diff --git a/build_system.sh b/build_system.sh
index 05eee17..0d5f5f6 100755
--- a/build_system.sh
+++ b/build_system.sh
@@ -16,7 +16,8 @@ OUTPUT_DIR="$DIR/output"
 ROOTFS_DIR="$BUILD_DIR/agnos-rootfs"
 ROOTFS_IMAGE="$BUILD_DIR/system.img.raw"
 ROOTFS_IMAGE_SIZE=10G
-SPARSE_IMAGE="$BUILD_DIR/system.img"
+SPARSE_IMAGE="$OUTPUT_DIR/system.img"
+SKIP_CHUNKS_IMAGE="$OUTPUT_DIR/system-skip-chunks.img"
 
 # Create temp dir if non-existent
 mkdir -p $BUILD_DIR $OUTPUT_DIR
@@ -87,7 +88,13 @@ sudo umount -l $ROOTFS_DIR
 
 # Sparsify
 echo "Sparsify image"
-img2simg $ROOTFS_IMAGE $SPARSE_IMAGE
-mv $SPARSE_IMAGE $OUTPUT_DIR
+TMP_SPARSE="$(mktemp)"
+img2simg $ROOTFS_IMAGE $TMP_SPARSE
+mv $TMP_SPARSE $SPARSE_IMAGE
+
+# Make image with skipped chunks
+TMP_SKIP="$(mktemp)"
+$DIR/tools/simg2dontcare.py $SPARSE_IMAGE $TMP_SKIP
+mv $TMP_SKIP $SKIP_CHUNKS_IMAGE
 
 echo "Done!"
diff --git a/flash_system.sh b/flash_system.sh
index f449106..9719924 100755
--- a/flash_system.sh
+++ b/flash_system.sh
@@ -1,29 +1,12 @@
 #!/bin/bash -e
+set -e
 
-# Config
-OUTPUT_DIR="output"
-ROOTFS_IMAGE="system.img"
-
-# Log colors
-GREEN="\033[0;32m"
-NO_COLOR='\033[0m'
-
-# Make sure we're in the correct spot
 DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null && pwd)"
 cd $DIR
 
-# Flash system
 fastboot --set-active=a
-
-if [ -d $HOME/openpilot/provisioning ]; then
-  DIMG=$(mktemp)
-  $DIR/tools/simg2dontcare.py $OUTPUT_DIR/$ROOTFS_IMAGE $DIMG
-  fastboot erase system_a
-  fastboot flash system_a $DIMG
-else
-  fastboot flash system_a $OUTPUT_DIR/$ROOTFS_IMAGE
-fi
-
+fastboot erase system_a
+fastboot flash system_a $OUTPUT_DIR/system-skip-chunks.img
 fastboot continue
 
-echo -e "${GREEN}Done!"
+echo "Done!"
diff --git a/internal/build_and_package.sh b/internal/build_and_package.sh
index ff6f8f8..2ec5bf3 100755
--- a/internal/build_and_package.sh
+++ b/internal/build_and_package.sh
@@ -19,5 +19,5 @@ fi
 
 ./build_kernel.sh
 ./build_system.sh
-internal/package_ota.sh
+internal/package_ota.py
 internal/ota_push.sh staging
diff --git a/internal/ota_push.sh b/internal/ota_push.sh
index f329059..443fcba 100755
--- a/internal/ota_push.sh
+++ b/internal/ota_push.sh
@@ -39,6 +39,14 @@ process_file() {
   local NAME=$1
   local HASH_RAW=$(cat $OTA_JSON | jq -r ".[] | select(.name == \"$NAME\") | .hash_raw")
   upload_file "$NAME-$HASH_RAW.img.xz"
+  upload_file "$NAME-$HASH_RAW.img.gz"
+
+  local ALT_URL=$(cat $OTA_JSON | jq -r ".[] | select(.name == \"$NAME\") | .alt.url")
+  if [ "$ALT_URL" != "null" ]; then
+    local ALT_FILE_NAME=$(basename $ALT_URL)
+    upload_file $ALT_FILE_NAME
+    upload_file {$ALT_FILE_NAME%.xz}.gz
+  fi
 
