diff --git a/crates/dwg-thumbnailer/macos/Info.plist b/crates/dwg-thumbnailer/macos/Info.plist
index 43b03fe66..8d48bd4a8 100644
--- a/crates/dwg-thumbnailer/macos/Info.plist
+++ b/crates/dwg-thumbnailer/macos/Info.plist
@@ -33,6 +33,14 @@
             <key>QLSupportedContentTypes</key>
             <array>
                 <string>com.autodesk.dwg</string>
+                <!-- QuickLook matches these against the UTI the *system*
+                     resolves for a .dwg file, which is whatever the
+                     highest-precedence installed declaration says. With
+                     AutoCAD installed that is its exported UTI below — list
+                     it too or thumbnails never render on those machines
+                     (#365). Other CAD apps' exported UTIs (e.g. BricsCAD's)
+                     can be appended here as they are identified. -->
+                <string>com.autodesk.autocad.dwg</string>
             </array>
             <key>QLThumbnailMinimumDimension</key>
             <integer>0</integer>
diff --git a/crates/dwg-thumbnailer/macos/entitlements.plist b/crates/dwg-thumbnailer/macos/entitlements.plist
new file mode 100644
index 000000000..663747a82
--- /dev/null
+++ b/crates/dwg-thumbnailer/macos/entitlements.plist
@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<!-- Entitlements for the DWG QuickLook Thumbnail Extension (.appex).
+
+     App Extensions MUST be sandboxed: without com.apple.security.app-sandbox
+     the extension registers (shows up in System Settings and pluginkit) but
+     secd/QuickLook refuses to launch it, so Finder simply never renders a
+     thumbnail — with no error anywhere (#365). The system hands the appex a
+     read-only security-scoped handle to the file being thumbnailed, so no
+     other entitlement is needed. -->
+<plist version="1.0">
+<dict>
+    <key>com.apple.security.app-sandbox</key>
+    <true/>
+</dict>
+</plist>
diff --git a/packaging/Info.plist b/packaging/Info.plist
index ecfb4d334..c6df654e1 100644
--- a/packaging/Info.plist
+++ b/packaging/Info.plist
@@ -40,6 +40,10 @@
             <key>LSItemContentTypes</key>
             <array>
                 <string>com.autodesk.dwg</string>
+                <!-- AutoCAD exports its own DWG UTI; when it is installed,
+                     .dwg files bind to this identifier instead, so claim it
+                     too or OCS never appears under "Open with". -->
+                <string>com.autodesk.autocad.dwg</string>
             </array>
             <key>LSHandlerRank</key>
             <string>Alternate</string>
@@ -54,10 +58,62 @@
             <key>LSItemContentTypes</key>
             <array>
                 <string>com.autodesk.dxf</string>
+                <string>com.autodesk.autocad.dxf</string>
             </array>
             <key>LSHandlerRank</key>
             <string>Alternate</string>
         </dict>
     </array>
+    <!-- Declare (import) the DWG/DXF UTIs we claim above. Without a
+         declaration Launch Services cannot map the .dwg/.dxf filename
+         extensions to these identifiers on systems where no other CAD app
+         declares them, so "Open with", default-handler binding and the
+         QuickLook thumbnail extension all silently fail (#365). An *import*
+         declaration defers to any app that *exports* the same UTI. -->
+    <key>UTImportedTypeDeclarations</key>
+    <array>
+        <dict>
+            <key>UTTypeIdentifier</key>
+            <string>com.autodesk.dwg</string>
+            <key>UTTypeDescription</key>
+            <string>AutoCAD Drawing</string>
+            <key>UTTypeConformsTo</key>
+            <array>
+                <string>public.data</string>
+            </array>
+            <key>UTTypeTagSpecification</key>
+            <dict>
+                <key>public.filename-extension</key>
+                <array>
+                    <string>dwg</string>
+                </array>
+                <key>public.mime-type</key>
+                <array>
+                    <string>image/vnd.dwg</string>
+                </array>
+            </dict>
+        </dict>
+        <dict>
+            <key>UTTypeIdentifier</key>
+            <string>com.autodesk.dxf</string>
+            <key>UTTypeDescription</key>
+            <string>AutoCAD DXF</string>
+            <key>UTTypeConformsTo</key>
+            <array>
+                <string>public.data</string>
+            </array>
+            <key>UTTypeTagSpecification</key>
+            <dict>
+                <key>public.filename-extension</key>
+                <array>
+                    <string>dxf</string>
+                </array>
+                <key>public.mime-type</key>
+                <array>
+                    <string>image/vnd.dxf</string>
+                </array>
+            </dict>
+        </dict>
+    </array>
 </dict>
 </plist>
diff --git a/packaging/build_macos_signed.sh b/packaging/build_macos_signed.sh
new file mode 100755
index 000000000..d4547f740
--- /dev/null
+++ b/packaging/build_macos_signed.sh
@@ -0,0 +1,136 @@
+#!/usr/bin/env bash
+# Build, sign, notarize and package OpenCADStudio.app + .dmg for macOS arm64.
