The captain-side duel-claim has two related forms:
Post-recruitment form (Ch 17) — “Keep your mitts out of [crewmate]‘s duel!” — enforces a structural rule: crewmate-fights are crewmate-fights, captain stays out, third parties stay out. The captain protects the duel-boundary.
Pre-recruitment form (Ch 36) — “Laugh at [pre-recruit] again… and I’ll kill you.” — publicly claims an unrecruited member as crew. The captain reads the recruit’s signature feat and responds with captain-grade lethal-intent defense. This is the pattern’s first staging — the recruit (Usopp) has not formally joined, but Luffy treats him as if he had.
The pre-recruitment alignment leak is the captain-side mirror of the recruit-side signature-feat. Each recruit acts the pattern (their signature feat); the captain claims the pattern (with lethal-intent public defense). Together they constitute the both-halves-of-recruitment-grammar-must-fire rule that governs every Straw Hat join.
Pattern projection: every later Straw Hat recruitment should have a moment where Luffy publicly claims the recruit before the formal join. The composition will repeat: straw-hat-shaded eyes, captain-grade lethal-intent verb, single-clause threat. Confirmed instances to watch for:
Sanji at Baratie (Ch 43+).
Chopper at Drum (Ch 130+).
Robin at Enies Lobby (Ch 398-399 — “BURN IT!”).
Franky at Water 7 (Ch 350+).
Brook at Thriller Bark.
Jinbei at Whole Cake / Wano.
The Ch 36 instance is the originating clean version of the pre-recruitment form. Worth flagging in subsequent recruitment chapters.