diff --git a/gradle.properties b/gradle.properties
index 158680d..5fda3b1 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -9,3 +9,14 @@ android.useAndroidX=true
 # Kotlin (built-in Kotlin is enabled by default in AGP 9; the Kotlin version is set
 # via the buildscript classpath in the root build.gradle.kts).
 kotlin.code.style=official
+
+# Gradle Managed Devices (app/build.gradle.kts `testOptions.managedDevices`): cap the number
+# of emulators Gradle boots at once to 1. The `e2e` device group spans api29-36 and
+# org.gradle.parallel=true is on, so without this cap a local `e2eGroupDebugAndroidTest` (or
+# the /preflight skill) can launch several emulators concurrently; they contend for the same
+# hardware VT-x/HAXM virtualization slot and hang at 0% CPU ("another emulator instance is
+# running"). This forces GMD emulator runs to be serial on a single machine. Does not affect
+# CI: the `e2e` matrix in .github/workflows/ci.yml runs each API level's emulator on its own
+# GitHub Actions runner via reactivecircus/android-emulator-runner + connectedDebugAndroidTest,
+# not via these Gradle Managed Device tasks, so this cap has no effect there either way.
+android.experimental.testOptions.managedDevices.maxConcurrentDevices=1