diff --git a/.github/workflows/mobile-ci.yml b/.github/workflows/mobile-ci.yml
index e0d05f8a2..826118867 100644
--- a/.github/workflows/mobile-ci.yml
+++ b/.github/workflows/mobile-ci.yml
@@ -177,7 +177,7 @@ jobs:
           sdkmanager "platform-tools" "platforms;android-36" "build-tools;36.0.0" "ndk;30.0.14904198"
 
       - name: Install system deps
-        run: sudo apt-get update && sudo apt-get install -y pkg-config libssl-dev libcap-dev clang cmake
+        run: sudo apt-get update && sudo apt-get install -y pkg-config libssl-dev libcap-dev clang lld cmake
 
       - name: Setup Zig (libghostty build)
         uses: mlugg/setup-zig@v2
@@ -382,3 +382,31 @@ jobs:
             ONLY_ACTIVE_ARCH=YES \
             -only-testing:LitterTests \
             test-without-building
+
+      # Runtime smoke test: install the freshly built app, launch it, and
+      # assert the process survives. Unit tests never boot the embedded iSH
+      # guest runtime, so they cannot catch silently-broken embeds (e.g.
+      # litter-ish's ARM64 vdso degrading to an empty stub when no
+      # lld-capable clang is available, which SIGABRTs the app at launch).
+      - name: Smoke test (app launches and stays alive)
+        run: |
+          set -euo pipefail
+          xcrun simctl bootstatus "$IOS_SIMULATOR_UDID" -b
+          APP="$(find "$HOME/Library/Developer/Xcode/DerivedData" \
+            -path "*Debug-iphonesimulator/Litter.app" -type d -print -quit 2>/dev/null || true)"
+          if [ -z "$APP" ]; then
+            echo "Could not locate Litter.app in DerivedData" >&2
+            exit 1
+          fi
+          echo "Using app: $APP"
+          xcrun simctl install "$IOS_SIMULATOR_UDID" "$APP"
+          xcrun simctl launch "$IOS_SIMULATOR_UDID" com.sigkitten.litter
+          sleep 10
+          if ! xcrun simctl spawn "$IOS_SIMULATOR_UDID" launchctl list 2>/dev/null \
+              | grep -q "UIKitApplication:com.sigkitten.litter"; then
+            echo "App exited or crashed within 10s of launch" >&2
+            xcrun simctl spawn "$IOS_SIMULATOR_UDID" log show --last 2m \
+              --predicate 'process == "Litter"' --style compact 2>/dev/null | tail -80 || true
+            exit 1
+          fi
+          echo "App is alive"
diff --git a/AGENTS.md b/AGENTS.md
index cc7adfe75..e32d19e79 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -97,7 +97,7 @@
 Before building on a new machine, verify:
 1. `xcode-select -p` must print `/Applications/Xcode.app/Contents/Developer`, not `/Library/Developer/CommandLineTools`. Fix with `sudo xcode-select -s /Applications/Xcode.app/Contents/Developer`. The Command Line Tools do not include iOS simulator SDKs.
 2. `cargo` and `rustc` must come from **rustup**, not Homebrew's `rust` formula. If `which cargo` points to `/opt/homebrew/bin/cargo` (a Homebrew standalone binary, not a rustup proxy), cross-compilation targets like `aarch64-apple-ios-sim` will fail even if `rustup target list` shows them installed. The Makefile prepends the rustup toolchain bin to PATH automatically, but standalone script runs and CI environments must also ensure the correct resolution. Either `brew uninstall rust` or put `~/.cargo/bin` (or the rustup toolchain bin from `rustup which cargo`) before `/opt/homebrew/bin` in PATH.
-3. `xcodegen` must be installed (`brew install xcodegen`). Required for Xcode project generation.
+3. Build toolchain dependencies must be installed via Homebrew: `brew install xcodegen meson ninja llvm lld` and `brew install zig@0.15`. `xcodegen` is required for Xcode project generation; `meson`/`ninja` are required by the embedded iSH build; and `llvm`/`lld` (not Apple's `/usr/bin/clang`, which lacks `lld`) are required to cross-compile the iSH ARM64 vdso. If `llvm`/`lld` are missing, `tools/scripts/check-ish-vdso.sh` fails the Rust build because litter-ish silently degrades the vdso to an empty stub that makes the app SIGABRT at launch.
 4. *(Optional)* `pymobiledevice3` enables `make ios-device-run` over Tailscale when the device is not on the local network. Install with `pipx install pymobiledevice3` (or `uv tool install pymobiledevice3`). Also requires Tailscale on both the Mac and the iOS device.
 
