diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 97a9528..c84ae8b 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -248,6 +248,16 @@ jobs:
             libxcb-keysyms1 libxcb-shape0 libxcb-image0 libxcb-render-util0 libxcb-randr0 \
             libxcb-xinerama0 libxcb-xkb1 libfontconfig1 libdbus-1-3 desktop-file-utils
 
+      # appimagetool and the AppImage runtime kept between builds: GitHub's release downloads
+      # sometimes answer 504 for a long time, which failed the 0.30.0 release build.
+      - name: Cache appimagetool and the AppImage runtime
+        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: |
+            build/appimagetool-x86_64.AppImage
+            build/runtime-x86_64
+          key: appimagetool-1.9.0-runtime-x86_64
+
       - name: Build (tests + AppImage)
         run: ./build_linux.sh
 
diff --git a/build_linux.sh b/build_linux.sh
index 3043d42..5e42adf 100755
--- a/build_linux.sh
+++ b/build_linux.sh
@@ -72,12 +72,50 @@ fi
 echo "[5/5] Packing the AppImage ..."
 ARCH="$(uname -m)"
 TOOL="build/appimagetool-$ARCH.AppImage"
+# appimagetool (packs the AppImage) and the AppImage runtime (the small program at the start of every
+# AppImage) come from GitHub's release downloads, which sometimes answer "504 Gateway Timeout" for
+# a long time (November 2026: the 0.30.0 release). So: a fixed appimagetool version, tried for a few
+# minutes; the runtime is fetched here too (appimagetool would fetch it itself, with no retries), and
+# if GitHub can't deliver it, the runtime inside appimagetool (itself an AppImage) is used. In CI both
+# are kept in GitHub's cache (build.yml), so most builds download neither.
+APPIMAGETOOL_VERSION="${APPIMAGETOOL_VERSION:-1.9.0}"
+RUNTIME="build/runtime-$ARCH"
+BASE="https://github.com/AppImage"
+mkdir -p build
+
+fetch() {   # fetch FILE ROUNDS URL... : the first URL that answers, trying ROUNDS times
+    local out="$1" rounds="$2" round url
+    shift 2
+    for ((round = 1; round <= rounds; round++)); do
+        for url in "$@"; do
+            if curl -fsSL --retry 2 --retry-delay 10 --retry-all-errors -o "$out.part" "$url"; then
+                mv "$out.part" "$out"
+                echo "  downloaded ${url##*/download/}"
+                return 0
+            fi
+            echo "  not available right now: ${url##*/download/} (round $round of $rounds)"
+        done
+        ((round < rounds)) && sleep 20
+    done
+    rm -f "$out.part"
+    return 1
+}
+
 if [[ ! -x "$TOOL" ]]; then
-    mkdir -p build
-    curl -fL -o "$TOOL" \
-        "https://github.com/AppImage/appimagetool/releases/download/continuous/appimagetool-$ARCH.AppImage"
+    fetch "$TOOL" 4 "$BASE/appimagetool/releases/download/$APPIMAGETOOL_VERSION/appimagetool-$ARCH.AppImage" \
+                    "$BASE/appimagetool/releases/download/continuous/appimagetool-$ARCH.AppImage" || {
+        echo "Could not download appimagetool from GitHub. Try again later." >&2
+        exit 1
+    }
     chmod +x "$TOOL"
 fi
+if [[ ! -s "$RUNTIME" ]]; then
+    if ! fetch "$RUNTIME" 2 "$BASE/type2-runtime/releases/download/continuous/runtime-$ARCH"; then
+        echo "  using the runtime inside appimagetool instead"
+        head -c "$("$TOOL" --appimage-offset)" "$TOOL" > "$RUNTIME"
+    fi
+fi
+
 APPDIR=build/AppDir
 rm -rf "$APPDIR"
 mkdir -p "$APPDIR/usr/lib"
@@ -95,7 +133,8 @@ UPDATE_INFO="${UPDATE_INFO:-gh-releases-zsync|sevagjb|PrintView3D|latest|PrintVi
 ZSYNC="PrintView3D-$ARCH.AppImage.zsync"
 rm -f "$ZSYNC" "dist/$ZSYNC"
 # APPIMAGE_EXTRACT_AND_RUN: works on machines without FUSE (containers, CI)
-APPIMAGE_EXTRACT_AND_RUN=1 ARCH="$ARCH" "$TOOL" --no-appstream -u "$UPDATE_INFO" "$APPDIR" "$OUT"
+APPIMAGE_EXTRACT_AND_RUN=1 ARCH="$ARCH" "$TOOL" --no-appstream --runtime-file "$RUNTIME" \
+    -u "$UPDATE_INFO" "$APPDIR" "$OUT"
 chmod +x "$OUT"
 if [[ -f "$ZSYNC" ]]; then
     mv "$ZSYNC" "dist/$ZSYNC"