Buy YES or NO contracts. Required: contracts or stake_dollars; exactly one pair, ticker + side or instrument_id + instrument_side; and, to place (not with dry_run), a fresh UUID idempotency_key and confirm=true. MARKET fills at the ask now. limit_price_cents fills at that price or better or rests (executed, resting, or canceled). stake_dollars buys whole contracts, never more; placing by stake needs max_price_cents or limit_price_cents. The balance is the limit. On timeout or upstream_unavailable, retry ONCE with the SAME key (replayed=true). Never retry a 4xx. A reused key for a different trade is idempotency_key_reused. get_positions (get_orders for a limit) is ground truth. route=best (default, members) picks the venue. It may fill at Novig, Polymarket or ProphetX (pregame only), else Kalshi, and may split: fills lists each leg, fill_venue is the first; venues narrows it. route=kalshi is one venue. rule_note when the contract differs. dry_run=true places nothing (about 20 a minute). Over the cash: insufficient_balance with details.max_contracts. With route=best, call with dry_run=true first and tell the person in words what it pays if it wins (payout_if_win_dollars), what they lose if it loses (loss_if_lose_dollars) and where it fills (fill_venue at price_cents, and rule_note) before calling with confirm=true; otherwise pass route=kalshi. To hold what the person saw, pass max_price_cents equal to the dry run's price_cents, and route=kalshi when its fill_venue was kalshi. stated_prob_pct (1-99) never changes the fill. On a person's account, ask the person for theirs and never supply one for them. Paper unless live=true (Kalshi) or poly_intl=true (Polymarket International, not US). Pass one. Rules: arena://rules/engine. This server places paper trades only: a call with live or poly_intl is refused (live_not_hosted).