fix: the swap decoder names where a WRAP_ETH pays and shows a V4 step's ETH as a maximum when the router may hold ETH (closes #665)

A `WRAP_ETH` that sets the input side and pays its WETH to anyone but
the router names that recipient on its `Moved to` line.

V4 pays a `SETTLE` of ETH from the router's own ETH, whichever payer it
names. When an earlier step may leave ETH in the router, the ETH that
the user pays a later V4 step gets lines of its own. Their
`Moved from the sender` line reads `Up to <amount>`, or
`Unknown (could not be decoded)` when the amount is not known. The
decoder never subtracts the router's ETH, so no line shows less ETH
than can leave the wallet. Without such an earlier step, the lines read
as before.

Model: opus-5-5

README.md

@@ -1265,7 +1265,9 @@ see Transaction Decoding), do not always read as a number. They can read:
   the same words. `Unlimited`, `All available (V4 open delta)` and
   `Whatever an earlier step sent to the pair (V2 already paid)` keep their
   wording. When a V2 or V3 exact-out step sets `Min. received`, that line shows
-  its `amountOut`, the exact amount it buys.
+  its `amountOut`, the exact amount it buys. When the router may hold ETH from
+  an earlier step, the `Moved from the sender` line of ETH that the user pays a
+  V4 step also reads `Up to <amount>` (see Transaction Decoding).
 - `All available (V4 open delta)`: the swap's `Amount` line, when the amount it
   shows is a V4 exact-in `amountIn` of zero. V4 reads that zero as "use the
   whole open delta", so the calldata states no quantity. Transaction Decoding
@@ -3412,47 +3414,43 @@ Currently supported:
   as a swap paid in ETH does, but only when that amount is not
   `CONTRACT_BALANCE` and the router held no WETH from an earlier step when the
   `WRAP_ETH` ran: after two `WRAP_ETH`s of 0.5 ETH into the router, a swap of 1
-  WETH from it reads as leaving the WETH there. An `UNWRAP_WETH` into the router
-  turns all the router's WETH into as much ETH. The decoder then knows how much
-  ETH the router holds only when it knew the WETH's amount and the router held
-  no ETH before. A V4 `SETTLE` of ETH spends the ETH the router holds, whoever
-  pays it (see below). A V2 or V3 exact-out step paid from the router spends an
-  amount the calldata does not state, at most its `amountInMax`, so the decoder
-  counts it as spending nothing the router holds. A `SETTLE` of `OPEN_DELTA`
-  (zero), and a `SETTLE_ALL`, pay only what the step still owes of their token
-  when they run. The decoder takes that to be the amounts the step's swap
-  actions state they spend of that token only when every action in the step
-  could be read, every swap action names the token it spends and the token it
-  buys (a swap action with an empty path swaps nothing), the `SETTLE` or
-  `SETTLE_ALL` is the step's only `SETTLE` or `SETTLE_ALL` of that token and
-  comes after every swap action, no swap action buys that token, no `TAKE`,
-  `TAKE_ALL` or `TAKE_PORTION` in the step takes it (a `TAKE` of a stated amount
-  adds to what the step owes), and the swap actions that spend it state a
-  quantity together; otherwise it does not know the amount, and such a `SETTLE`
-  from the router counts as spending nothing. So the rest of a V4 `SETTLE` of
-  more than its swap spends, taken to the router, the WETH of a `WRAP_ETH`, and
-  a token a swap pays the router on the way to the output, each stays in the
-  router when later steps do not spend all of it, where anyone can take it in a
-  later transaction. The router sends ETH as written, so a `TRANSFER` of
-  `CONTRACT_BALANCE` of ETH always reverts and pays out nothing. A step paying a
-  token to the V2 pair that the very next step spends, as a V2 exact-in of that
-  token whose `amountIn` is `ALREADY_PAID` (zero) or `CONTRACT_BALANCE`, is a
-  hop, not a payout: Uniswap's mixed routes pay the next V2 pair directly, and
-  the router enforces that swap's own minimum. The decoder checks that the
-  address paid is that pair, deriving it from the V2 factory and pair init code
-  hash the network's routers were deployed with and the first two tokens of that
-  step's path; a payment to any other address is named. (Sepolia's
-  `UniversalRouterV1_2_V2Support` was deployed with placeholders for both
-  values, so no V2 swap step can succeed on that router.) Every other payment
-  counts, in whichever step and of whatever token, a share of the output
-  included: an interface fee paid to another address by `PAY_PORTION`,
-  `TRANSFER` or `TAKE_PORTION` is named, and the minimum reads `Min. paid out`,
-  because the swap's minimum includes the fee. The minimum reads `Min. received`
-  only when some step pays the transaction's sender (the From address on
-  TxApproval, the sender the explorer reports on TransactionDetail), no
-  `Paid to` line is shown, and no permit or batch grants an allowance to a
-  spender other than the router (see below). `Paid to` lines after `Token Out`
-  name each other address paid; one reads
+  WETH from it reads as leaving the WETH there. A V2 or V3 exact-out step paid
+  from the router spends an amount the calldata does not state, at most its
+  `amountInMax`, so the decoder counts it as spending nothing the router holds.
