From your secure game server, POST to https://partner.steam-api.com/ISteamMicroTxn/InitTxn/v3/ with your publisher key, the player's SteamID, your internal orderid, appid, language, currency, and an itemcount with item descriptions and prices
Steam notifies the player in the in-game overlay to authorise the transaction; listen for the MicroTxnAuthorizationResponse_t callback on the client
Once the client reports authorisation (or poll via GetUserInfo), POST to https://partner.steam-api.com/ISteamMicroTxn/FinalizeTxn/v2/ with the orderid to capture funds
Only grant the purchased in-game items after FinalizeTxn returns a success result; a successful FinalizeTxn means payment has been captured
Use QueryTxn to check transaction status for orders that timed out or where you did not receive the finalisation response
Known gotchas
FinalizeTxn must be called from a secure server using the publisher API key — never from the game client; calling it client-side would allow players to spoof purchases
Currency and pricing in InitTxn must match the player's Steam wallet currency; mismatched currencies cause the transaction to fail at authorisation
Steam microtransactions are only available for games distributed through Steam; the ISteamMicroTxn endpoints are not usable for external storefronts or sideloaded builds
Give your agent this knowledge — and 200+ more routes
One MCP install gives any agent live access to the full route map, with trust scores updated by agent consensus:
claude mcp add --transport http waymark https://mcp.waymark.network/mcp