run-simulator

name: run-simulator description: Build, install, launch, and screenshot an iOS app in the Simulator to verify a change visually. Use when the user wants to run the app, see a change live, screenshot the running app, or confirm a UI fix actually works (not just that it compiles). allowed-tools: [Bash, Read, Glob]

Run in Simulator

Launches the actual app in the iOS Simulator and drives it far enough to see what a user would see. Building proves the code compiles; this skill proves it runs. The payoff is a screenshot of the live app that you read back to confirm the change — a blank or crashed frame is a failure, not a pass.

This skill is generic. Nothing about a specific app is hardcoded — the scheme, simulator, product path, and bundle id are all discovered at runtime.

When This Skill Activates

Use this skill when the user:

Asks to "run", "launch", or "open" the app in the simulator
Wants to see a change working, not just compile it
Asks for a screenshot of the running app
Wants to verify a UI fix visually before committing or pushing
Says "does this actually work?" about a view/flow they just changed

Do not use it for unit/UI test runs (xcodebuild test) — that's a different goal. This is about meeting the app as a user would.

Process

Run the steps in order. Each step's output feeds the next, so don't hardcode values a previous step can discover.

1. Discover the project and scheme

Prefer a workspace over a bare project when both exist (CocoaPods/SPM setups often require the workspace):

# Find the container
ls *.xcworkspace 2>/dev/null || ls *.xcodeproj 2>/dev/null

# List schemes (use -workspace X.xcworkspace OR -project X.xcodeproj)
xcodebuild -list -project <App>.xcodeproj 2>/dev/null

Pick the app scheme (usually matches the app name). If several schemes look plausible and none clearly matches, ask the user which to run rather than guessing.

2. Pick a simulator destination

Never hardcode a device name — the named device may not exist on this Mac (e.g. assuming "iPhone 16" when only "iPhone 17 Pro" is installed fails with "Unable to find a device matching the provided destination specifier"). List what's actually available and prefer one already booted:

# Already-booted sim, if any (fastest — skip the boot wait)
xcrun simctl list devices booted

# Otherwise, available iPhones to choose from
xcrun simctl list devices available | grep -i iphone

Choose a booted device if present; otherwise pick a recent iPhone from the available list and remember its name for the destination string.

3. Build for the simulator

xcodebuild build \
  -project <App>.xcodeproj \
  -scheme <Scheme> \
  -destination 'platform=iOS Simulator,name=<SimName>' \
  2>&1 | tail -5

Read the tail for ** BUILD SUCCEEDED **. Note that -quiet suppresses that success line, so either drop -quiet for the confirming run or grep for error: explicitly.
On failure, surface the error: lines — don't proceed to install a stale or nonexistent build.

4. Resolve the built `.app` and its bundle id

Don't guess the DerivedData path — ask the build system for it:

# Product directory + name from the resolved build settings
eval $(xcodebuild -project <App>.xcodeproj -scheme <Scheme> \
  -destination 'platform=iOS Simulator,name=<SimName>' \
  -showBuildSettings 2>/dev/null \
  | awk -F' = ' '/ TARGET_BUILD_DIR =/{print "DIR=\""$2"\""} / FULL_PRODUCT_NAME =/{print "NAME=\""$2"\""}')
APP="$DIR/$NAME"
echo "APP: $APP"

# Bundle id straight from the built Info.plist
BID=$(/usr/libexec/PlistBuddy -c "Print :CFBundleIdentifier" "$APP/Info.plist")
echo "Bundle id: $BID"

5. Boot, install, launch

SIM="<SimName>"
xcrun simctl boot "$SIM" 2>/dev/null   # no-op if already booted
open -a Simulator                      # bring the window forward
xcrun simctl bootstatus "$SIM" -b      # block until fully booted

xcrun simctl install "$SIM" "$APP"
xcrun simctl launch "$SIM" "$BID"

simctl launch prints <bundleid>: <pid> on success. A non-zero exit or an error string here means the app failed to start — investigate before screenshotting.

6. Screenshot and verify (and drive if needed)

xcrun simctl io "$SIM" screenshot /tmp/sim-shot.png

Then Read /tmp/sim-shot.png and actually look at it:

Blank/white frame, a crash dialog, or the launch screen stuck → failure.
The expected screen → success; describe what confirms the change.

If the change lives behind navigation, drive there before judging. Tap by point or describe to the user what to navigate to:

# Tap a point (x y in points) — useful for hitting a known tab/button
xcrun simctl io "$SIM" tap <x> <y>          # (where supported)
# Re-screenshot after each interaction
xcrun simctl io "$SIM" screenshot /tmp/sim-shot-2.png

For deep links, xcrun simctl openurl "$SIM" "<scheme>://<path>". If precise tapping isn't available, take the screenshot at the landing screen and tell the user the exact taps to reach the target view.

Output Format

Report, concisely:

What ran — scheme, simulator name/OS, bundle id.
Build result — succeeded / failed (with the error tail if failed).
Launch result — pid on success, or the failure reason.
What the screenshot shows — the screen reached and whether it confirms the change. Embed/Read the screenshot so the user sees it too.
Anything the user must do — e.g. taps needed to reach a gated screen.

Reliability Notes (common failure modes)

Wrong device name → "Unable to find a device matching the destination." Always list available sims (step 2) instead of assuming a model number.
-quiet hides success → it removes ** BUILD SUCCEEDED **; grep for error: or run the verifying build without -quiet.
Stale install → if behavior looks unchanged, xcrun simctl uninstall "$SIM" "$BID" then reinstall; or xcrun simctl shutdown "$SIM" && xcrun simctl erase "$SIM" for a clean slate (destroys sim data — confirm first).
Workspace vs project → if the build complains about missing packages/pods, re-run with -workspace <App>.xcworkspace instead of -project.
First boot is slow → simctl bootstatus -b blocks correctly; a fixed sleep does not and races the install.
Launch screen "stuck" → on a cold sim the app may still be initializing; re-screenshot after a short wait before declaring failure.

References

xcrun simctl help — full simulator control surface (boot, install, launch, io, openurl, spawn).
man xcodebuild — build, -list, -showBuildSettings, -destination.
Apple: Running your app in Simulator — https://developer.apple.com/documentation/xcode/running-your-app-in-the-simulator-or-on-a-device