fix: the swap decoder reads a V2 already-paid zero and a zero balance check as the router does (closes #415)

A V2 exact-in amountIn of zero is the router's ALREADY_PAID marker: an
earlier step sent the tokens to the pair and the swap spends all of them.
Amount showed 0.0000 for it; it now reads "Whatever an earlier step sent
to the pair (V2 already paid)", in the style of the V4 open delta line.

A BALANCE_CHECK_ERC20 passes whenever the balance is at least minBalance,
so a zero one guarantees nothing. It no longer replaces a minimum an
earlier step stated; a nonzero one sets the output side as before.

README's Display Consistency text and TODO.md are updated to match.

Model: opus-5-5

README.md

@@ -887,7 +887,9 @@ Truncation stays truncation: `0.99999` shows as `0.9999`, never rounded up. The
 rule still renders a genuine zero as `0.0000`. Two lines of a swap say a zero in
 words instead: `Min. received` reads `None (no minimum guaranteed)` for a zero
 minimum, and `Amount` reads `All available (V4 open delta)` when the amount it
-shows is a V4 exact-in `amountIn` of zero.
+shows is a V4 exact-in `amountIn` of zero and
+`Whatever an earlier step sent to the pair (V2 already paid)` when it is a V2
+exact-in `amountIn` of zero.
 
 The rule and its exception live in `src/shared/amountDisplay.js` as
 `truncateAmount()` and `truncateAmountNeverZero()`. Everything the approval and
@@ -980,7 +982,8 @@ read:
   exact-out step, whichever step set the line, including the `WRAP_ETH` of a
   swap paid in ETH and a `PERMIT2_PERMIT`. The swap spends at most that figure,
   not necessarily all of it; the wait, success and error screens show it with
-  the same words. `Unlimited` and `All available (V4 open delta)` keep their
+  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 exact-out step sets `Min. received`, that line shows its
   `amountOut`, the exact amount it buys.
 - `All available (V4 open delta)`: the swap's `Amount` line, when the amount it
@@ -991,11 +994,21 @@ read:
   V2 exact-out, `WRAP_ETH` or V4 swap step. A V2 exact-out step gives its
   `amountInMax`, and a V4 swap step the `amountIn` of its first readable
   exact-in action.
+- `Whatever an earlier step sent to the pair (V2 already paid)`: the swap's
+  `Amount` line, when the amount it shows is a V2 exact-in `amountIn` of zero.
+  The router reads that zero as "the pair already holds the input tokens": the
+  step pays nothing itself and swaps whatever an earlier step sent to the pair,
+  so the calldata states no quantity. A V3 exact-in `amountIn` of zero has no
+  such meaning and is shown as a zero.
 - `None (no minimum guaranteed)`: the swap's `Min. received` line, when the
   minimum it shows is zero, whether a V2, V3 or V4 swap's minimum or a
   `BALANCE_CHECK_ERC20` step's `minBalance`. Before
   [#359](https://git.eeqj.de/sneak/AutistMask/issues/359), a zero `minBalance`
-  read `0.0000` when the token's scale was known.
+  read `0.0000` when the token's scale was known. The router passes a balance
+  check whenever the balance is at least `minBalance`, so a zero `minBalance`
+  guarantees nothing: it sets `Token Out` and `Min. received` only when no
+  earlier step stated a minimum, and otherwise leaves that step's token and
+  figure in place. A nonzero `minBalance` sets both lines, as a swap step does.
 
 The swap's `Token In` and `Token Out` lines name a currency, not an amount; each
 reads `Unknown (not named in the calldata)` when the decoder found no token for

TODO.md

@@ -45,6 +45,15 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-04: The swap decoder reads two router zeros the way the router does
+  ([#415](https://git.eeqj.de/sneak/AutistMask/issues/415)). A V2 exact-in
+  `amountIn` of zero means an earlier step already sent the tokens to the pair;
+  `Amount` showed `0.0000` for it and now reads
+  `Whatever an earlier step sent to the pair (V2 already paid)`. A
+  `BALANCE_CHECK_ERC20` with a zero `minBalance` guarantees nothing, yet it
+  replaced the minimum an earlier swap step stated, so `Min. received` read
+  `None (no minimum guaranteed)`; it now leaves that minimum in place. A nonzero
+  `minBalance` still sets the output side.
 - 2026-10-04: The signature screen shows a personal message as the bytes that
   are signed ([#403](https://git.eeqj.de/sneak/AutistMask/issues/403)). It
   showed only the decoded text, with bidirectional and zero-width characters