   # if [ "$NAME" == "system" ]; then
   #   local CAIBX_FILE_NAME="system-$HASH_RAW.caibx"
diff --git a/internal/package_ota.py b/internal/package_ota.py
new file mode 100755
index 0000000..f6ad7f1
--- /dev/null
+++ b/internal/package_ota.py
@@ -0,0 +1,102 @@
+#!/usr/bin/env python3
+import json
+import os
+import subprocess
+from copy import deepcopy
+from pathlib import Path
+from tempfile import NamedTemporaryFile
+
+ROOT = Path(__file__).parent.parent
+OUTPUT_DIR = ROOT / "output"
+OTA_OUTPUT_DIR = OUTPUT_DIR / "ota"
+FIRMWARE_DIR = ROOT / "agnos-firmware"
+
+AGNOS_UPDATE_URL = os.getenv("AGNOS_UPDATE_URL", "https://commadist.azureedge.net/agnosupdate")
+AGNOS_STAGING_UPDATE_URL = os.getenv("AGNOS_STAGING_UPDATE_URL", "https://commadist.azureedge.net/agnosupdate-staging")
+
+
+def checksum(fn):
+  return subprocess.check_output(["sha256sum", fn]).decode().split()[0]
+
+
+def compress(fin, fout) -> None:
+  subprocess.check_call(f"xz -T4 -vc {fin} > {fout}", shell=True)
+  subprocess.check_call(f"gzip -vc {fin} > {fout.with_suffix('.gz')}", shell=True)
+
+
+def process_file(fn, name, sparse=False, full_check=True, has_ab=True, alt=None):
+  print(name)
+  hash_raw = hash = checksum(fn)
+  size = fn.stat().st_size
+  print(f"  {size} bytes, hash {hash}")
+
+  if sparse:
+    with NamedTemporaryFile() as tmp_f:
+      print("  converting sparse image to raw")
+      subprocess.check_call(["simg2img", fn, tmp_f.name])
+      hash_raw = checksum(tmp_f.name)
+      size = Path(tmp_f.name).stat().st_size
+      print(f"  {size} bytes, hash {hash} (raw)")
+
+  print("  compressing")
+  xz_fn = OTA_OUTPUT_DIR / f"{fn.stem}-{hash_raw}.img.xz"
+  compress(fn, xz_fn)
+
+  ret = {
+    "name": name,
+    "url": "{remote_url}/" + xz_fn.name,
+    "hash": hash,
+    "hash_raw": hash_raw,
+    "size": size,
+    "sparse": sparse,
+    "full_check": full_check,
+    "has_ab": has_ab,
+  }
+
+  if alt is not None:
+    print("  calculating alt")
+    alt_hash = checksum(alt)
+    alt_size = alt.stat().st_size
+    print(f"  {alt_size} bytes, hash {alt_hash} (alt)")
+
+    print("  compressing alt")
+    alt_xz_fn = OTA_OUTPUT_DIR / f"{alt.stem}-{hash_raw}.img.xz"
+    compress(alt, alt_xz_fn)
+
+    ret["alt"] = {
+      "hash": alt_hash,
+      "url": "{remote_url}/" + alt_xz_fn.name,
+    }
+
+  return ret
+
+
+if __name__ == "__main__":
+  OTA_OUTPUT_DIR.mkdir(parents=True, exist_ok=True)
+
+  files = [
+    process_file(OUTPUT_DIR / "boot.img", "boot"),
+    process_file(FIRMWARE_DIR / "abl.bin", "abl"),
+    process_file(FIRMWARE_DIR / "xbl.bin", "xbl"),
