } {
+  return {
+    time: (d.getUTCHours() << 11) | (d.getUTCMinutes() << 5) | (d.getUTCSeconds() >> 1),
+    date: ((Math.max(d.getUTCFullYear(), 1980) - 1980) << 9) | ((d.getUTCMonth() + 1) << 5) | d.getUTCDate(),
+  };
+}
+
+const UTF8_NAMES = 0x0800;
+const LIMIT = 0xffffffff;
+
+export function zipStore(entries: readonly ZipEntry[]): Uint8Array {
+  const names = entries.map((e) => new TextEncoder().encode(e.name));
+  const local = entries.reduce((sum, e, i) => sum + 30 + names[i]!.length + e.bytes.length, 0);
+  const central = names.reduce((sum, n) => sum + 46 + n.length, 0);
+  if (local + central + 22 > LIMIT) throw new ZipError("These covers add up to more than 4 GiB; select fewer.");
+
+  const out = new Uint8Array(local + central + 22);
+  const view = new DataView(out.buffer);
+  const offsets: number[] = [];
+  const crcs: number[] = [];
+  let at = 0;
+  entries.forEach((e, i) => {
+    const name = names[i]!;
+    const { time, date } = dosTime(e.modified);
+    const crc = crc32(e.bytes);
+    offsets.push(at);
+    crcs.push(crc);
+    view.setUint32(at, 0x04034b50, true);
+    view.setUint16(at + 4, 20, true);
+    view.setUint16(at + 6, UTF8_NAMES, true);
+    view.setUint16(at + 8, 0, true);
+    view.setUint16(at + 10, time, true);
+    view.setUint16(at + 12, date, true);
+    view.setUint32(at + 14, crc, true);
+    view.setUint32(at + 18, e.bytes.length, true);
+    view.setUint32(at + 22, e.bytes.length, true);
+    view.setUint16(at + 26, name.length, true);
+    view.setUint16(at + 28, 0, true);
+    out.set(name, at + 30);
+    out.set(e.bytes, at + 30 + name.length);
+    at += 30 + name.length + e.bytes.length;
+  });
+  const centralStart = at;
+  entries.forEach((e, i) => {
+    const name = names[i]!;
+    const { time, date } = dosTime(e.modified);
+    view.setUint32(at, 0x02014b50, true);
+    view.setUint16(at + 4, 20, true);
+    view.setUint16(at + 6, 20, true);
+    view.setUint16(at + 8, UTF8_NAMES, true);
+    view.setUint16(at + 10, 0, true);
+    view.setUint16(at + 12, time, true);
+    view.setUint16(at + 14, date, true);
+    view.setUint32(at + 16, crcs[i]!, true);
+    view.setUint32(at + 20, e.bytes.length, true);
+    view.setUint32(at + 24, e.bytes.length, true);
+    view.setUint16(at + 28, name.length, true);
+    // extra, comment, disk number, internal attributes: 0
+    view.setUint32(at + 38, 0, true);
+    view.setUint32(at + 42, offsets[i]!, true);
+    out.set(name, at + 46);
+    at += 46 + name.length;
+  });
+  view.setUint32(at, 0x06054b50, true);
+  view.setUint16(at + 8, entries.length, true);
+  view.setUint16(at + 10, entries.length, true);
+  view.setUint32(at + 12, at - centralStart, true);
+  view.setUint32(at + 16, centralStart, true);
+  return out;
+}
diff --git a/test/zip.test.ts b/test/zip.test.ts
new file mode 100644
index 0000000..07f3017
--- /dev/null
+++ b/test/zip.test.ts
@@ -0,0 +1,37 @@
+import { expect, test } from "bun:test";
+import { mkdtempSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { ZipError, crc32, zipStore, type ZipEntry } from "../src/core/zip";
+
+const text = (s: string) => new TextEncoder().encode(s);
+
+test("crc32 matches the standard check value", () => {
+  expect(crc32(text("123456789"))).toBe(0xcbf43926);
+  expect(crc32(new Uint8Array())).toBe(0);
+});
+
+test("a stored archive passes unzip -t and gives back the exact bytes", async () => {
+  const image = new Uint8Array(70_000).map((_, i) => (i * 31) & 0xff);
+  const entries: ZipEntry[] = [
+    { name: "03_poster_zh-Hant_480x623.jpg", bytes: image, modified: new Date("2026-10-15T12:34:56Z") },
+    { name: "covers.txt", bytes: text("Tower of God — 탑\n"), modified: new Date("2026-10-15T12:34:56Z") },
+  ];
+  const zip = zipStore(entries);
+  // General-purpose flag bit 11: names are UTF-8.
+  expect(new DataView(zip.buffer, zip.byteOffset).getUint16(6, true) & 0x0800).toBe(0x0800);
+
+  const path = join(mkdtempSync(join(tmpdir(), "mbseed-zip-")), "covers.zip");
+  await Bun.write(path, zip);
+  const check = Bun.spawnSync(["unzip", "-t", path]);
+  expect(check.exitCode).toBe(0);
+  const out = Bun.spawnSync(["unzip", "-p", path, "03_poster_zh-Hant_480x623.jpg"]);
+  expect(new Uint8Array(out.stdout)).toEqual(image);
+  const txt = Bun.spawnSync(["unzip", "-p", path, "covers.txt"]);
+  expect(new TextDecoder().decode(txt.stdout)).toBe("Tower of God — 탑\n");
+});
+
+test("refuses archives past 4 GiB instead of writing a broken one", () => {
+  const huge = { length: 2 ** 32 } as unknown as Uint8Array;
+  expect(() => zipStore([{ name: "big.jpg", bytes: huge, modified: new Date() }])).toThrow(ZipError);
+});