+  A `SETTLE` of `OPEN_DELTA` (zero), and a `SETTLE_ALL`, pay only what the step
+  still owes of their token when they run. The decoder takes that to be the
+  amounts the step's swap actions state they spend of that token only when every
+  action in the step could be read, every swap action names the token it spends
+  and the token it buys (a swap action with an empty path swaps nothing), the
+  `SETTLE` or `SETTLE_ALL` is the step's only `SETTLE` or `SETTLE_ALL` of that
+  token and comes after every swap action, no swap action buys that token, no
+  `TAKE`, `TAKE_ALL` or `TAKE_PORTION` in the step takes it (a `TAKE` of a
+  stated amount adds to what the step owes), and the swap actions that spend it
+  state a quantity together; otherwise it does not know the amount, and such a
+  `SETTLE` from the router counts as spending nothing. So the rest of a V4
+  `SETTLE` of more than its swap spends, taken to the router, the WETH of a
+  `WRAP_ETH`, and a token a swap pays the router on the way to the output, each
+  stays in the router when later steps do not spend all of it, where anyone can
+  take it in a later transaction. The router sends ETH as written, so a
+  `TRANSFER` of `CONTRACT_BALANCE` of ETH always reverts and pays out nothing. A
+  step paying a token to the V2 pair that the very next step spends, as a V2
+  exact-in of that token whose `amountIn` is `ALREADY_PAID` (zero) or
+  `CONTRACT_BALANCE`, is a hop, not a payout: Uniswap's mixed routes pay the
+  next V2 pair directly, and the router enforces that swap's own minimum. The
+  decoder checks that the address paid is that pair, deriving it from the V2
+  factory and pair init code hash the network's routers were deployed with and
+  the first two tokens of that step's path; a payment to any other address is
+  named. (Sepolia's `UniversalRouterV1_2_V2Support` was deployed with
+  placeholders for both values, so no V2 swap step can succeed on that router.)
+  Every other payment counts, in whichever step and of whatever token, a share
+  of the output included: an interface fee paid to another address by
+  `PAY_PORTION`, `TRANSFER` or `TAKE_PORTION` is named, and the minimum reads
+  `Min. paid out`, because the swap's minimum includes the fee. The minimum
+  reads `Min. received` only when some step pays the transaction's sender (the
+  From address on TxApproval, the sender the explorer reports on
+  TransactionDetail), no `Paid to` line is shown, and no permit or batch grants
+  an allowance to a spender other than the router (see below). `Paid to` lines
+  after `Token Out` name each other address paid; one reads
   `The bridge, which pays the deposit's recipient on another network` for a
   bridge deposit; one reads, for example,
   `The router, 2000.0000 USDT from the sender (later steps may not spend all of it)`
@@ -3481,89 +3479,89 @@ Currently supported:
   of any amount but `OPEN_DELTA` pays that amount, and a `SETTLE` of
   `OPEN_DELTA` or a `SETTLE_ALL` pays what the step owes, as above. So a V4 step
   that swaps 400 USDT, with a `SETTLE` of 5,000 USDT paid by the user and the
-  rest taken to another address, reads `5000.0000 USDT`. V4 pays ETH from the
-  router's own ETH, whoever a `SETTLE` names as payer. So when a V4 step would
-  take ETH from the user's wallet, ETH the router already holds pays first. The
-  wallet pays only the rest. For example, an `UNWRAP_WETH` into the router
-  leaves ETH there. When the decoder does not know how much ETH the router
-  holds, or how much the step would take, it counts the router as paying all of
-  that ETH. A later step paid from the router spends what the router holds and
-  takes nothing, and so does a V2 exact-in whose `amountIn` is `ALREADY_PAID`. A
-  later take of `Token In` is added to the total while both are quantities: a V3
-  exact-in of 600 USDT and a V2 exact-in of 400 USDT, both paid by the user,
-  read `1000.0000 USDT`. When the decoder does not know a take's amount, the
-  total is not known and there is no `Amount` line. A `PERMIT2_PERMIT` names
-  `Token In` only when its amount, `Unlimited` included, stands as the `Amount`
-  figure in place of the total, which it does only in the plain shape of
-  Uniswap's own routes: it comes before the step that sets the input side, it is
-  the only `PERMIT2_PERMIT` in the transaction and there is no
-  `PERMIT2_PERMIT_BATCH`, its spender is the router, the router may not let it
-  fail, no step before it takes its token, a `PERMIT2_TRANSFER_FROM` of it
-  included, or a token the decoder does not know, and the decoder reads every
-  step after it in full. Permit2 then lets the router take no more of the token
-  than that amount, so the permit covers every take of it. A step before the
-  permit takes under whatever allowance the router already had, so it may take
-  more than the permit's amount, which would then read lower than what leaves
-  the user's wallet: a `PERMIT2_TRANSFER_FROM` of 2,000 USDT into the router,
-  then a permit of 600 USDT to the router and a V3 exact-in of 2,000 USDT paid
-  from the router, read `Amount 2000.0000 USDT`. A later permit or batch may
-  raise what Permit2 lets the router take, and so may a step the decoder does
-  not read in full: a command the decoder does not read (listed at the end of
-  this section), a step whose input it cannot read, in whole or in part, and a
-  step it fails on. Every other `PERMIT2_PERMIT`, before the step that sets the
-  input side or after it, leaves `Token In` and the `Amount` line as they read
-  with no permit, unless it lets a spender other than the router take `Token In`
-  (see below): a permit of 5,000 DAI to another address and then a V3 exact-in
-  of 600 USDT read `Swap USDT → USDC` with `Amount 600.0000 USDT`, and so do
-  that exact-in and then a permit of 5,000 DAI to the router. When a permit's
-  amount stands and the step that sets the input side spends a token other than
-  the permit's, or one the decoder does not know, that step's amount gets lines
-  of its own, unless it states no amount and the user pays it none of its input
-  token. Each take the total does not count gives three lines after the `Amount`
-  line (after `Token In` when there is none): one of a token other than
-  `Token In`; one of `Token In` after an amount that is not a quantity (a V4
-  open delta, `ALREADY_PAID`, which keep their wording on the `Amount` line, or
-  no stated amount, as of a V4 exact-out step); and one whose amount the decoder
-  does not know. When there is no `Amount` line, each take the total counts gets
-  them too, so that every amount the decoder reads is shown: the amount the step
