If you set pmxt_api_key on a Polymarket or Opinion exchange instance, trade methods now route through trade.pmxt.dev/v0/* and execute via PMXT's PreFundedEscrow custody.
pmxt_api_key is not app-scoped per-user auth. The key holder can read any user's escrow data and forward signed orders for any wallet. Writes are still gated by the user's EIP-712 signature against their own wallet — the key alone cannot move funds. Reads (fetch_balance, fetch_positions, etc.) are NOT gated by signature.
Treat pmxt_api_key as a backend secret: keep it on a server, never ship it to a browser bundle, never log it.
In venue-direct mode (pmxt_api_key=None), fetch_balance() returns the wallet's CLOB-proxy USDC balance at the venue.
In hosted mode (pmxt_api_key=set), fetch_balance() returns the wallet's USDC balance in PMXT's PreFundedEscrow contract on Polygon — the funds available for hosted trading.
Same Balance shape; different source. If you've been catching fetch_balance() calls and relying on venue-direct semantics, audit your usage.
Position.outcome_label, entry_price, current_price, unrealized_pnl are now Optional. Venue-direct mode still populates them; hosted mode surfaces None when the server doesn't have the data. If your code accesses these fields, handle None.