diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 51b8650730..e6922ab8a0 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -122,6 +122,7 @@ jobs:
     defaults:
       run:
         shell: C:\msys64\msys2_shell.cmd -mingw64 -defterm -no-start -here -c ". <(cygpath '{0}')"
+        working-directory: ./pkg/urbit
 
     steps:
       - uses: actions/checkout@v2
@@ -130,10 +131,9 @@ jobs:
 
       # echo suppresses pacman prompt
       - run: echo|./configure
-        working-directory: ./pkg/urbit
+        env:
+          CACHIX_CACHE: ares
+          CACHIX_AUTH_TOKEN: ${{ secrets.CACHIX_AUTH_TOKEN }}
 
       - run: make build/urbit build/urbit-worker
-        working-directory: ./pkg/urbit
-
       - run: build/urbit -d -B ../../bin/brass.pill -F zod && curl -f --data '{"source":{"dojo":"+hood/exit"},"sink":{"app":"hood"}}' http://localhost:12321
-        working-directory: ./pkg/urbit
diff --git a/pkg/urbit/compat/mingw/poor-mans-nix-shell.sh b/pkg/urbit/compat/mingw/poor-mans-nix-shell.sh
new file mode 100644
index 0000000000..fa6762cc2a
--- /dev/null
+++ b/pkg/urbit/compat/mingw/poor-mans-nix-shell.sh
@@ -0,0 +1,118 @@
+# ensure required mingw packages are installed
+mpkgs=(cmake curl gcc jq libsigsegv libuv make wslay)
+pacman -S --needed autoconf automake-wrapper libtool patch ${mpkgs[@]/#/mingw-w64-x86_64-}
+
+declare -a cdirs
+declare -a ldirs
+sources=(../../nix/sources.json ../../nix/sources-mingw.json)
+
+hex2nixbase32 () {
+  local digits='0123456789abcdfghijklmnpqrsvwxyz'
+  local bits=0
+  local left=0 # number of bits left in $bits
+  local i=0
+  while ((1))
+  do
+    while ((left>=5))
+    do
+      echo -n ${digits:$((bits&31)):1}
+      bits=$((bits>>5))
+      left=$((left-5))
+    done
+    if ((i == ${#1}))
+    then
+      break
+    fi
+    char=0x${1:i:2}
+    i=$((i+2))
+    bits=$((bits|(char<<(left))))
+    left=$((left+8))
+  done
+  echo -n ${digits:$bits:1}
+}
+
+buildnixdep () {
+  local cache=https://app.cachix.org/api/v1/cache/${CACHIX_CACHE-}
+  local deriver=urbit-mingw-build
+  local hash=
+  local dir
+  if [ -n "$url" ]
+  then
+    local patch=compat/mingw/$key.patch
+    # create 'store hash' from sources.json data and patch
+    read hash _ < <((
+    jq -Sscj --arg key "$key" --arg deriver "$deriver" 'add|to_entries|.[]|select(.key==$key)|{($deriver):.value}' ${sources[@]}
+    [ -e $patch ] && cat $patch)|sha256sum)
+    hash=$(hex2nixbase32 $hash)
+    dir=../$hash-$key
+    if [ -e $dir/.mingw~ ]
+    then
+      # dependency present, don't reupload
+      hash=
+    else
+      # dependency absent, check the binary cache if configured
+      if [ -n "${CACHIX_CACHE-}" ]
+      then
+        echo Checking binary cache for $hash-$key...
+        narinfo="$cache/${hash}.narinfo"
+        if curl -fLI "$narinfo"
+        then
+          url="$cache/$(curl -fL -H "Accept: application/json" "$narinfo"|jq -r '.url')"
+          echo Found $url
+          strip=0
+          hash=
+        fi
+      fi
+      mkdir -p $dir
+      pushd $dir
+      curl -fL "$url"|(tar --strip $strip -xzf - || true)
+      popd
+    fi
+  else
+    # local dependency
+    dir=../$key
+  fi
+  # patch and build the dependency if necessary
+  # and append dependency paths to -I and -L arrays
+  . <(jq -sr --arg key "$key" --arg dir "$dir" 'add|to_entries|.[]|select(.key==$key)|"
+pushd \($dir)
+if [ ! -e .mingw~ ]
+then" + ("../urbit/compat/mingw/\($key).patch"|"
+  [ -e \(.) ] && patch -p 1 <\(.)") + "
+  \(.value.mingw.prepare//"")
+  make \(.value.mingw.make//"")
+  touch .mingw~
+fi
+popd
+\(.value.mingw.include//"."|if type == "array" then . else [.] end|map("cdirs+=(-I\($dir)/\(.))")|join("\n"))
+\(.value.mingw.lib//"."|if type == "array" then . else [.] end|map("ldirs+=(-L\($dir)/\(.))")|join("\n"))"' ${sources[@]})
+
+  # if configured, upload freshly built dependency to binary cache
+  if [ -n "$hash" -a -n "${CACHIX_AUTH_TOKEN-}" ]
+  then
+    (
+    echo Uploading freshly built $hash-$key to binary cache...
+    tar -C $dir -czf $hash.tar .
+    local size=$(stat -c '%s' $hash.tar)
+    read filehash _ < <(sha256sum $hash.tar)
+    curl -fL -H "Content-Type: application/gzip" -H "Authorization: Bearer $CACHIX_AUTH_TOKEN" --data-binary @"$hash.tar" "$cache/nar"
+    curl -fL -H "Content-Type: application/json" -H "Authorization: Bearer $CACHIX_AUTH_TOKEN" --data-binary @- "$cache/${hash}.narinfo" <<EOF
+{
+  "cStoreHash": "$hash",
+  "cStoreSuffix": "$key",
+  "cNarHash": "sha256:$(hex2nixbase32 $filehash)",
+  "cNarSize": $size,
+  "cFileHash": "$filehash",
+  "cFileSize": $size,
+  "cReferences": [],
+  "cDeriver": "$deriver"
+}
+EOF
+    echo Done. ) || true
+    rm $hash.tar || true
+  fi
+}
+
+# I have to go over the sources files several times
+# because jq does not have a way to invoke external programs
+. <(jq -sr 'add|to_entries|.[]|select(.value.mingw)|"key=\(.key|@sh) url=\(.value.url//""|@sh) strip=\(.value.mingw.strip+1) buildnixdep"' ${sources[@]})
diff --git a/pkg/urbit/configure b/pkg/urbit/configure
index 60a3cf508e..3bec9bc85b 100755
--- a/pkg/urbit/configure
+++ b/pkg/urbit/configure
@@ -93,38 +93,12 @@ esac
 # TODO Determine if the target cpu is little or big endian.
 case $(tr A-Z a-z <<< $os) in
   *mingw*)
-     # ensure required mingw packages are installed
-     mpkgs=(cmake curl gcc jq libsigsegv libuv make wslay)
-     pacman -S --needed autoconf automake-wrapper libtool patch ${mpkgs[@]/#/mingw-w64-x86_64-}
+     # set up dependencies and create cdirs and ldirs arrays
+     . compat/mingw/poor-mans-nix-shell.sh
 
