Gather the list of items to purchase and map each to its itemid, description, quantity, and amount (in the currency's smallest unit)
Call the ISteamMicroTxn/InitTxn/v3 endpoint using indexed parameters: itemid[0], qty[0], amount[0], description[0] for the first item, itemid[1], qty[1], amount[1], description[1] for the second, and so on
Pass the required top-level parameters including the user's Steam ID, the app ID, the language, and the currency code
Parse the returned transaction ID (orderid) and steam transaction handle from the InitTxn response
Redirect the user to the Steam payment overlay or page using the returned transaction handle to complete authorization
After the user authorizes, call ISteamMicroTxn/FinalizeTxn/v2 with the orderid to capture and complete the transaction
Known gotchas
There is no CartPurchase method in ISteamMicroTxn — multi-item purchases are done in a single InitTxn call using indexed array-style parameters (itemid[0], itemid[1], etc.)
Amounts must be provided in the smallest unit of the given currency (e.g., cents for USD); using decimal amounts will cause validation errors
The transaction is not complete after InitTxn — FinalizeTxn must be called after user authorization or the purchase will never settle
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