 ## Build System
diff --git a/apps/ios/scripts/build-rust.sh b/apps/ios/scripts/build-rust.sh
index 8e55f240d..45a1a6854 100755
--- a/apps/ios/scripts/build-rust.sh
+++ b/apps/ios/scripts/build-rust.sh
@@ -302,6 +302,15 @@ copy_macabi_artifact() {
     -output "$GENERATED_MACABI_DIR/libcodex_mobile_client.a"
 }
 
+# Fail the build instead of shipping an app whose embedded iSH runtime has a
+# broken (empty-stub) ARM64 vdso. litter-ish silently degrades the vdso to an
+# empty placeholder when no lld-capable clang is available; at runtime that
+# makes the app SIGABRT as soon as a guest process hits a signal.
+check_ish_vdso_for() {
+  local triple="$1"
+  "$REPO_DIR/tools/scripts/check-ish-vdso.sh" "$CARGO_TARGET_DIR_EFFECTIVE" "$triple" "$PROFILE"
+}
+
 echo "==> Preparing codex submodule..."
 "$SCRIPT_DIR/sync-codex.sh" "$SYNC_MODE"
 
@@ -329,15 +338,18 @@ rustup target add i686-unknown-linux-musl aarch64-unknown-linux-musl
 if [ "$DEVICE_ONLY" -eq 1 ]; then
   echo "==> Building codex-mobile-client for aarch64-apple-ios ($PROFILE)..."
   cargo rustc --manifest-path "$RUST_BRIDGE_DIR/Cargo.toml" -p codex-mobile-client $CARGO_PROFILE_FLAG --target aarch64-apple-ios --crate-type staticlib $CARGO_FEATURES
+  check_ish_vdso_for aarch64-apple-ios
   copy_device_artifact
 elif [ "$SIM_ONLY" -eq 1 ]; then
   echo "==> Building codex-mobile-client for aarch64-apple-ios-sim ($PROFILE)..."
   cargo rustc --manifest-path "$RUST_BRIDGE_DIR/Cargo.toml" -p codex-mobile-client $CARGO_PROFILE_FLAG --target aarch64-apple-ios-sim --crate-type staticlib $CARGO_FEATURES
+  check_ish_vdso_for aarch64-apple-ios-sim
   copy_sim_artifact "$CARGO_TARGET_DIR_EFFECTIVE/aarch64-apple-ios-sim/$PROFILE/libcodex_mobile_client.a"
 elif [ "$MACABI_ONLY" -eq 1 ]; then
   if [ "$FAST_MACABI" -eq 1 ]; then
     echo "==> Building codex-mobile-client for $MACABI_HOST_TARGET ($PROFILE)..."
     cargo rustc --manifest-path "$RUST_BRIDGE_DIR/Cargo.toml" -p codex-mobile-client $CARGO_PROFILE_FLAG --target "$MACABI_HOST_TARGET" --crate-type staticlib $CARGO_FEATURES
+    check_ish_vdso_for "$MACABI_HOST_TARGET"
     copy_if_changed "$CARGO_TARGET_DIR_EFFECTIVE/$MACABI_HOST_TARGET/$PROFILE/libcodex_mobile_client.a" \
       "$GENERATED_MACABI_DIR/libcodex_mobile_client.a"
   else
@@ -367,6 +379,8 @@ elif [ "$MACABI_ONLY" -eq 1 ]; then
     fi
     [ "$FAILED" -eq 0 ] || exit 1
 
+    check_ish_vdso_for aarch64-apple-ios-macabi
+    check_ish_vdso_for x86_64-apple-ios-macabi
     copy_macabi_artifact
   fi
 else
@@ -417,6 +431,11 @@ else
   fi
   [ "$FAILED" -eq 0 ] || exit 1
 
+  check_ish_vdso_for aarch64-apple-ios
+  check_ish_vdso_for aarch64-apple-ios-sim
+  check_ish_vdso_for aarch64-apple-ios-macabi
+  check_ish_vdso_for x86_64-apple-ios-macabi
+
   copy_device_artifact
   copy_sim_artifact "$CARGO_TARGET_DIR_EFFECTIVE/aarch64-apple-ios-sim/$PROFILE/libcodex_mobile_client.a"
   copy_macabi_artifact
diff --git a/tools/scripts/build-android-rust.sh b/tools/scripts/build-android-rust.sh
index c4663f6ed..390348dc8 100755
--- a/tools/scripts/build-android-rust.sh
+++ b/tools/scripts/build-android-rust.sh
@@ -110,4 +110,12 @@ echo "==> Building codex_mobile_client Android shared libs..."
 cd "$WORKSPACE_DIR"
 cargo ndk "${ABI_ARGS[@]}" -o "$OUT_DIR" build --profile "$RUST_PROFILE" -p codex-mobile-client
 