src/shared/uniswap.js

@@ -84,17 +84,31 @@ function present(value) {
 //
 // `amountOutMinimum` gets no such mapping: V4Router compares it directly
 // (`if (amountOut < params.amountOutMinimum) revert V4TooLittleReceived`), so
-// a zero minimum is a literal zero slippage floor and is stated as one. Nor do
-// the V2/V3 paths have it — universal-router's `V3SwapRouter.v3SwapExactInput`
+// a zero minimum is a literal zero slippage floor and is stated as one. Nor
+// does the V3 path have it — universal-router's `V3SwapRouter.v3SwapExactInput`
 // special-cases only `ActionConstants.CONTRACT_BALANCE` (1<<255), never zero —
-// so a zero `amountIn` there is a literal zero and is displayed as one.
+// so a zero V3 `amountIn` is a literal zero and is displayed as one. The V2
+// exact-in path gives zero a meaning of its own: see ALREADY_PAID.
 const OPEN_DELTA = Symbol("v4-open-delta");
 
-// The two amount lines that state a fact instead of a quantity. Same register
-// as UNNAMED_CURRENCY — a sentence in the value slot, so it cannot be misread
-// as a number — and deliberately not a third phrasing of "not named": these
-// say different things.
+// The Universal Router's V2 exact-in spells "the pair already holds the input
+// tokens" as an amount of zero: universal-router
+// `contracts/libraries/Constants.sol` declares
+// `uint256 internal constant ALREADY_PAID = 0` ("Used for identifying cases
+// when a v2 pair has already received input tokens"), and
+// `V2SwapRouter.v2SwapExactInput` makes no payment of its own when `amountIn`
+// equals it. The swap then spends whatever an earlier step sent to the pair.
+// As with OPEN_DELTA, the calldata states no quantity, and "0.0000" would say
+// that nothing is swapped.
+const ALREADY_PAID = Symbol("v2-already-paid");
+
+// The amount lines that state a fact instead of a quantity. Same register as
+// UNNAMED_CURRENCY — a sentence in the value slot, so it cannot be misread as a
+// number — and deliberately not another phrasing of "not named": these say
+// different things.
 const OPEN_DELTA_AMOUNT = "All available (V4 open delta)";
+const ALREADY_PAID_AMOUNT =
+    "Whatever an earlier step sent to the pair (V2 already paid)";
 const NO_MINIMUM = "None (no minimum guaranteed)";
 
 // Permit2 amounts are uint160; the maximum is Permit2's "unbounded".
@@ -185,6 +199,7 @@ function decodeBalanceCheck(input) {
 // Decode V2_SWAP_EXACT_IN (command 0x08) input bytes.
 // ABI: (address recipient, uint256 amountIn, uint256 amountOutMin,
 //        address[] path, bool payerIsUser)
+// A zero `amountIn` is read the way the router reads it, as ALREADY_PAID.
 function decodeV2SwapExactIn(input) {
     try {
         const d = coder.decode(
@@ -192,7 +207,7 @@ function decodeV2SwapExactIn(input) {
             input,
         );
         return {
-            amountIn: d[1],
+            amountIn: d[1] === 0n ? ALREADY_PAID : d[1],
             amountOutMin: d[2],
             tokenIn: d[3][0],
             tokenOut: d[3][d[3].length - 1],
@@ -502,7 +517,13 @@ function decode(data, toAddress, sources) {
 
                 if (cmdId === 0x0e) {
                     const b = decodeBalanceCheck(inputs[i]);
-                    if (b) setOutput(b.token, b.minBalance);
+                    // The router passes this check whenever the owner holds at
+                    // least minBalance, so a zero one guarantees nothing and
+                    // does not replace a minimum an earlier step stated. Any
+                    // other minBalance sets the output side as a swap does.
+                    if (b && !(b.minBalance === 0n && present(minOutput))) {
+                        setOutput(b.token, b.minBalance);
+                    }
                 }
 
                 if (cmdId === 0x00) {
@@ -623,14 +644,20 @@ function decode(data, toAddress, sources) {
         }
 
         if (present(inputAmount)) {
-            // Two amounts need no scale to describe and are named rather than
-            // formatted: V4's open delta, which is not a quantity at all (see
-            // OPEN_DELTA), and an unbounded permit. The open-delta test comes
-            // first — the sentinel is not a bigint and cannot be compared with
-            // one.
+            // Three amounts need no scale to describe and are named rather
+            // than formatted: V4's open delta and 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.
             let amount;
             if (inputAmount === OPEN_DELTA) {
                 amount = { raw: OPEN_DELTA_AMOUNT, display: OPEN_DELTA_AMOUNT };
+            } else if (inputAmount === ALREADY_PAID) {
+                amount = {
+                    raw: ALREADY_PAID_AMOUNT,
+                    display: ALREADY_PAID_AMOUNT,
+                };
             } else if (inputAmount >= MAX_UINT160) {
                 amount = { raw: "Unlimited", display: "Unlimited" };
             } else if (hasV2ExactOut) {

tests/uniswap.test.js

@@ -831,6 +831,104 @@ describe("uniswap decoder", () => {
         expect(detail(result, "Min. received").value).toBe("0.9900 USDC");
     });
 
