diff --git a/scripts/lib/mkcpio.py b/scripts/lib/mkcpio.py
new file mode 100644
index 00000000..a14360a4
--- /dev/null
+++ b/scripts/lib/mkcpio.py
@@ -0,0 +1,69 @@
+#!/usr/bin/env python3
+"""Write a newc-format cpio archive of a directory tree to stdout.
+
+Used by scripts/build-linux-bootstrap.sh to pack the Nucleic Linux bootstrap initramfs without a
+`cpio` binary (which macOS lacks by default). Emits the SVR4/newc format the Linux kernel's initramfs
+loader expects; handles regular files, directories, and symlinks. Byte-identical on Linux and macOS.
+"""
+import os
+import stat
+import sys
+
+
+def field(n: int) -> bytes:
+    return b"%08X" % (n & 0xFFFFFFFF)
+
+
+def emit(out, name: bytes, st, data: bytes):
+    # newc header: magic + 13 eight-hex fields, then name (NUL-terminated, 4-byte padded), then data
+    # (4-byte padded).
+    hdr = b"070701"
+    hdr += field(st.st_ino if st else 0)
+    hdr += field(st.st_mode if st else 0)
+    hdr += field(0)  # uid — normalize to root so the initramfs is reproducible
+    hdr += field(0)  # gid
+    hdr += field(st.st_nlink if st else 1)
+    hdr += field(0)  # mtime — normalize for reproducibility
+    hdr += field(len(data))
+    hdr += field(0) * 4  # devmajor/minor, rdevmajor/minor
+    namebytes = name + b"\x00"
+    hdr += field(len(namebytes))
+    hdr += field(0)  # check (unused for newc)
+    out.write(hdr)
+    out.write(namebytes)
+    pad = (-(len(hdr) + len(namebytes))) % 4
+    out.write(b"\x00" * pad)
+    out.write(data)
+    out.write(b"\x00" * ((-len(data)) % 4))
+
+
+def main():
+    rootdir = sys.argv[1]
+    out = sys.stdout.buffer
+    entries = []
+    for dirpath, dirnames, filenames in os.walk(rootdir):
+        for d in sorted(dirnames):
+            entries.append(os.path.join(dirpath, d))
+        for f in sorted(filenames):
+            entries.append(os.path.join(dirpath, f))
+    for path in entries:
+        rel = os.path.relpath(path, rootdir).encode()
+        st = os.lstat(path)
+        if stat.S_ISLNK(st.st_mode):
+            data = os.readlink(path).encode()
+        elif stat.S_ISDIR(st.st_mode):
+            data = b""
+        else:
+            with open(path, "rb") as fh:
+                data = fh.read()
+        emit(out, rel, st, data)
+    # Trailer.
+    class Z:
+        st_ino = 0
+        st_mode = 0
+        st_nlink = 1
+    emit(out, b"TRAILER!!!", Z(), b"")
+
+
+if __name__ == "__main__":
+    main()