Router

SwapRouter assembles the whole route, so a caller passes an address and an amount and nothing else. A coin never trades against ETH or USDG directly — it trades against a leveraged token, which the quote book prices — so every trade is the same hops, hidden:

ETH --v3--> USDG --quote book (v4)--> leveraged token --v3 or v4--> COIN

The last hop depends on where the coin currently lives: the v3 curve before graduation, the v4 pool after it. The router reads which from the launcher rather than keeping its own registry, so a coin that graduates mid-flight routes correctly on the very next block with nothing to update.

Launched coins

function buyWithUsdg(address coin, uint256 usdgIn, uint256 minCoinOut, address to)
    external returns (uint256 coinOut);

function buyWithEth(address coin, uint256 minCoinOut, address to)
    external payable returns (uint256 coinOut);

function sellForUsdg(address coin, uint256 coinIn, uint256 minUsdgOut, address to)
    external returns (uint256 usdgOut);

function sellForEth(address coin, uint256 coinIn, uint256 minEthOut, address to)
    external returns (uint256 ethOut);

Selling pulls the coin, so it needs an allowance on the router first. Buying with USDG needs one on USDG; buying with ETH needs none.

A buy larger than the curve can absorb is trimmed rather than reverted. You receive what the curve could sell and the remainder is refunded in whatever you paid with, with a BuyResized event recording both figures.

Leveraged tokens

The same four, against the assets the quote book makes a market in:

function buyLtWithUsdg(address lt, uint256 usdgIn, uint256 minLtOut, address to)
    external returns (uint256 ltOut);

function buyLtWithEth(address lt, uint256 minLtOut, address to)
    external payable returns (uint256 ltOut);

function sellLtForUsdg(address lt, uint256 ltIn, uint256 minUsdgOut, address to)
    external returns (uint256 usdgOut);

function sellLtForEth(address lt, uint256 ltIn, uint256 minEthOut, address to)
    external returns (uint256 ethOut);

One book hop, no curve and no graduated pool. Whether an asset is tradable is asked of the hook itself rather than a list held by the router, so an asset it does not quote — or whose book is switched off — is refused by name with NotQuoted(lt).

Launching

function launch(string calldata name, string calldata symbol, uint256 firstBuyNumeraire)
    external returns (address token, address pool);

firstBuyNumeraire is optional and denominated in the leveraged token. Anything the curve cannot absorb is refunded.

Quoting a trade

Simulate the call you are about to send. Nothing else gives the same number: the book reprices every ten seconds and a fill executes at whatever quote is live when it lands, so a figure derived from an indexed price is a different figure from the one the caller gets.

For a payable call, a node rejects the simulation outright when the sender cannot cover value + gas, before any contract logic runs. Quote it through eth_call with the caller's balance overridden for that one call — what a buy returns does not depend on who is asking.

Errors worth decoding

Attach these to your ABI or a revert arrives as "unknown reason":