diff --git a/package.json b/package.json
index 4bed5fa15..fa842edc6 100644
--- a/package.json
+++ b/package.json
@@ -30,9 +30,10 @@
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
+    "test": "bash tests/sync-agent-rules.test.sh",
     "lint": "eslint",
     "typecheck": "tsc --noEmit",
-    "check": "npm run lint && npm run typecheck && npm run build"
+    "check": "npm run test && npm run lint && npm run typecheck && npm run build"
   },
   "dependencies": {
     "@base-ui/react": "^1.3.0",
diff --git a/scripts/sync-agent-rules.sh b/scripts/sync-agent-rules.sh
index 661affc81..73d1149cd 100644
--- a/scripts/sync-agent-rules.sh
+++ b/scripts/sync-agent-rules.sh
@@ -42,7 +42,8 @@ resolve_imports() {
         cat "$resolved"
         echo ""
       else
-        echo "<!-- Import not found: $import_path -->"
+        echo "Error: AGENTS.md import not found: $import_path" >&2
+        return 1
       fi
     else
       echo "$line"
diff --git a/tests/sync-agent-rules.test.sh b/tests/sync-agent-rules.test.sh
new file mode 100644
index 000000000..4553b05e2
--- /dev/null
+++ b/tests/sync-agent-rules.test.sh
@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+FIXTURE_ROOT="$(mktemp -d)"
+trap 'rm -rf "$FIXTURE_ROOT"' EXIT
+
+mkdir -p "$FIXTURE_ROOT/scripts"
+cp "$REPO_ROOT/scripts/sync-agent-rules.sh" "$FIXTURE_ROOT/scripts/"
+printf '# Fixture instructions\n\n@docs/missing-guide.md\n' > "$FIXTURE_ROOT/AGENTS.md"
+
+set +e
+OUTPUT="$(bash "$FIXTURE_ROOT/scripts/sync-agent-rules.sh" 2>&1)"
+STATUS=$?
+set -e
+
+if [[ $STATUS -eq 0 ]]; then
+  echo "Expected sync-agent-rules.sh to reject a missing @file import" >&2
+  exit 1
+fi
+
+if [[ "$OUTPUT" != *"docs/missing-guide.md"* ]]; then
+  echo "Expected the failure to name the missing import" >&2
+  exit 1
+fi
+
+echo "Missing @file imports fail closed"