+# Fail the build instead of shipping an app whose embedded iSH runtime has a
+# broken (empty-stub) ARM64 vdso; at runtime that SIGABRTs as soon as a guest
+# process hits a signal. Mirrors the iOS guard in apps/ios/scripts/build-rust.sh.
+TARGET_DIR="${CARGO_TARGET_DIR:-$WORKSPACE_DIR/target}"
+for target in "${RUST_TARGETS[@]}"; do
+  "$REPO_DIR/tools/scripts/check-ish-vdso.sh" "$TARGET_DIR" "$target" "$RUST_PROFILE"
+done
+
 echo "==> Done. Android JNI libs are in: $OUT_DIR"
diff --git a/tools/scripts/check-ish-vdso.sh b/tools/scripts/check-ish-vdso.sh
new file mode 100755
index 000000000..354ff0631
--- /dev/null
+++ b/tools/scripts/check-ish-vdso.sh
@@ -0,0 +1,75 @@
+#!/usr/bin/env bash
+# Validate that the ARM64 guest vdso embedded by litter-ish is a real aarch64
+# ELF rather than the empty placeholder that litter-ish's
+# vdso/arm64/meson.build creates when it cannot find an lld-capable clang
+# (e.g. only Apple's clang on PATH). An empty vdso makes the app SIGABRT at
+# launch as soon as a guest process hits a signal, so the build must fail
+# loudly instead of silently shipping a broken embed.
+#
+# Usage: check-ish-vdso.sh <cargo-target-dir> <rust-target-triple> <profile>
+set -euo pipefail
+
+CARGO_TARGET_DIR="${1:?missing cargo target dir}"
+TRIPLE="${2:?missing rust target triple}"
+PROFILE="${3:?missing cargo profile}"
+
+install_hint() {
+  case "$(uname -s)" in
+    Darwin)
+      echo "install LLVM's clang and lld, then rebuild: brew install llvm lld"
+      ;;
+    Linux)
+      if command -v apt-get >/dev/null 2>&1; then
+        echo "install clang and lld, then rebuild: apt-get install clang lld"
+      elif command -v dnf >/dev/null 2>&1; then
+        echo "install clang and lld, then rebuild: dnf install clang lld"
+      else
+        echo "install an lld-capable clang (clang + lld) and rebuild"
+      fi
+      ;;
+    *)
+      echo "install an lld-capable clang (clang + lld) and rebuild"
+      ;;
+  esac
+}
+
+HINT="$(install_hint)"
+
+# Build-script outputs live under <target>/<triple>/<profile>/build/ for
+# custom profiles, but historically landed at <target>/<triple>/build/. Search
+# both layouts so the check is resilient to cargo layout changes.
+VDSO_DIR=""
+for base in \
+  "$CARGO_TARGET_DIR/$TRIPLE/$PROFILE/build" \
+  "$CARGO_TARGET_DIR/$TRIPLE/build"
+do
+  VDSO_DIR="$(find "$base" -type d -path "*ish-embed-host-*/out/meson-build/vdso/arm64" 2>/dev/null | head -n 1 || true)"
+  [ -n "$VDSO_DIR" ] && break
+done
+
+if [ -z "$VDSO_DIR" ]; then
+  # No embed build output for this target/profile (e.g. cleaned cache). The
+  # stub degradation always leaves the meson build dir behind, so there is
+  # nothing to validate here.
+  exit 0
+fi
+
+VDSO="$VDSO_DIR/libvdso.so.elf"
+if [ ! -f "$VDSO" ] || [ ! -s "$VDSO" ]; then
+  echo "ERROR: iSH ARM64 vdso is missing or empty at $VDSO" >&2
+  echo "       litter-ish's vdso/arm64/meson.build builds an empty stub when it cannot find" >&2
+  echo "       an lld-capable clang; $HINT" >&2
+  exit 1
+fi
+
+VDSO_KIND="$(file -b "$VDSO")"
+case "$VDSO_KIND" in
+  *"ELF 64-bit"*"ARM aarch64"*) ;;
+  *)
+    echo "ERROR: iSH ARM64 vdso at $VDSO is not an aarch64 ELF: $VDSO_KIND" >&2
+    echo "       The app will SIGABRT at launch when a guest process hits a signal; $HINT" >&2
+    exit 1
+    ;;
+esac
+
+echo "==> iSH ARM64 vdso OK ($(stat -f '%z' "$VDSO") bytes, $VDSO_KIND)"