iterating-on-voltius-android-emulator

Iterating on Voltius Android (Docker emulator)

Overview

No phone, no cable. A headless x86_64 Android emulator runs in Docker (Dockerfile.android-emulator), and the tauri-android MCP runs inside the same container via docker exec, so adb is local — none of the real-phone Windows-firewall / ADB_SERVER_SOCKET pain applies. Same MCP, same tool names, same selector rules as the on-device flow (iterating-on-voltius-android); only the transport differs.

Use this for fast no-device iteration / CI. Use the real-phone skill when a feature depends on actual hardware (real keychain, USB serial, camera) — the emulator's sandbox still differs from a real device, so a cross-compile or an emulator pass is not proof a feature works on real hardware. The proof is still: drive it, read the screenshot, check logcat.

Requires /dev/kvm (KVM accel — verified on x86_64 WSL2). The MCP itself needs no changes: it is device-agnostic (ANDROID_SERIAL selects the target, adb is local, CDP host defaults to 127.0.0.1).

Arch: the Dockerfile and the orchestrator both pick the ABI from the build host's uname -m — x86_64 host → x86_64 image + x86_64 APK; ARM host → arm64-v8a image + aarch64 APK. KVM only accelerates a same-arch guest, so the ABI must match the host. On ARM-WSL2 two things are unverified (check on that box): ls /dev/kvm must exist (Windows-on-ARM nested-virt exposure is less mature), and sdkmanager "emulator" must pull a working linux-aarch64 emulator host binary.

Bring it up (one command)

scripts/android-emulator.sh          # build image → build matching-arch APK → run → wait boot → install + launch
scripts/android-emulator.sh --no-apk # just boot the emulator (install the APK yourself later)

First run downloads the SDK + system image and bakes the AVD into the image (slow once, cached after). Cold boot ~1-2 min. The script auto-builds the APK for the host arch — don't hardcode it.

Manual equivalents (also documented at the bottom of Dockerfile.android-emulator):

docker build -f Dockerfile.android-emulator -t voltius-emulator .
docker run -d --name android-emulator --device /dev/kvm -p 5555:5555 -v "$(pwd):/project" voltius-emulator
docker logs -f android-emulator     # wait for "boot completed"
scripts/android-build.sh x86_64   # match the host: x86_64 on Intel/AMD, aarch64 on ARM → "universal" APK
docker exec android-emulator adb install -r \
  /project/src-tauri/gen/android/app/build/outputs/apk/universal/debug/app-universal-debug.apk
docker exec android-emulator adb shell am start -n com.voltius.app/.MainActivity

Register the MCP

claude mcp add tauri-android-emu --scope user \
  --env PLATFORM=android --env ANDROID_SERIAL=emulator-5554 \
  -- docker exec -i android-emulator npx -y "github:VoltiusApp/mcp-tauri-automation#android-backend"

No ADB_SERVER_SOCKET and no ANDROID_CDP_HOST — adb and CDP are local to the container. Mid-session gotcha: a freshly-added MCP's tools are NOT callable until the session reloads (tool registry is fixed at startup). Restart Claude Code or /reload-plugins, then the mcp__tauri-android-emu__* tools appear. claude mcp list shows connected ≠ tools loaded.

Drive loop

1. launch_app (appPath ignored) → starts the activity, finds the WebView socket, adb forwards it, attaches CDP. Confirm get_app_state → isRunning: true.
2. click_element / type_text / press_key / wait_for_element / get_element_text — CSS selectors only (no :contains/XPath); grep the component, don't guess.
3. capture_screenshot → full frame via adb exec-out screencap → Read the file and look. Blank/old frame = launch failed. Save shots under ./screenshots/ (gitignored), e.g. docker exec android-emulator sh -c 'adb exec-out screencap -p' > screenshots/NN-what.png — keep them out of the repo root so they never get committed.
4. execute_tauri_command → probe a backend command via window.__TAURI__ invoke.

Native dialogs (permission prompts, system file picker) are OUTSIDE the WebView — CDP can't touch them. Drive with docker exec android-emulator adb shell input tap <x> <y> / … input keyevent <KEY> at coordinates read from a screenshot.

Root-causing on the emulator

docker exec android-emulator adb logcat -d -t 200 | grep -iE 'voltius|RustStdoutStderr|panic|FATAL'

A startup panic with no WebView socket = a host-integration feature failed early. Reproduce → logcat → fix → re-deploy (scripts/android-build.sh x86_64 + adb install -r) → re-drive.

Common mistakes