diff --git a/.claude/scripts/check-messages.sh b/.claude/scripts/check-messages.sh
index b553afa..31a1bc3 100644
--- a/.claude/scripts/check-messages.sh
+++ b/.claude/scripts/check-messages.sh
@@ -22,19 +22,38 @@ if [ -f "$IDENTITY_FILE" ]; then
   USER_ALIAS=$(jq -r '.user // empty' "$IDENTITY_FILE" 2>/dev/null)
 fi
 
+# Resolve a Python interpreter for the JSON sanitizer. Prefer the value in
+# identity.json (.python.command — set during machine onboarding); fall back
+# to a PATH lookup. On ACG Windows boxes the Microsoft Store `python3` alias
+# is disabled and `py` is the canonical launcher, so an unconditional
+# `python3` call would silently fail and cause every coord message to be
+# dropped. If no Python is available at all, the sanitizer falls back to
+# `tr -d` (lossy on real \n/\t in string fields but keeps messages visible).
+PY=""
+if [ -f "$IDENTITY_FILE" ]; then
+  PY=$(jq -r '.python.command // empty' "$IDENTITY_FILE" 2>/dev/null)
+fi
+if [ -z "$PY" ]; then
+  PY=$(command -v python3 || command -v py || command -v python)
+fi
+
 # Sanitize JSON: the coord API sometimes returns message bodies with unescaped
 # control chars (U+0000-U+001F) — invalid JSON per RFC 8259, which makes jq
 # abort with "Invalid string: control characters ... must be escaped". Round-
 # trip through python's json.loads(strict=False) (which accepts them) and
 # re-emit properly escaped JSON so jq downstream works on every payload.
 sanitize_json() {
-  python3 -c '
+  if [ -n "$PY" ]; then
+    "$PY" -c '
 import json, sys
 try:
     print(json.dumps(json.loads(sys.stdin.read(), strict=False)))
 except Exception:
     pass
 ' 2>/dev/null
+  else
+    tr -d '\000-\037'
+  fi
 }
 
 # --- Unread messages ---------------------------------------------------------