-  that sets the input side states, when it is a quantity and the user pays that
-  step (a V2 or V3 step whose `payerIsUser` is true, a V4 step the user pays
-  some of its input token, or a `WRAP_ETH`, whose ETH comes from the
-  transaction's value), and each later take of `Token In` added to it. A step
-  that sets the input side and is paid from the router takes nothing from the
-  user's wallet and gets none. A `PERMIT2_PERMIT` the decoder cannot read, then
-  V3 exact-ins of 600 USDT and 400 DAI paid by the user, give lines for both
-  amounts. When the step that sets the input side states no amount, the user
-  pays it some of its input token, and there is no `Amount` line, that step gets
-  them too whenever another take's lines give a figure, or a
-  `PERMIT2_TRANSFER_FROM` of its token gives one, on the lines below or, into
-  the router, on its `Paid to` line, so that figure cannot read as all that
-  leaves. When it states no amount and the user pays it none of its input token,
-  as when the router pays it, that step takes none of that token from the user's
-  wallet and never gets them, with a permit or without. They come in step order,
-  except that the step that sets the input side comes first when it has them,
-  followed, when there is no `Amount` line, by the other takes the total counts:
-  `Moved from the sender`, its amount, worded as the `Amount` line words one, or
+  rest taken to another address, reads `5000.0000 USDT`. V4 pays a `SETTLE` of
+  ETH from the router's own ETH, whichever payer it names. So when the router
+  may hold ETH from an earlier step (see above), the ETH that the user pays a V4
+  step gets lines of its own (see below). A later step paid from the router
+  spends what the router holds and takes nothing, and so does a V2 exact-in
+  whose `amountIn` is `ALREADY_PAID`. A later take of `Token In` is added to the
+  total while both are quantities: a V3 exact-in of 600 USDT and a V2 exact-in
+  of 400 USDT, both paid by the user, read `1000.0000 USDT`. When the decoder
+  does not know a take's amount, the total is not known and there is no `Amount`
+  line. A `PERMIT2_PERMIT` names `Token In` only when its amount, `Unlimited`
+  included, stands as the `Amount` figure in place of the total, which it does
+  only in the plain shape of Uniswap's own routes: it comes before the step that
+  sets the input side, it is the only `PERMIT2_PERMIT` in the transaction and
+  there is no `PERMIT2_PERMIT_BATCH`, its spender is the router, the router may
+  not let it fail, no step before it takes its token, a `PERMIT2_TRANSFER_FROM`
+  of it included, or a token the decoder does not know, and the decoder reads
+  every step after it in full. Permit2 then lets the router take no more of the
+  token than that amount, so the permit covers every take of it. A step before
+  the permit takes under whatever allowance the router already had, so it may
+  take more than the permit's amount, which would then read lower than what
+  leaves the user's wallet: a `PERMIT2_TRANSFER_FROM` of 2,000 USDT into the
+  router, then a permit of 600 USDT to the router and a V3 exact-in of 2,000
+  USDT paid from the router, read `Amount 2000.0000 USDT`. A later permit or
+  batch may raise what Permit2 lets the router take, and so may a step the
+  decoder does not read in full: a command the decoder does not read (listed at
+  the end of this section), a step whose input it cannot read, in whole or in
+  part, and a step it fails on. Every other `PERMIT2_PERMIT`, before the step
+  that sets the input side or after it, leaves `Token In` and the `Amount` line
+  as they read with no permit, unless it lets a spender other than the router
+  take `Token In` (see below): a permit of 5,000 DAI to another address and then
+  a V3 exact-in of 600 USDT read `Swap USDT → USDC` with `Amount 600.0000 USDT`,
+  and so do that exact-in and then a permit of 5,000 DAI to the router. When a
+  permit's amount stands and the step that sets the input side spends a token
+  other than the permit's, or one the decoder does not know, that step's amount
+  gets lines of its own, unless it states no amount and the user pays it none of
+  its input token. Each take the total does not count gives three lines after
+  the `Amount` line (after `Token In` when there is none): one of a token other
+  than `Token In`; one of `Token In` after an amount that is not a quantity (a
+  V4 open delta, `ALREADY_PAID`, which keep their wording on the `Amount` line,
+  or no stated amount, as of a V4 exact-out step); one whose amount the decoder
+  does not know; and one of ETH that the user pays a V4 step when the router may
+  hold ETH. When there is no `Amount` line, each take the total counts gets them
+  too, so that every amount the decoder reads is shown: the amount the step that
+  sets the input side states, when it is a quantity and the user pays that step
+  (a V2 or V3 step whose `payerIsUser` is true, a V4 step the user pays some of
+  its input token, or a `WRAP_ETH`, whose ETH comes from the transaction's
+  value), and each later take of `Token In` added to it. A step that sets the
+  input side and is paid from the router takes nothing from the user's wallet
+  and gets none. A `PERMIT2_PERMIT` the decoder cannot read, then V3 exact-ins
+  of 600 USDT and 400 DAI paid by the user, give lines for both amounts. When
+  the step that sets the input side states no amount, the user pays it some of
+  its input token, and there is no `Amount` line, that step gets them too
+  whenever another take's lines give a figure, or a `PERMIT2_TRANSFER_FROM` of
+  its token gives one, on the lines below or, into the router, on its `Paid to`
+  line, so that figure cannot read as all that leaves. When it states no amount
+  and the user pays it none of its input token, as when the router pays it, that
+  step takes none of that token from the user's wallet and never gets them, with
+  a permit or without. They come in step order, except that the step that sets
+  the input side comes first when it has them, followed, when there is no
+  `Amount` line, by the other takes the total counts: `Moved from the sender`,
+  its amount, worded as the `Amount` line words one, or
   `Unknown (could not be decoded)` when the decoder does not know it;
   `Token moved`, giving the token's address as the `Token In` line does; and
-  `Moved to`, which reads `The swap step that spends it`. For a `WRAP_ETH` that
-  pays its WETH to anyone but the router, `Moved to` names that recipient
-  instead: `The sender` for `address(1)`, and otherwise the address. A token the