+    process_file(FIRMWARE_DIR / "xbl_config.bin", "xbl_config"),
+    process_file(FIRMWARE_DIR / "devcfg.bin", "devcfg"),
+    process_file(FIRMWARE_DIR / "aop.bin", "aop"),
+    process_file(OUTPUT_DIR / "system.img", "system", sparse=True, full_check=False, alt=OUTPUT_DIR / "system-skip-chunks.img"),
+  ]
+  configs = [
+    (AGNOS_UPDATE_URL, "ota.json"),
+    (AGNOS_STAGING_UPDATE_URL, "ota-staging.json"),
+  ]
+
+  for remote_url, output_fn in configs:
+    processed_files = []
+    for f in deepcopy(files):
+      f["url"] = f["url"].format(remote_url=remote_url)
+      if "alt" in f:
+        f["alt"]["url"] = f["alt"]["url"].format(remote_url=remote_url)
+      processed_files.append(f)
+
+    with open(OTA_OUTPUT_DIR / output_fn, "w") as out:
+      json.dump(processed_files, out, indent=2)
+
+  print("Done")
diff --git a/internal/package_ota.sh b/internal/package_ota.sh
deleted file mode 100755
index 01d2da8..0000000
--- a/internal/package_ota.sh
+++ /dev/null
@@ -1,108 +0,0 @@
-#!/bin/bash -e
-
-# Make sure we're in the correct spot
-DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null && pwd)"
-
-ROOT="$DIR/../"
-TMP_DIR="/tmp/agnos-builder-tmp"
-OUTPUT_DIR="$ROOT/output"
-OTA_OUTPUT_DIR="$OUTPUT_DIR/ota"
-FIRMWARE_DIR="$ROOT/agnos-firmware"
-
-AGNOS_UPDATE_URL=${AGNOS_UPDATE_URL:-https://commadist.azureedge.net/agnosupdate}
-AGNOS_STAGING_UPDATE_URL=${AGNOS_STAGING_UPDATE_URL:-https://commadist.azureedge.net/agnosupdate-staging}
-OUTPUT_JSON="$OTA_OUTPUT_DIR/ota.json"
-OUTPUT_STAGING_JSON="$OTA_OUTPUT_DIR/ota-staging.json"
-
-# Make sure archive dir is empty
-rm -rf $OTA_OUTPUT_DIR && mkdir -p $OTA_OUTPUT_DIR
-
-process_file() {
-  local FILE=$1
-  local NAME=$2
-  local SPARSE=${3:-false}
-  local FULL_CHECK=${4:-true}
-  local HAS_AB=${5:-true}
-
-  echo "Hashing $NAME..."
-  local HASH=$(sha256sum $FILE | cut -c 1-64)
-  local SIZE=$(wc -c < $FILE)
-  echo "  $HASH ($SIZE bytes)"
-
-  local HASH_RAW=$HASH
-  if [ "$NAME" == "system" ]; then
-    echo "Converting system to raw..."
-    local FILE_RAW=/tmp/system.img.raw
-    simg2img $FILE $FILE_RAW
-
-    echo "Hashing system raw..."
-    HASH_RAW=$(sha256sum $FILE_RAW | cut -c 1-64)
-    SIZE=$(wc -c < $FILE_RAW)
-    echo "  $HASH_RAW ($SIZE bytes) (raw)"
-
-
-    # echo "Creating system casync files"
-    # casync make --compression=xz --store $OTA_OUTPUT_DIR/system-$HASH_RAW $OTA_OUTPUT_DIR/system-$HASH_RAW.caibx $FILE_RAW
-
-    rm $FILE_RAW
-  fi
-
-  echo "Compressing $NAME..."
-  local FILENAME=$NAME-$HASH_RAW.img.xz
-  local ARCHIVE=$OTA_OUTPUT_DIR/$FILENAME