      # support running off a tarball that doesn't contain binary pills
      (( $(stat -c %s ../../bin/ivory.pill) > 512 )) || curl -L https://github.com/urbit/urbit/raw/urbit-v$URBIT_VERSION/bin/ivory.pill > ../../bin/ivory.pill
 
-     # poor man's nix-shell
-     declare -a cdirs
-     declare -a ldirs
-     . <(jq -sr '
-add|to_entries|.[]|select(.value.mingw)|.key as $key|"../\(.key)" as $dir|(if .value.url then "
-if [ ! -d \($dir) ]
-then
-  mkdir -p \($dir)
-  pushd \($dir)
-  curl -L \(.value.url)|(tar --strip \(.value.mingw.strip+1) -xzf - || true)
-  popd
-fi" else "" end) + "
-pushd \($dir)
-if [ ! -e .mingw~ ]
-then" + ("../urbit/compat/mingw/\($key).patch"|"
-  [ -e \(.) ] && patch -p 1 <\(.)") + "
-  \(.value.mingw.prepare//"")
-  touch .mingw~
-fi
-make \(.value.mingw.make//"")
-popd
-\(.value.mingw.include//"."|if type == "array" then .[] else . end|"cdirs+=(-I../\($key)/\(.))
-")\(.value.mingw.lib//"."|if type == "array" then .[] else . end|"ldirs+=(-L../\($key)/\(.))
-")"' ../../nix/sources.json ../../nix/sources-mingw.json)
-
      [ -e include/ca-bundle.h ] || xxd /etc/pki/ca-trust/extracted/openssl/ca-bundle.trust.crt include_ca_bundle_crt >include/ca-bundle.h
      [ -e include/ivory.h     ] || xxd ../../bin/ivory.pill u3_Ivory_pill >include/ivory.h