Local Build Agent Skill

Local Build Workflow (APK and .app)

Overview

Generate two shell scripts that build release artifacts for an Expo / React Native project using the platform toolchains directly:

• scripts/build-android.sh → runs ./gradlew assembleRelease and copies the APK into builds/
• scripts/build-ios.sh → runs xcodebuild for the iOS Simulator and copies the .app into builds/

Both scripts replace the previous artifact on each run, so the output path is always predictable.

Why this exists: Debug builds on an Expo project include expo-dev-client and launch the "Development Servers" picker instead of the app UI. For e2e runs, teammate previews, or real device installs, you need a Release build, and that's what these scripts produce.

Step 0: Confirm the project is a React Native / Expo app

Before doing anything else, verify the current directory is an Expo/React Native project:

• Check for package.json with expo in dependencies.
• Check for app.json or app.config.ts / app.config.js.

If neither exists, stop and tell the user the skill only works inside an Expo project.

Step 1: Read the app identity from the Expo config

The scripts need three values specific to the user's project. Do not hardcode "YourApp" or assume any name. Read them from the config.

Values to extract

| Value | Where to find it | Used for |

|-------|------------------|----------|

| name | app.config.ts / app.config.js / app.json → expo.name | iOS scheme and .app filename |

| slug | expo.slug | APK filename (fallback: lowercased name) |

| ios.bundleIdentifier | expo.ios.bundleIdentifier | xcrun simctl launch command |

| android.package | expo.android.package | adb shell am start command |

How to read each config format

• app.json — plain JSON, parse directly. The top-level key is usually expo.
• app.config.ts / app.config.js — exports a function that returns the config. Read the file and extract the literal values from the returned object. Do not try to execute it. If a value is computed dynamically, ask the user what to use.

Verify the iOS scheme exists on disk

The iOS scheme file is generated from expo.name during expo prebuild. Confirm it's there before wiring it into the script:

ls ios/*.xcworkspace 2>/dev/null
ls ios/*.xcodeproj/xcshareddata/xcschemes/*.xcscheme 2>/dev/null

The .xcworkspace name and the .xcscheme filename (without the extension) are what you pass to xcodebuild -workspace and -scheme. They usually match expo.name, but if they don't, use what's on disk.

If ios/ or android/ doesn't exist yet, run:

npx expo prebuild

Then re-check the scheme.

Step 2: Create the Android build script

Write scripts/build-android.sh. Replace <SLUG> with the value from Step 1.

#!/usr/bin/env bash
set -euo pipefail

HERE="$(cd "$(dirname "$0")" && pwd)"
ROOT="$(cd "$HERE/.." && pwd)"
ARTIFACT="$ROOT/android/app/build/outputs/apk/release/app-release.apk"
DEST_DIR="$ROOT/builds"
DEST="$DEST_DIR/<SLUG>.apk"

echo "==> Building Android release APK"
cd "$ROOT/android"
./gradlew assembleRelease

if [ ! -f "$ARTIFACT" ]; then
  echo "Build did not produce $ARTIFACT" >&2
  exit 1
fi

mkdir -p "$DEST_DIR"
rm -f "$DEST"
cp "$ARTIFACT" "$DEST"

echo "Done: $DEST"

Notes:

• assembleRelease is required. assembleDebug launches expo-dev-client instead of the app.
• If the project has no release signing configured, Gradle falls back to the debug keystore, which is fine for local testing but not acceptable for Play Store submission.

Step 3: Create the iOS build script

Write scripts/build-ios.sh. Replace <SCHEME> with the value from Step 1 (it's the same string used for -workspace <SCHEME>.xcworkspace and -scheme <SCHEME>, and the output file is <SCHEME>.app).

#!/usr/bin/env bash
set -euo pipefail

HERE="$(cd "$(dirname "$0")" && pwd)"
ROOT="$(cd "$HERE/.." && pwd)"
ARTIFACT="$ROOT/ios/build/Build/Products/Release-iphonesimulator/<SCHEME>.app"
DEST_DIR="$ROOT/builds"
DEST="$DEST_DIR/<SCHEME>.app"

echo "==> Building iOS Simulator .app (Release)"
cd "$ROOT/ios"
xcodebuild \
  -workspace <SCHEME>.xcworkspace \
  -scheme <SCHEME> \
  -configuration Release \
  -sdk iphonesimulator \
  -derivedDataPath build \
  CODE_SIGNING_ALLOWED=NO \
  -quiet

if [ ! -d "$ARTIFACT" ]; then
  echo "Build did not produce $ARTIFACT" >&2
  exit 1
fi

mkdir -p "$DEST_DIR"
rm -rf "$DEST"
cp -R "$ARTIFACT" "$DEST"

echo "Done: $DEST"

Notes:

• -sdk iphonesimulator produces a simulator .app. It will not run on a physical device.
• CODE_SIGNING_ALLOWED=NO is safe for simulator builds and avoids the need for a signing identity.
• A .app is a directory bundle, not a file, so cp -R is required.

Step 4: Make the scripts executable

chmod +x scripts/build-android.sh scripts/build-ios.sh

Step 5: Add builds/ to .gitignore

Append this to .gitignore (check it's not already there):

# Local build artifacts
builds/

Step 6: Tell the user how to use them

Give the user this summary:

Built two scripts for you:

  ./scripts/build-android.sh   → builds/<slug>.apk
  ./scripts/build-ios.sh       → builds/<scheme>.app

Install and run:

  # Android (connected device or emulator)
  adb install -r builds/<slug>.apk
  adb shell am start -n <android.package>/.MainActivity

  # iOS Simulator
  xcrun simctl boot "iPhone 16" 2>/dev/null; open -a Simulator
  xcrun simctl install booted builds/<scheme>.app
  xcrun simctl launch booted <ios.bundleIdentifier>

Substitute the real values from Step 1 when producing the message so the user can copy/paste directly.

Important notes

• Release, not Debug. Debug builds launch expo-dev-client, which is the launcher screen with the "Development Servers" list. For anything where you want the real UI, you need Release.
• iOS builds are simulator-only here. Device builds need provisioning profiles and a signing identity, which is out of scope for this skill. If the user asks for a device build, point them at eas build or manual Xcode archive.
• Android signing is debug-key only. The APK is fine for local testing and e2e, but do not use it for Play Store submission. For production, set up a release keystore or use eas build.
• Values are project-specific. Never hardcode "YourApp" or the Platano example values. Always read from app.config.ts / app.json and verify against files on disk.

Troubleshooting

"Build did not produce ..."

The xcodebuild or gradlew command succeeded but the artifact isn't where the script expects. Usually means:

• The scheme or workspace name is wrong (check ios/*.xcworkspace and ios/*.xcodeproj/xcshareddata/xcschemes/)
• A custom derivedDataPath or output directory is configured in a build-script or Gradle override

xcodebuild: error: The workspace ... does not exist

Run npx expo prebuild to regenerate ios/. If you already have an ios/ folder but no workspace, the CocoaPods install likely failed — run cd ios && pod install.

APK installs but opens expo-dev-client launcher

The script ran assembleDebug somewhere in the chain. Make sure Step 2's script calls assembleRelease.

.app opens and shows "Development Servers"

Same issue on iOS — the script was built with -configuration Debug instead of Release. Check Step 3's script.

xcrun simctl install says "incorrect executable format"

The user tried to open the .app directly on macOS. Simulator .app bundles only run inside xcrun simctl, not as a macOS application. Use the xcrun simctl install booted <path> command.