-  decoder does not know gives the `Moved from the sender` line alone. A step may
-  also take from the user's wallet what the decoder did not read: a V4 step
-  whose input or one of whose `SETTLE`s or `SETTLE_ALL`s it cannot read, which
-  the router still runs; a V2 or V3 exact-in or exact-out step or a
-  `PERMIT2_TRANSFER_FROM` whose input it cannot read, such as a V3 step whose
-  path is shorter than one token address or has bytes after its last hop; every
-  `PERMIT2_TRANSFER_FROM_BATCH`; every `EXECUTE_SUB_PLAN`, wherever it stands;
-  and every step it fails on. Each such step gives one `Moved from the sender`
-  line among those, reading `Unknown (could not be decoded)`. That line, or any
-  line for a token the decoder does not know, leaves no `Amount` line, whose
-  figure might be too low, and such a step before a `PERMIT2_PERMIT` keeps the
-  permit's amount from standing. Then each `PERMIT2_TRANSFER_FROM` but one into
-  the router, in step order, gives three lines: `Moved from the sender`, the
-  amount it moves, for example `1000000.0000 USDT`; `Token moved`, giving the
-  token's address as the `Token In` line does; and `Moved to`, which reads
+  `Moved to`, which reads `The swap step that spends it`. For ETH that the user
+  pays a V4 step when the router may hold ETH, `Moved from the sender` reads
+  `Up to <amount>` for a known amount. For a `WRAP_ETH` that pays its WETH to
+  anyone but the router, `Moved to` names that recipient instead: `The sender`
+  for `address(1)`, and otherwise the address. A token the decoder does not know
+  gives the `Moved from the sender` line alone. A step may also take from the
+  user's wallet what the decoder did not read: a V4 step whose input or one of
+  whose `SETTLE`s or `SETTLE_ALL`s it cannot read, which the router still runs;
+  a V2 or V3 exact-in or exact-out step or a `PERMIT2_TRANSFER_FROM` whose input
+  it cannot read, such as a V3 step whose path is shorter than one token address
+  or has bytes after its last hop; every `PERMIT2_TRANSFER_FROM_BATCH`; every
+  `EXECUTE_SUB_PLAN`, wherever it stands; and every step it fails on. Each such
+  step gives one `Moved from the sender` line among those, reading
+  `Unknown (could not be decoded)`. That line, or any line for a token the
+  decoder does not know, leaves no `Amount` line, whose figure might be too low,
+  and such a step before a `PERMIT2_PERMIT` keeps the permit's amount from
+  standing. Then each `PERMIT2_TRANSFER_FROM` but one into the router, in step
+  order, gives three lines: `Moved from the sender`, the amount it moves, for
+  example `1000000.0000 USDT`; `Token moved`, giving the token's address as the
+  `Token In` line does; and `Moved to`, which reads
   `The V2 pair the next swap spends` for a hop and `The sender` for `address(1)`
   on every router but `UniversalRouterV1`, and otherwise gives the address. A
   move into the router needs none of them, because its `Paid to` line and

src/shared/uniswap.js

@@ -1340,15 +1340,18 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
         };
 
         // What the steps take from the sender beyond the amount of the step
-        // that sets the input side, as { token, amount }, in step order.
-        // After the loop each joins the Amount line or gets lines of its own
-        // (see permitStands).
+        // that sets the input side, as { token, amount, atMost }, in step
+        // order. After the loop each joins the Amount line or gets lines of
+        // its own (see permitStands). A take with `atMost` set takes at most
+        // its amount, and always gets lines of its own.
         const userPaid = [];
         // A swap step the user pays for (`payerIsUser` true) takes `amount`
-        // of `token` from the sender. A V2 exact-in of ALREADY_PAID takes
-        // nothing.
-        const addUserPaid = (token, amount) => {
-            if (amount !== ALREADY_PAID) userPaid.push({ token, amount });
+        // of `token` from the sender, or at most that when `atMost` is set. A
+        // V2 exact-in of ALREADY_PAID takes nothing.
+        const addUserPaid = (token, amount, atMost = false) => {
+            if (amount !== ALREADY_PAID) {
+                userPaid.push({ token, amount, atMost });
+            }
         };
         // A step may take from the sender what this decoder did not read: a
         // V4 step whose input or one of whose SETTLEs or SETTLE_ALLs it
@@ -1403,11 +1406,9 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
         // token, an UNWRAP_WETH of WETH, a WRAP_ETH of CONTRACT_BALANCE of
         // ETH, a TRANSFER, PAY_PORTION, PAY_PORTION_FULL_PRECISION or
         // ACROSS_V4_DEPOSIT_V3 of all of it, or a swap step or V4 SETTLE paid
-        // from the router (see spendFromRouter()). A V4 SETTLE of ETH paid by
-        // the user spends the router's ETH too (see the V4_SWAP branch). The
-        // value is the amount the router holds, in base units, when a
-        // WRAP_ETH stated it, otherwise null. An UNWRAP_WETH into the router
-        // turns that amount of WETH into the same amount of ETH.
+        // from the router (see spendFromRouter()). The value is the amount the
+        // router holds, in base units, when a WRAP_ETH stated it, otherwise
+        // null.
         const heldByRouter = new Map();
         const isRouter = (recipient) => {
             const r = recipient.toLowerCase();
@@ -1415,8 +1416,8 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
         };
         // A swap step or V4 SETTLE pays `amount` of `token` from the router's
         // own balance. CONTRACT_BALANCE pays all of it. Any other quantity
-        // counts against a known amount the router holds, and the steps pay
-        // all of it once their amounts reach that one: a swap paid in ETH wraps
+        // counts against the amount a WRAP_ETH stated, and the steps pay all
+        // of it once their amounts reach that one: a swap paid in ETH wraps
         // what its swap steps then spend. An amount that is not a quantity,
         // such as the null decodeV4Swap() gives a SETTLE whose payment it
         // cannot tell, or one of a token whose amount is not known, pays out
@@ -1824,45 +1825,20 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
                 } else if (command === "V4_SWAP") {
                     const v4 = decodeV4Swap(inputs[i]);
                     const inputSetBefore = inputEstablished;
-                    // What the user pays the step, as decodeV4Swap() gives
-                    // it, less the ETH the router already holds (below).
