diff --git a/scripts/check.sh b/scripts/check.sh
new file mode 100755
index 0000000..739cc8e
--- /dev/null
+++ b/scripts/check.sh
@@ -0,0 +1,113 @@
+#!/usr/bin/env bash
+# Project integrity check after a refactor.
+# Runs fmt, clippy, build and tests for every Cargo world plus the Wear OS Gradle build.
+# All stages run even if earlier ones fail. A summary table is printed at the end.
+# Exit code equals the number of FAIL stages (WARN stages do not count).
+
+set -u
+set -o pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+export JAVA_HOME="/opt/android-studio/jbr"
+
+# Colors only when stdout is a terminal
+if [[ -t 1 ]]; then
+  C_RESET=$'\e[0m'
+  C_RED=$'\e[31m'
+  C_GREEN=$'\e[32m'
+  C_YELLOW=$'\e[33m'
+  C_BOLD=$'\e[1m'
+else
+  C_RESET=""
+  C_RED=""
+  C_GREEN=""
+  C_YELLOW=""
+  C_BOLD=""
+fi
+
+# Stage results, parallel arrays
+STAGE_NAMES=()
+STAGE_STATUS=()
+STAGE_SECONDS=()
+FAIL_COUNT=0
+
+# run_stage <label> <working-dir> <severity> <cmd...>
+#   severity = "fail" -> non-zero exit marks FAIL and increments FAIL_COUNT
+#   severity = "warn" -> non-zero exit marks WARN, does not affect exit code
+run_stage() {
+  local label="$1"; shift
+  local workdir="$1"; shift
+  local severity="$1"; shift
+
+  echo
+  echo "${C_BOLD}=== ${label} ===${C_RESET}"
+  echo "  dir: ${workdir}"
+  echo "  cmd: $*"
+
+  local start=$SECONDS
+  local rc=0
+  ( cd "$workdir" && "$@" ) || rc=$?
+  local dur=$(( SECONDS - start ))
+
+  STAGE_NAMES+=("$label")
+  STAGE_SECONDS+=("$dur")
+
+  if [[ $rc -eq 0 ]]; then
+    STAGE_STATUS+=("PASS")
+    echo "${C_GREEN}-> PASS${C_RESET} (${dur}s)"
+  elif [[ "$severity" == "warn" ]]; then
+    STAGE_STATUS+=("WARN")
+    echo "${C_YELLOW}-> WARN (rc=${rc})${C_RESET} (${dur}s) -- not counted as failure"
+  else
+    STAGE_STATUS+=("FAIL")
+    FAIL_COUNT=$(( FAIL_COUNT + 1 ))
+    echo "${C_RED}-> FAIL (rc=${rc})${C_RESET} (${dur}s)"
+  fi
+}
+
+# fmt is executed (not --check), so the working tree may be modified.
+# clippy escalates warnings to errors per project convention.
+# test uses -j 4 to avoid linker thrashing on a many-crate workspace.
+cargo_world() {
+  local world_label="$1"
+  local dir="$2"
+  run_stage "${world_label} :: fmt"    "$dir" fail cargo fmt
+  run_stage "${world_label} :: clippy" "$dir" fail cargo clippy --all-targets --all-features -- -D warnings
+  run_stage "${world_label} :: build"  "$dir" fail cargo build --all-targets
+  run_stage "${world_label} :: test"   "$dir" fail cargo test -j 4
+}
+
+cargo_world "common"          "${REPO_ROOT}/common"
+cargo_world "server"          "${REPO_ROOT}/server"
+cargo_world "clients/desktop" "${REPO_ROOT}/clients/desktop"
+cargo_world "experiments"     "${REPO_ROOT}/experiments"
+
+# Wear OS (Gradle). lintDebug is WARN-only because legacy Android Lint findings
+# would otherwise turn the script red regardless of the refactor under review.
+# Instrumented tests are intentionally excluded (AVD MockWebServer hang, see memory).
+WEAROS_DIR="${REPO_ROOT}/clients/wearos"
+run_stage "wearos :: assembleDebug"     "$WEAROS_DIR" fail ./gradlew assembleDebug
+run_stage "wearos :: lintDebug"         "$WEAROS_DIR" warn ./gradlew lintDebug
+run_stage "wearos :: testDebugUnitTest" "$WEAROS_DIR" fail ./gradlew testDebugUnitTest
+
+# Summary
+echo
+echo "${C_BOLD}========== SUMMARY ==========${C_RESET}"
+total=0
+for i in "${!STAGE_NAMES[@]}"; do
+  name="${STAGE_NAMES[$i]}"
+  status="${STAGE_STATUS[$i]}"
+  dur="${STAGE_SECONDS[$i]}"
+  total=$(( total + dur ))
+  case "$status" in
+    PASS) color="$C_GREEN" ;;
+    WARN) color="$C_YELLOW" ;;
+    FAIL) color="$C_RED" ;;
+    *)    color="" ;;
+  esac
+  printf "  %s%-4s%s %5ds  %s\n" "$color" "$status" "$C_RESET" "$dur" "$name"
+done
+echo "  ----"
+printf "  total: %ds across %d stages, %d failure(s)\n" "$total" "${#STAGE_NAMES[@]}" "$FAIL_COUNT"
+
+exit "$FAIL_COUNT"