Regulated Exchanges: The Clean Case
Kalshi and regulated peers issue 1099 forms; profits from event contracts on CFTC-regulated exchanges are generally treated under capital-asset rules rather than gambling-income rules — meaning losses net against gains cleanly on your return, without the itemization trap that punishes gambling losses. Active traders should confirm the exact characterization with a professional, but the paperwork arrives pre-organized either way.
Crypto Platforms: Two Tax Layers
Polymarket profits are taxable income like any trading profit — and the USDC itself adds a layer: acquiring, holding and disposing of crypto for funding creates its own (usually minor, since stablecoins barely move) capital gains events, plus the general crypto reporting obligations. Every deposit, trade and withdrawal should be logged; crypto tax software handles the USDC legs while your trade log covers the market P&L.
Record-Keeping That Works
Whatever the venue: export trade histories regularly (platforms change, records vanish), keep your own log of funding flows, and reconcile annually against platform statements. Cross-platform traders should track per-venue P&L separately — different tax characterizations may apply to each. None of this is tax advice; a professional who has seen event-contract returns before is worth finding for meaningful volume.