-                    // Empty when the step cannot be read.
-                    const paidByUser = [];
+                    // V4Router pays a SETTLE of ETH from the router's own
+                    // ETH, whichever payer it names. When the router may hold
+                    // ETH from an earlier step, that ETH may pay some of the
+                    // ETH that the user pays this step. The sender then pays
+                    // at most the step's amount.
+                    const routerMayHoldEth = heldByRouter.has(
+                        "0x0000000000000000000000000000000000000000",
+                    );
                     if (v4) {
                         // Each SETTLE paid from the router spends the amount
                         // decodeV4Swap() says it pays.
                         for (const s of v4.settledFromRouter) {
                             spendFromRouter(s.currency, s.amount);
                         }
-                        // V4Router pays ETH from the router's own ETH,
-                        // whoever a SETTLE names as payer. So ETH the router
-                        // holds from an earlier step pays first for the ETH
-                        // the user would pay the step. The user pays only the
-                        // rest. When this decoder does not know how much ETH
-                        // the router holds, or how much the user would pay,
-                        // it counts the router as paying all of that ETH.
-                        for (const p of v4.paidByUser) {
-                            if (
-                                p.currency !==
-                                    "0x0000000000000000000000000000000000000000" ||
-                                !heldByRouter.has(p.currency)
-                            ) {
-                                paidByUser.push(p);
-                                continue;
-                            }
-                            const held = heldByRouter.get(p.currency);
-                            if (
-                                typeof held === "bigint" &&
-                                typeof p.amount === "bigint" &&
-                                p.amount > held
-                            ) {
-                                paidByUser.push({
-                                    currency: p.currency,
-                                    amount: p.amount - held,
-                                });
-                            }
-                            spendFromRouter(p.currency, p.amount);
-                        }
                         for (const t of v4.takes) {
                             if (!isHop(i, t.currency, t.recipient)) {
                                 addPayee(t.recipient, t.currency);
@@ -1887,10 +1863,10 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
                         commandNames[i] += " (could not be decoded)";
                     }
                     // The step's amount, if it sets the input side: what the
-                    // user pays it of its input currency (see paidByUser),
-                    // or, when the user pays none of it, what its swap
-                    // actions spend of it, whoever pays that.
-                    const paidIn = paidByUser.find(
+                    // user pays it of its input currency (see
+                    // decodeV4Swap()), or, when the user pays none of it,
+                    // what its swap actions spend of it, whoever pays that.
+                    const paidIn = v4?.paidByUser.find(
                         (p) => p.currency === v4.tokenIn,
                     );
                     const amountIn = paidIn ? paidIn.amount : v4?.amountIn;
@@ -1944,9 +1920,17 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
                     // router still runs it.
                     const setInputHere = inputEstablished && !inputSetBefore;
                     if (setInputHere) inputPaidByUser = paidIn !== undefined;
-                    for (const p of paidByUser) {
-                        if (setInputHere && p === paidIn) continue;
-                        addUserPaid(p.currency, p.amount);
+                    if (v4) {
+                        for (const p of v4.paidByUser) {
+                            if (setInputHere && p === paidIn) continue;
+                            addUserPaid(
+                                p.currency,
+                                p.amount,
+                                routerMayHoldEth &&
+                                    p.currency ===
+                                        "0x0000000000000000000000000000000000000000",
+                            );
+                        }
                     }
                     if (!v4 || v4.settleUnread) {
                         spentUnread();
@@ -1994,23 +1978,15 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
                         addPayee(u.recipient);
                         // It pays out all the router's WETH, as ETH. Into the
                         // router, the router then holds as ETH the WETH a
-                        // step paid it. That amount is known when the router
-                        // held a known amount of WETH and no ETH before.
-                        const heldWeth = WETH_ADDRESSES.filter((w) =>
+                        // step paid it.
+                        const heldWeth = WETH_ADDRESSES.some((w) =>
                             heldByRouter.has(w),
                         );
-                        const ethAmount =
-                            heldWeth.length === 1 &&
-                            !heldByRouter.has(
-                                "0x0000000000000000000000000000000000000000",
-                            )
-                                ? heldByRouter.get(heldWeth[0])
-                                : null;
-                        heldWeth.forEach((w) => heldByRouter.delete(w));
-                        if (isRouter(u.recipient) && heldWeth.length > 0) {
+                        WETH_ADDRESSES.forEach((w) => heldByRouter.delete(w));
+                        if (isRouter(u.recipient) && heldWeth) {
                             heldByRouter.set(
                                 "0x0000000000000000000000000000000000000000",
-                                ethAmount,
+                                null,
                             );
                         }
                     } else {
@@ -2064,7 +2040,8 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
         // being of another token or coming after an amount that is not a
         // quantity (a V4 open delta, ALREADY_PAID, or none stated), gets
         // lines of its own. One whose amount is not known also leaves the
-        // total unknown, with no figure.
+        // total unknown, with no figure. A take of at most its amount (see
+        // userPaid) is never added, and gets lines of its own too.
