diff --git a/plugins/ralph-loop/hooks/stop-hook.sh b/plugins/ralph-loop/hooks/stop-hook.sh
index 7edf13b7..6873772d 100755
--- a/plugins/ralph-loop/hooks/stop-hook.sh
+++ b/plugins/ralph-loop/hooks/stop-hook.sh
@@ -27,9 +27,19 @@ COMPLETION_PROMISE=$(echo "$FRONTMATTER" | grep '^completion_promise:' | sed 's/
 # Session isolation: the state file is project-scoped, but the Stop hook
 # fires in every Claude Code session in that project. If another session
 # started the loop, this session must not block (or touch the state file).
-# Legacy state files without session_id fall through (preserves old behavior).
 STATE_SESSION=$(echo "$FRONTMATTER" | grep '^session_id:' | sed 's/session_id: *//' || true)
 HOOK_SESSION=$(echo "$HOOK_INPUT" | jq -r '.session_id // ""')
+
+# Lazy claim: CLAUDE_CODE_SESSION_ID env var is not available to plugin
+# scripts at setup time, but the stop hook receives session_id via stdin
+# JSON. On first invocation, claim the loop for the current session.
+if [[ -z "$STATE_SESSION" ]] && [[ -n "$HOOK_SESSION" ]]; then
+  TEMP_FILE="${RALPH_STATE_FILE}.tmp.$$"
+  sed "s/^session_id:.*/session_id: $HOOK_SESSION/" "$RALPH_STATE_FILE" > "$TEMP_FILE"
+  mv "$TEMP_FILE" "$RALPH_STATE_FILE"
+  STATE_SESSION="$HOOK_SESSION"
+fi
+
 if [[ -n "$STATE_SESSION" ]] && [[ "$STATE_SESSION" != "$HOOK_SESSION" ]]; then
   exit 0
 fi