+    // https://git.eeqj.de/sneak/AutistMask/issues/415 — the router's V2
+    // exact-in reads an amountIn of zero as universal-router
+    // Constants.ALREADY_PAID: an earlier step sent the tokens to the pair, and
+    // the swap uses all of them. Against 375998b this read "0.0000 USDT".
+    test("a V2 exact-in already-paid amountIn is named, not printed as zero", () => {
+        const data = buildExecute(
+            "0x08",
+            [
+                encodeV2SwapExactIn(
+                    USER_ADDR,
+                    0n, // Constants.ALREADY_PAID
+                    500000000000000n,
+                    [USDT_ADDR, WETH_ADDR],
+                ),
+            ],
+            9999999999n,
+        );
+
+        const result = uniswap.decode(data, ROUTER_ADDR);
+        expect(result).not.toBeNull();
+
+        expect(detail(result, "Token In").value).toContain("USDT");
+        expect(detail(result, "Amount").value).toBe(
+            "Whatever an earlier step sent to the pair (V2 already paid)",
+        );
+        expect(detail(result, "Amount").rawValue).toBe(
+            "Whatever an earlier step sent to the pair (V2 already paid)",
+        );
+        expect(detail(result, "Min. received").value).toBe("0.0005 WETH");
+    });
+
+    // https://git.eeqj.de/sneak/AutistMask/issues/415 — the router passes a
+    // BALANCE_CHECK_ERC20 whenever the balance is at least minBalance, so a
+    // zero one guarantees nothing. Against 375998b it replaced the swap's
+    // output side: Token Out = USDC, Min. received = "None (no minimum
+    // guaranteed)".
+    test("a zero balance check keeps the minimum a swap step stated", () => {
+        const data = buildExecute(
+            solidityPacked(["uint8", "uint8"], [0x08, 0x0e]),
+            [
+                encodeV2SwapExactIn(USER_ADDR, 1000000n, 500000000000000n, [
+                    USDT_ADDR,
+                    WETH_ADDR,
+                ]),
+                encodeBalanceCheck(USER_ADDR, USDC_ADDR, 0n),
+            ],
+            9999999999n,
+        );
+
+        const result = uniswap.decode(data, ROUTER_ADDR);
+        expect(result).not.toBeNull();
+
+        expect(detail(result, "Token Out").value).toContain("WETH");
+        expect(detail(result, "Min. received").value).toBe("0.0005 WETH");
+    });
+
+    // A nonzero balance check still replaces the output side, as before.
+    test("a nonzero balance check replaces the minimum a swap step stated", () => {
+        const data = buildExecute(
+            solidityPacked(["uint8", "uint8"], [0x08, 0x0e]),
+            [
+                encodeV2SwapExactIn(USER_ADDR, 1000000n, 500000000000000n, [
+                    USDT_ADDR,
+                    WETH_ADDR,
+                ]),
+                encodeBalanceCheck(USER_ADDR, USDC_ADDR, 2000000n),
+            ],
+            9999999999n,
+        );
+
+        const result = uniswap.decode(data, ROUTER_ADDR);
+        expect(result).not.toBeNull();
+
+        expect(detail(result, "Token Out").value).toContain("USDC");
+        expect(detail(result, "Min. received").value).toBe("2.0000 USDC");
+    });
+
+    // With no minimum stated before it, a zero balance check is what sets the
+    // output side, and it guarantees nothing.
+    test("a zero balance check with no earlier minimum states no minimum", () => {
+        const data = buildExecute(
+            solidityPacked(["uint8", "uint8"], [0x0b, 0x0e]),
+            [
+                encodeWrapEth(ROUTER_ADDR, 1000000000000000000n),
+                encodeBalanceCheck(USER_ADDR, USDT_ADDR, 0n),
+            ],
+            9999999999n,
+        );
+
+        const result = uniswap.decode(data, ROUTER_ADDR);
+        expect(result).not.toBeNull();
+
+        expect(detail(result, "Token Out").value).toContain("USDT");
+        expect(detail(result, "Min. received").value).toBe(
+            "None (no minimum guaranteed)",
+        );
+    });
+
     // Pins what https://git.eeqj.de/sneak/AutistMask/pulls/356 changed without
     // testing: a non-swap execute() carrying only PERMIT2_PERMIT names no
     // output currency, so it says so and titles itself "Uniswap Swap" rather