         //
         // A PERMIT2_PERMIT names Token In only when its amount stands as the
         // Amount figure in place of the total, which it does only in the
@@ -2099,10 +2076,10 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
             permit.beforeEveryTake &&
             !stepsUnread.some((step) => step > permit.step);
         // What the steps take that the Amount line does not give, as
-        // { token, amount, recipient }. A null token is a step this decoder
-        // did not read (see spentUnread()), or a token it does not know. Only
-        // the take of a WRAP_ETH that pays elsewhere has a recipient (see
-        // wrapEthRecipient).
+        // { token, amount, atMost, recipient }. A null token is a step this
+        // decoder did not read (see spentUnread()), or a token it does not
+        // know. `atMost` is as in `userPaid`. Only the take of a WRAP_ETH that
+        // pays elsewhere has a recipient (see wrapEthRecipient).
         const spentFromSender = [];
         let total = null;
         // The takes from the sender that `total` adds up, each a quantity, in
@@ -2148,6 +2125,7 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
             if (ofTokenIn && permitStands) continue;
             if (
                 ofTokenIn &&
+                !t.atMost &&
                 typeof total === "bigint" &&
                 typeof t.amount === "bigint"
             ) {
@@ -2252,8 +2230,9 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
         // V2's already-paid zero, neither of which is a quantity at all (see
         // OPEN_DELTA and ALREADY_PAID), and an unbounded permit. Those two
         // tests come first — the sentinels are not bigints and cannot be
-        // compared with one.
-        const spentText = (amount, info) => {
+        // compared with one. `atMost` says the amount is a maximum (see
+        // userPaid).
+        const spentText = (amount, info, atMost = false) => {
             if (amount === OPEN_DELTA) {
                 return { raw: OPEN_DELTA_AMOUNT, display: OPEN_DELTA_AMOUNT };
             }
@@ -2266,7 +2245,7 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
             if (amount >= MAX_UINT160) {
                 return { raw: "Unlimited", display: "Unlimited" };
             }
-            if (hasV2OrV3ExactOut) {
+            if (hasV2OrV3ExactOut || atMost) {
                 // A V2 or V3 exact-out swap spends at most this figure,
                 // whichever steps gave it (an amountInMax, the WRAP_ETH of a
                 // swap paid in ETH, a permit, or several steps' total), so it
@@ -2320,13 +2299,14 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
         // of the lines below for a PERMIT2_TRANSFER_FROM. An amount this
         // decoder does not know is said so, and a step it did not read gets
         // that one line alone. A take with no recipient goes to the swap step
-        // that spends it. A take with a recipient goes there, and MSG_SENDER
-        // there is the sender.
+        // that spends it. A take with a recipient goes to that recipient;
+        // `MSG_SENDER` means the sender.
         for (const s of spentFromSender) {
             details.push({
                 label: "Moved from the sender",
                 value: present(s.amount)
-                    ? spentText(s.amount, tokenInfo(s.token, sources)).display
+                    ? spentText(s.amount, tokenInfo(s.token, sources), s.atMost)
+                          .display
                     : UNDECODED_CURRENCY,
             });
             if (present(s.token)) {

tests/uniswap.test.js

@@ -1428,8 +1428,8 @@ describe("uniswap decoder", () => {
         ];
 
         // execute() of these command bytes and inputs, decoded as sent by
-        // the user.
-        function decodeSteps(commands, inputs) {
+        // the user to `router`.
+        function decodeSteps(commands, inputs, router = ROUTER_ADDR) {
             const data = buildExecute(
                 solidityPacked(
                     commands.map(() => "uint8"),
@@ -1438,7 +1438,7 @@ describe("uniswap decoder", () => {
                 inputs,
                 9999999999n,
             );
-            return uniswap.decode(data, ROUTER_ADDR, "0x1", {}, USER_ADDR);
+            return uniswap.decode(data, router, "0x1", {}, USER_ADDR);
         }
 
         // The first step: a V3 exact-in of 600 USDT for USDC, paid by the
@@ -2470,13 +2470,20 @@ describe("uniswap decoder", () => {
             expect(linesBetweenTokens(result)).toEqual(lines);
         });
 
-        // https://git.eeqj.de/sneak/AutistMask/issues/665 — V4Router pays ETH
-        // from the router's own ETH, whoever a SETTLE names as payer. ETH the
-        // router holds from an UNWRAP_WETH into it pays first, so the sender
-        // pays only the ETH beyond it. On next each row also took the V4
-        // step's whole ETH from the sender: 1 ETH more in the first two, and
-        // 1.5 ETH in the last.
-        const unwrapIntoRouter = encodeWrapEth(ROUTER_ADDR, 0n); // UNWRAP_WETH, same encoding
+        // https://git.eeqj.de/sneak/AutistMask/issues/665 — V4Router pays a
+        // SETTLE of ETH from the router's own ETH, whichever payer it names.
+        // After an earlier step that may leave ETH in the router, the ETH that
+        // the user pays a V4 step reads as a maximum, on lines of its own.
+        // Every row reads at least the ETH that can leave the wallet. The two
+        // rows marked "reads as on next" read the same on next. On next every
+        // other row added that ETH to the Amount line, or read it as an exact
+        // amount.
+        //
+        // ADDRESS_THIS is the router on every release.
+        const ADDRESS_THIS = "0x0000000000000000000000000000000000000002";
+        const WRAP_INTO_ROUTER = encodeWrapEth(ADDRESS_THIS, ONE_ETH);
+        // UNWRAP_WETH has the layout of WRAP_ETH.
+        const UNWRAP_INTO_ROUTER = encodeWrapEth(ADDRESS_THIS, 0n);
         // A V4 step swapping `amount` of ETH for USDC, with a SETTLE of
         // OPEN_DELTA by the user.
         const v4EthForUsdc = (amount) =>
@@ -2492,42 +2499,257 @@ describe("uniswap decoder", () => {
                     encodeV4TakeAll(USDC_ADDR, 0n),
                 ],
             );
+        const V4_ONE_ETH = v4EthForUsdc(ONE_ETH);
+        // A TRANSFER or PAY_PORTION of `amount` of `token` to another
+        // address.
