diff --git a/README.md b/README.md
index 0a519a0..265b8da 100644
--- a/README.md
+++ b/README.md
@@ -150,6 +150,9 @@ Needs JDK 11+, Python 3, `d8` and `apksigner.jar` (Android SDK build-tools),
 to `com.minamp.bc` that Plexamp Lite's `tools/setup_toolchain.sh` prepares (`BC_SRC`, default
 `~/android-toolchain/bcsrc`). No Gradle.
 
+With build-tools 34's `d8.jar` and `apksigner.jar` and `apktool_2.9.3.jar` in `~/android-toolchain/calendar-tools`
+(where they are on this Mac), just `./build.sh`. Elsewhere:
+
 ```sh
 D8=d8 APKTOOL=~/apktool.jar APKSIGNER=$SDK/build-tools/34.0.0/lib/apksigner.jar ./build.sh
 # add ANDROID_JAR=$SDK/platforms/android-10/android.jar to compile against the real platform
diff --git a/build.sh b/build.sh
index 17ac89c..b05ba1f 100755
--- a/build.sh
+++ b/build.sh
@@ -11,7 +11,17 @@
 set -eu
 cd "$(dirname "$0")"
 OUT=build
+# defaults: the copies in ~/android-toolchain/calendar-tools (build-tools 34's d8.jar and apksigner.jar, apktool 2.9.3)
+CT=${CALENDAR_TOOLS:-$HOME/android-toolchain/calendar-tools}
+if [ -z "${D8:-}" ] && [ -f "$CT/d8.jar" ]; then D8="java -cp $CT/d8.jar com.android.tools.r8.D8"; fi
 D8=${D8:-d8}
+if [ -z "${APKTOOL:-}" ] && [ -f "$CT/apktool_2.9.3.jar" ]; then APKTOOL=$CT/apktool_2.9.3.jar; fi
+if [ -z "${APKSIGNER:-}" ] && [ -f "$CT/apksigner.jar" ]; then APKSIGNER=$CT/apksigner.jar; fi
+if ! command -v javac >/dev/null 2>&1; then
+  for j in "${JAVA_HOME:-}" /opt/homebrew/opt/openjdk@17 /opt/homebrew/opt/openjdk; do
+    if [ -n "$j" ] && [ -x "$j/bin/javac" ]; then JAVA_HOME=$j; PATH=$j/bin:$PATH; export JAVA_HOME PATH; break; fi
+  done
+fi
 APKTOOL=${APKTOOL:?set APKTOOL=/path/to/apktool.jar}
 APKSIGNER=${APKSIGNER:?set APKSIGNER=/path/to/apksigner.jar}
 KS=${KEYSTORE:-gingercal.keystore}