diff --git a/companion/prepare.sh b/companion/prepare.sh
new file mode 100755
index 0000000..b1d8b8e
--- /dev/null
+++ b/companion/prepare.sh
@@ -0,0 +1,74 @@
+#!/usr/bin/env bash
+#
+# Builds the embeddable simulator runner tarball.
+#
+# Regenerates the Xcode project, builds the UI test bundle for a generic
+# simulator destination (so no booted simulator is needed), then stages a
+# relocatable test root: the runner app alongside its xctestrun. A port
+# placeholder is injected into the xctestrun so the driver can substitute the
+# real port before launching. The staged tree is packaged as a tarball.
+#
+# Output: ../internal/driver/ioscompanion/runnerassets/assets/runner-1.0.0.tar.gz
+
+set -euo pipefail
+
+# Keep the archiver from emitting AppleDouble (._name) sidecar entries that
+# would otherwise duplicate extended attributes into the payload.
+export COPYFILE_DISABLE=1
+
+VERSION="1.0.0"
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+ASSETS_DIR="${SCRIPT_DIR}/../internal/driver/ioscompanion/runnerassets/assets"
+OUTPUT="${ASSETS_DIR}/runner-${VERSION}.tar.gz"
+
+cd "${SCRIPT_DIR}"
+
+# Regenerate the project from its spec. The .xcodeproj is gitignored, so a
+# fresh checkout must produce it before building.
+/opt/homebrew/bin/xcodegen --spec project.yml
+
+# Build the UI test bundle for a generic simulator destination so no booted
+# simulator is required; products land under build/Build/Products/.
+xcodebuild build-for-testing \
+  -project CompanionRunner.xcodeproj \
+  -scheme CompanionRunner \
+  -destination 'generic/platform=iOS Simulator' \
+  -derivedDataPath build \
+  CODE_SIGNING_ALLOWED=NO
+
+PRODUCTS="${SCRIPT_DIR}/build/Build/Products"
+
+STAGE="$(mktemp -d)"
+trap 'rm -rf "${STAGE}"' EXIT
+
+# The xctestrun name embeds the simulator SDK version, so resolve it rather
+# than hardcoding it.
+XCTESTRUN="$(find "${PRODUCTS}" -maxdepth 1 -name '*.xctestrun' -print -quit)"
+if [ -z "${XCTESTRUN}" ]; then
+  echo "no xctestrun under ${PRODUCTS}" >&2
+  exit 1
+fi
+cp "${XCTESTRUN}" "${STAGE}/runner.xctestrun"
+
+# Copy the runner app into the staged products directory. The .swiftmodule
+# sibling is build-time only and is not copied.
+mkdir -p "${STAGE}/Debug-iphonesimulator"
+cp -R \
+  "${PRODUCTS}/Debug-iphonesimulator/CompanionRunnerUITests-Runner.app" \
+  "${STAGE}/Debug-iphonesimulator/CompanionRunnerUITests-Runner.app"
+
+# Inject the port placeholder the driver substitutes at launch time.
+/usr/libexec/PlistBuddy \
+  -c "Add :CompanionRunnerUITests:EnvironmentVariables:COMPANION_PORT string __COMPANION_PORT__" \
+  "${STAGE}/runner.xctestrun"
+
+STAGED_BYTES="$(find "${STAGE}" -type f -exec stat -f%z {} + | awk '{sum += $1} END {print sum}')"
+
+mkdir -p "${ASSETS_DIR}"
+tar -czf "${OUTPUT}" -C "${STAGE}" .
+
+SHA="$(shasum -a 256 "${OUTPUT}" | awk '{print $1}')"
+
+echo "wrote ${OUTPUT}"
+echo "staged uncompressed size: ${STAGED_BYTES} bytes"
+echo "sha256: ${SHA}"