+        const payOut = (token, amount) =>
+            coder.encode(
+                ["address", "address", "uint256"],
+                [token, OTHER_ADDR, amount],
+            );
+        // An ACROSS_V4_DEPOSIT_V3 of `amount` of ETH (`useNative` set), from
+        // the sender to the sender on chain id 8453.
+        const acrossEth = (amount) =>
+            coder.encode(
+                [
+                    "tuple(address,address,address,address,uint256,uint256,uint256,address,uint32,uint32,uint32,bytes,bool)",
+                ],
+                [
+                    [
+                        USER_ADDR,
+                        USER_ADDR,
+                        WETH_ADDR,
+                        WETH_ADDR,
+                        amount,
+                        1n,
+                        8453n,
+                        ZERO_ADDR,
+                        0,
+                        0,
+                        0,
+                        "0x",
+                        true,
+                    ],
+                ],
+            );
+        // The lines of ETH the sender pays a V4 step, of `amount`.
+        const ethLines = (amount) => [
+            ["Moved from the sender", amount],
+            ["Token moved", "ETH"],
+            ["Moved to", "The swap step that spends it"],
+        ];
         test.each([
             {
-                kind: "a V3 exact-in of 600 USDT for WETH paid to the router, an UNWRAP_WETH into the router, then a V4 swap of 1 ETH that the user settles",
-                commands: [0x00, 0x0c, 0x10],
-                inputs: [
-                    encodeV3SwapExactIn(ROUTER_ADDR, USDT_600, 1n, [
-                        USDT_ADDR,
-                        WETH_ADDR,
-                    ]),
-                    unwrapIntoRouter,
-                    v4EthForUsdc(ONE_ETH),
-                ],
-                lines: [["Amount", "600.0000 USDT"]],
+                kind: "a WRAP_ETH of 1 ETH into the router, then a V4 swap of 1 ETH that the user settles, reads as on next",
+                commands: [0x0b, 0x10],
+                inputs: [WRAP_INTO_ROUTER, V4_ONE_ETH],
+                lines: [["Amount", "2.0000 ETH"]],
             },
             {
                 kind: "a WRAP_ETH of 1 ETH into the router, an UNWRAP_WETH into the router, then a V4 swap of 1 ETH that the user settles",
                 commands: [0x0b, 0x0c, 0x10],
-                inputs: [
-                    encodeWrapEth(ROUTER_ADDR, ONE_ETH),
-                    unwrapIntoRouter,
-                    v4EthForUsdc(ONE_ETH),
+                inputs: [WRAP_INTO_ROUTER, UNWRAP_INTO_ROUTER, V4_ONE_ETH],
+                lines: [
+                    ["Amount", "1.0000 ETH"],
+                    ...ethLines("Up to 1.0000 ETH"),
                 ],
-                lines: [["Amount", "1.0000 ETH"]],
             },
             {
-                kind: "a WRAP_ETH of 1 ETH into the router, an UNWRAP_WETH into the router, then a V4 swap of 1.5 ETH that the user settles",
+                kind: "the same with a TRANSFER of 1 ETH to another address after the UNWRAP_WETH",
+                commands: [0x0b, 0x0c, 0x05, 0x10],
+                inputs: [
+                    WRAP_INTO_ROUTER,
+                    UNWRAP_INTO_ROUTER,
+                    payOut(ZERO_ADDR, ONE_ETH),
+                    V4_ONE_ETH,
+                ],
+                lines: [
+                    ["Amount", "1.0000 ETH"],
+                    ...ethLines("Up to 1.0000 ETH"),
+                ],
+            },
+            {
+                kind: "the same with a PAY_PORTION of 5,000 bips of WETH to another address before the UNWRAP_WETH",
+                commands: [0x0b, 0x06, 0x0c, 0x10],
+                inputs: [
+                    WRAP_INTO_ROUTER,
+                    payOut(WETH_ADDR, 5000n),
+                    UNWRAP_INTO_ROUTER,
+                    V4_ONE_ETH,
+                ],
+                lines: [
+                    ["Amount", "1.0000 ETH"],
+                    ...ethLines("Up to 1.0000 ETH"),
+                ],
+            },
+            {
+                kind: "the same with a WRAP_ETH of 1 ETH to another address after the UNWRAP_WETH",
+                commands: [0x0b, 0x0c, 0x0b, 0x10],
+                inputs: [
+                    WRAP_INTO_ROUTER,
+                    UNWRAP_INTO_ROUTER,
+                    encodeWrapEth(OTHER_ADDR, ONE_ETH),
+                    V4_ONE_ETH,
+                ],
+                lines: [
+                    ["Amount", "1.0000 ETH"],
+                    ...ethLines("Up to 1.0000 ETH"),
+                ],
+            },
+            {
+                kind: "the same on UniversalRouterV2_1_2 with an ACROSS_V4_DEPOSIT_V3 of 1 ETH after the UNWRAP_WETH",
+                router: ROUTER_V2_1_2_ADDR,
+                commands: [0x0b, 0x0c, 0x40, 0x10],
+                inputs: [
+                    WRAP_INTO_ROUTER,
+                    UNWRAP_INTO_ROUTER,
+                    acrossEth(ONE_ETH),
+                    V4_ONE_ETH,
+                ],
+                lines: [
+                    ["Amount", "1.0000 ETH"],
+                    ...ethLines("Up to 1.0000 ETH"),
+                ],
+            },
+            {
+                kind: "a V3 exact-in of 600 USDT that the user pays, a WRAP_ETH of 1 ETH into the router, an UNWRAP_WETH into the router, then a V4 swap of 1 ETH that the user settles",
+                commands: [0x00, 0x0b, 0x0c, 0x10],
+                inputs: [
+                    V3_600,
+                    WRAP_INTO_ROUTER,
+                    UNWRAP_INTO_ROUTER,
+                    V4_ONE_ETH,
+                ],
+                lines: [
+                    ["Amount", "600.0000 USDT"],
+                    ...ethLines("Up to 1.0000 ETH"),