+#
+# Mirrors the `build-macos` job in .github/workflows/release.yml, then goes
+# further: instead of the CI's ad-hoc signature it produces a Developer ID
+# signed, hardened-runtime, notarized and stapled bundle. Ad-hoc signing is
+# what breaks the QuickLook thumbnail extension and "Open with" registration
+# in the field (#365): appexes must be sandboxed *and* validly signed before
+# macOS will run them.
+#
+# Inputs (environment):
+#   DEVELOPER_ID    "Developer ID Application: ..." identity. Auto-derived
+#                   from the keychain when unset. Set to "-" for an ad-hoc
+#                   build (CI parity, no notarization).
+#   NOTARY_PROFILE  notarytool keychain profile. Unset → skip notarization.
+#   VERSION         Bundle version. Default: version from Cargo.toml.
+#
+# Output: dist/OpenCADStudio.app and dist/OpenCADStudio-v<VERSION>-macos-arm64.dmg
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+TARGET=aarch64-apple-darwin
+DIST=dist
+VERSION="${VERSION:-$(sed -n 's/^version = "\(.*\)"/\1/p' Cargo.toml | head -1)}"
+
+if [ -z "${DEVELOPER_ID:-}" ]; then
+    DEVELOPER_ID="$(security find-identity -v -p codesigning \
+        | sed -n 's/.*"\(Developer ID Application: .*\)"/\1/p' | head -1)"
+fi
+[ -n "$DEVELOPER_ID" ] || { echo "No Developer ID identity found; set DEVELOPER_ID (or '-' for ad-hoc)." >&2; exit 1; }
+echo "==> Version $VERSION, signing as: $DEVELOPER_ID"
+
+echo "==> cargo build (app + thumbnailer staticlib)"
+cargo build --release --target "$TARGET"
+# The staticlib crate-type is only emitted when the crate is built as a
+# target (as a plain dependency cargo produces just the rlib).
+cargo build --release --target "$TARGET" -p dwg-thumbnailer
+
+echo "==> icons"
+rm -rf "$DIST" && mkdir -p "$DIST"
+ICONSET="$DIST/OpenCADStudio.iconset"
+mkdir -p "$ICONSET"
+for SIZE in 16 32 64 128 256 512 1024; do
+    rsvg-convert -w $SIZE -h $SIZE assets/logo.svg -o "$ICONSET/icon_${SIZE}x${SIZE}.png"
+done
+for BASE in 16 32 128 256 512; do
+    cp "$ICONSET/icon_$((BASE * 2))x$((BASE * 2)).png" "$ICONSET/icon_${BASE}x${BASE}@2x.png"
+done
+iconutil -c icns "$ICONSET" -o "$DIST/AppIcon.icns"
+for T in dwg dxf; do
+    SET="$DIST/$T.iconset"
+    mkdir -p "$SET"
+    for SIZE in 16 32 64 128 256 512 1024; do
+        rsvg-convert -w $SIZE -h $SIZE "assets/mimetypes/image-vnd.$T.svg" \
+            -o "$SET/icon_${SIZE}x${SIZE}.png"
+    done
+    for BASE in 16 32 128 256 512; do
+        cp "$SET/icon_$((BASE*2))x$((BASE*2)).png" "$SET/icon_${BASE}x${BASE}@2x.png"
+    done
+    iconutil -c icns "$SET" -o "$DIST/$(echo "$T" | tr a-z A-Z).icns"
+done
+
+echo "==> QuickLook thumbnail extension (.appex)"
+EXT="$DIST/DWGThumbnail.appex"
+rm -rf "$EXT" && mkdir -p "$EXT/Contents/MacOS"
+swiftc \
+    -sdk "$(xcrun --sdk macosx --show-sdk-path)" \
+    -target arm64-apple-macos11 \
+    -O -parse-as-library -application-extension \
+    -module-name DWGThumbnail \
+    -import-objc-header crates/dwg-thumbnailer/macos/dwg_thumbnailer.h \