-  xz -vc $FILE > $ARCHIVE
-
-  cat <<EOF | tee -a $OUTPUT_JSON $OUTPUT_STAGING_JSON > /dev/null
-  {
-    "name": "$NAME",
-    "hash": "$HASH",
-    "hash_raw": "$HASH_RAW",
-    "size": $SIZE,
-    "sparse": $SPARSE,
-    "full_check": $FULL_CHECK,
-    "has_ab": $HAS_AB,
-EOF
-
-  cat <<EOF >> $OUTPUT_JSON
-    "url": "$AGNOS_UPDATE_URL/$FILENAME"
-EOF
-  cat <<EOF >> $OUTPUT_STAGING_JSON
-    "url": "$AGNOS_STAGING_UPDATE_URL/$FILENAME"
-EOF
-
-#   if [ $NAME == "system" ]; then
-#     cat <<EOF >> $OUTPUT_JSON
-#     "casync_caibx": "$AGNOS_UPDATE_URL/$NAME-$HASH_RAW.caibx",
-#     "casync_store": "$AGNOS_UPDATE_URL/$NAME-$HASH_RAW"
-# EOF
-#     cat <<EOF >> $OUTPUT_STAGING_JSON
-#     "casync_caibx": "$AGNOS_STAGING_UPDATE_URL/$NAME-$HASH_RAW.caibx",
-#     "casync_store": "$AGNOS_STAGING_UPDATE_URL/$NAME-$HASH_RAW"
-# EOF
-#   fi
-
-  cat <<EOF | tee -a $OUTPUT_JSON $OUTPUT_STAGING_JSON > /dev/null
-  },
-EOF
-}
-
-cd $ROOT
-
-echo "[" > $OUTPUT_JSON
-echo "[" > $OUTPUT_STAGING_JSON
-
-process_file "$OUTPUT_DIR/boot.img" boot
-process_file "$FIRMWARE_DIR/abl.bin" abl
-process_file "$FIRMWARE_DIR/xbl.bin" xbl
-process_file "$FIRMWARE_DIR/xbl_config.bin" xbl_config
-process_file "$FIRMWARE_DIR/devcfg.bin" devcfg
-process_file "$FIRMWARE_DIR/aop.bin" aop
-process_file "$OUTPUT_DIR/system.img" system true false true
-
-# remove trailing comma
-sed -i "$ s/.$//" $OUTPUT_JSON $OUTPUT_STAGING_JSON
-
-echo "]" >> $OUTPUT_JSON
-echo "]" >> $OUTPUT_STAGING_JSON
-
-echo "Done!"
diff --git a/internal/pull_ota.sh b/internal/pull_ota.sh
new file mode 100755
index 0000000..aafc54b
--- /dev/null
+++ b/internal/pull_ota.sh
@@ -0,0 +1,42 @@
+#!/bin/bash -e
+
+# Make sure we're in the correct directory
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null && pwd)"
+
+# Constants
+OUTPUT_DIR="$DIR/../output"
+
+if [ "$1" == "" ]; then
+  echo "Supply the URL to the OTA JSON as first argument!"
+  exit 1
+fi
+
+OTA_JSON=$(mktemp)
+wget $1 -O $OTA_JSON
+
+mkdir -p $OUTPUT_DIR
+cd $OUTPUT_DIR
+
+download_image() {
+  local name=$1
+  local alt=${2:-""}
+
+  local url=$(cat $OTA_JSON | jq -r ".[] | select(.name == \"$name\") | $alt.url")
+  if [ "$url" == "null" ]; then
+    return
+  fi
+
+  local hash_raw=$(cat $OTA_JSON | jq -r ".[] | select(.name == \"$name\") | .hash_raw")
+  local file_name=$(basename $url .xz)
+  file_name=${file_name//-$hash_raw/}
+
+  echo "Downloading $file_name..."
+  curl $url | xz -d > $file_name
+}
+
+for name in boot system; do
+  download_image $name
+  download_image $name ".alt"
+done
+
+echo "Done!"