+                ],
+            },
+            {
+                kind: "a V3 exact-in of 600 USDT for WETH paid to the router, an UNWRAP_WETH into the router, then a V4 swap of 1 ETH that the user settles",
+                commands: [0x00, 0x0c, 0x10],
+                inputs: [
+                    encodeV3SwapExactIn(ADDRESS_THIS, USDT_600, 1n, [
+                        USDT_ADDR,
+                        WETH_ADDR,
+                    ]),
+                    UNWRAP_INTO_ROUTER,
+                    V4_ONE_ETH,
+                ],
+                lines: [
+                    ["Amount", "600.0000 USDT"],
+                    ...ethLines("Up to 1.0000 ETH"),
+                ],
+            },
+            {
+                kind: "a V4 swap of 600 USDT that takes ETH into the router, then a V4 swap of 5 ETH that the user settles",
+                commands: [0x10, 0x10],
+                inputs: [
+                    encodeV4Swap(
+                        new Uint8Array([
+                            V4_SWAP_EXACT_IN_SINGLE_ID,
+                            V4_SETTLE_ID,
+                            V4_TAKE_ID,
+                        ]),
+                        [
+                            encodeV4ExactInSingle(
+                                USDT_ADDR,
+                                ZERO_ADDR,
+                                USDT_600,
+                                1n,
+                            ),
+                            settle(USDT_ADDR, 0n, true),
+                            take(ZERO_ADDR, ADDRESS_THIS, 0n),
+                        ],
+                    ),
+                    v4EthForUsdc(5n * ONE_ETH),
+                ],
+                lines: [
+                    ["Amount", "600.0000 USDT"],
+                    ...ethLines("Up to 5.0000 ETH"),
+                ],
+            },
+            {
+                kind: "a WRAP_ETH of 1 ETH into the router, an UNWRAP_WETH into the router, then a V4 exact-out paid in ETH with a SETTLE_ALL of at most 3 ETH, reads as on next",
                 commands: [0x0b, 0x0c, 0x10],
                 inputs: [
-                    encodeWrapEth(ROUTER_ADDR, ONE_ETH),
-                    unwrapIntoRouter,
-                    v4EthForUsdc((ONE_ETH * 3n) / 2n),
+                    WRAP_INTO_ROUTER,
+                    UNWRAP_INTO_ROUTER,
+                    encodeV4Swap(
+                        new Uint8Array([
+                            V4_SWAP_EXACT_OUT_SINGLE_ID,
+                            V4_SETTLE_ALL_ID,
+                            V4_TAKE_ALL_ID,
+                        ]),
+                        [
+                            encodeV4ExactOutSingle(
+                                ZERO_ADDR,
+                                USDC_ADDR,
+                                390000000n,
+                                3n * ONE_ETH,
+                            ),
+                            encodeV4SettleAll(ZERO_ADDR, 3n * ONE_ETH),
+                            encodeV4TakeAll(USDC_ADDR, 0n),
+                        ],
+                    ),
+                ],
+                lines: [
+                    ...wrapLines("The swap step that spends it"),
+                    ...ethLines(UNDECODED),
                 ],
-                lines: [["Amount", "1.5000 ETH"]],
             },
-        ])("$kind", ({ commands, inputs, lines }) => {
-            const result = decodeSteps(commands, inputs);
+            {
+                kind: "a WRAP_ETH of 1 ETH into the router, a V3 exact-out of at most 1 WETH paid from the router, an UNWRAP_WETH into the router, then a V4 swap of 1 ETH that the user settles",
+                commands: [0x0b, 0x01, 0x0c, 0x10],
+                inputs: [
+                    WRAP_INTO_ROUTER,
+                    encodeV3SwapExactOut(
+                        USER_ADDR,
+                        390000000n,
+                        ONE_ETH,
+                        [USDC_ADDR, WETH_ADDR],
+                        false,
+                    ),
+                    UNWRAP_INTO_ROUTER,
+                    V4_ONE_ETH,
+                ],
+                lines: [
+                    ["Amount", "Up to 1.0000 ETH"],
+                    ...ethLines("Up to 1.0000 ETH"),
+                ],
+            },
+            {
+                kind: "a WRAP_ETH of 1 ETH into the router, a V4 exact-out paid from the router's WETH, an UNWRAP_WETH into the router, then a V4 swap of 1 ETH that the user settles",
+                commands: [0x0b, 0x10, 0x0c, 0x10],
+                inputs: [
+                    WRAP_INTO_ROUTER,
+                    encodeV4Swap(
+                        new Uint8Array([
+                            V4_SWAP_EXACT_OUT_SINGLE_ID,
+                            V4_SETTLE_ID,
+                            V4_TAKE_ALL_ID,
+                        ]),
+                        [
+                            encodeV4ExactOutSingle(
+                                WETH_ADDR,
+                                USDC_ADDR,
+                                390000000n,
+                                ONE_ETH,
+                            ),
+                            settle(WETH_ADDR, 0n, false),
+                            encodeV4TakeAll(USDC_ADDR, 0n),
+                        ],
+                    ),
+                    UNWRAP_INTO_ROUTER,
+                    V4_ONE_ETH,
+                ],
+                lines: [
+                    ["Amount", "1.0000 ETH"],
+                    ...ethLines("Up to 1.0000 ETH"),
+                ],
+            },
+        ])("$kind", ({ router, commands, inputs, lines }) => {
+            const result = decodeSteps(commands, inputs, router);
             expect(linesBetweenTokens(result)).toEqual(lines);
         });