+    crates/dwg-thumbnailer/macos/ThumbnailProvider.swift \
+    -L "target/$TARGET/release" -ldwg_thumbnailer \
+    -framework QuickLookThumbnailing -framework CoreGraphics \
+    -framework ImageIO -framework Foundation -framework Security \
+    -framework SystemConfiguration -liconv \
+    -Xlinker -e -Xlinker _NSExtensionMain \
+    -o "$EXT/Contents/MacOS/DWGThumbnail"
+sed "s/__VERSION__/$VERSION/g" crates/dwg-thumbnailer/macos/Info.plist > "$EXT/Contents/Info.plist"
+
+echo "==> assemble .app"
+APP="$DIST/OpenCADStudio.app"
+rm -rf "$APP"
+mkdir -p "$APP/Contents/MacOS" "$APP/Contents/Resources" "$APP/Contents/PlugIns"
+cp "target/$TARGET/release/OpenCADStudio" "$APP/Contents/MacOS/OpenCADStudio"
+chmod +x "$APP/Contents/MacOS/OpenCADStudio"
+cp "$DIST/AppIcon.icns" "$DIST/DWG.icns" "$DIST/DXF.icns" "$APP/Contents/Resources/"
+cp -R "$EXT" "$APP/Contents/PlugIns/"
+sed "s/__VERSION__/$VERSION/g" packaging/Info.plist > "$APP/Contents/Info.plist"
+
+echo "==> codesign"
+if [ "$DEVELOPER_ID" = "-" ]; then
+    # CI-parity ad-hoc signature; cannot be notarized.
+    codesign --force --deep --sign - --timestamp=none "$APP"
+else
+    # Inside-out: the appex first (sandbox entitlement is REQUIRED for a
+    # QuickLook extension to run), then the outer bundle. Hardened runtime
+    # and a secure timestamp on every layer for notarization.
+    codesign --force --timestamp --options runtime \
+        --entitlements crates/dwg-thumbnailer/macos/entitlements.plist \
+        -s "$DEVELOPER_ID" "$APP/Contents/PlugIns/DWGThumbnail.appex"
+    codesign --force --timestamp --options runtime \
+        -s "$DEVELOPER_ID" "$APP"
+fi
+codesign --verify --strict --verbose=2 "$APP"
+
+echo "==> dmg"
+DMG="$DIST/OpenCADStudio-v$VERSION-macos-arm64.dmg"
+rm -f "$DMG"
+for i in 1 2 3 4 5; do
+    if hdiutil create -volname "Open CAD Studio" -srcfolder "$APP" -ov -format UDZO "$DMG"; then
+        break
+    fi
+    echo "hdiutil failed (attempt $i), retrying..." >&2
+    sleep 3
+done
+[ -f "$DMG" ] || { echo "hdiutil failed permanently" >&2; exit 1; }
+
+if [ -n "${NOTARY_PROFILE:-}" ] && [ "$DEVELOPER_ID" != "-" ]; then
+    echo "==> notarize (this can take a while; a first submission from a new"
+    echo "    Developer ID can be held for hours — that is normal)"
+    SUBMIT_OUT="$(xcrun notarytool submit "$DMG" --keychain-profile "$NOTARY_PROFILE" --wait 2>&1 | tee /dev/stderr)"
+    SUBMISSION_ID="$(echo "$SUBMIT_OUT" | sed -n 's/^[[:space:]]*id: \([0-9a-f-]*\)$/\1/p' | head -1)"
+    # Always pull the log — it names the exact per-file cause on Invalid.
+    [ -n "$SUBMISSION_ID" ] && xcrun notarytool log "$SUBMISSION_ID" \
+        --keychain-profile "$NOTARY_PROFILE" || true
+    echo "$SUBMIT_OUT" | grep -q "status: Accepted" || { echo "Notarization NOT accepted" >&2; exit 1; }
+    xcrun stapler staple "$DMG"
+    xcrun stapler staple "$APP"
+    spctl -a -vvv "$APP"
+else
+    echo "==> NOTARY_PROFILE unset (or ad-hoc build): skipping notarization"
+fi
+
+shasum -a 256 "$DMG" > "$DMG.sha256"
+echo "==> done: $DMG"