harden: a token scale above 80 decimal places is refused as unknown (closes #350)

toDecimals() accepted any uint8 scale, but formatUnits() and parseUnits()
refuse more than 80 decimal places. A token reporting 81 to 255 made the
formatter throw, and the catch in the swap decoder and in the ERC-20 decoder
turned that into an undecoded approval screen with nothing saying why.

MAX_DECIMALS is now 80, the formatter's own limit, so such a scale is
treated exactly like an unknown one: both approval paths show the base-unit
amount with the scale stated as unknown. The balance list, the history list
and the Send screen use the same check.

Model: opus-5-5

README.md

@@ -926,7 +926,10 @@ rule: the ERC-20 `transfer`/`approve` line (`src/popup/views/approval.js`) and
 the swap's `Amount` and `Min. received` lines (`src/shared/uniswap.js`). The
 token permission warning on the signature screen takes the same rule for its
 amounts. An unbounded allowance or permit needs no scale to describe and is
-still shown as `Unlimited`.
+still shown as `Unlimited`. A source's answer counts only if it is a whole
+number from 0 to 80: `decimals()` returns a `uint8`, but `formatUnits()` cannot
+format more than 80 decimal places, so a token that reports 81 to 255 is shown
+as one whose scale nothing knows.
 
 The rule holds only if nothing invents a scale UPSTREAM of it. Those three
 sources are read as authoritative, so a value written into one of them cannot be

TODO.md

@@ -45,6 +45,15 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-04: A token that reports more than 80 decimal places has no known
+  scale ([#350](https://git.eeqj.de/sneak/AutistMask/issues/350)). The shared
+  scale check `toDecimals()` accepted any `uint8`, but `formatUnits()` throws
+  above 80, so such a token left a swap or an ERC-20 call on the approval screen
+  undecoded, with nothing saying why. The check now stops at 80, and both
+  approval paths show the base-unit amount with the scale stated as unknown. The
+  balance list and the history list use the same check, so the same token no
+  longer stops an address's token balances from refreshing or its history from
+  loading.
 - 2026-10-04: Debug mode no longer writes RPC API keys to the console
   ([#410](https://git.eeqj.de/sneak/AutistMask/issues/410)). `debugFetch` logged
   every request's full URL and body, so an RPC endpoint with a key in its path

src/shared/approvalAmount.js

@@ -23,11 +23,11 @@
 // disputed is refused rather than guessed at.
 
 // Solidity's decimals() is a uint8, and every source here is ultimately
-// reporting that call's result. toDecimals() is that check, shared with the
-// send path rather than copied: the bundled list stores numbers, the
-// explorer's copy arrives as a string, and a token the user added by hand
-// carries whatever lookupTokenInfo() got back, so the accepted types are
-// enumerated rather than coerced.
+// reporting that call's result. toDecimals() is that check, stopping at the 80
+// places formatUnits() accepts, and shared with the send path rather than
+// copied: the bundled list stores numbers, the explorer's copy arrives as a
+// string, and a token the user added by hand carries whatever lookupTokenInfo()
+// got back, so the accepted types are enumerated rather than coerced.
 const { toDecimals } = require("./transferAmount");
 const { TOKEN_BY_ADDRESS } = require("./tokenList");
 const { isSpoofedSymbol } = require("./symbolSpoof");

src/shared/transferAmount.js

@@ -27,9 +27,11 @@
 
 const { parseUnits } = require("ethers");
 
-// Solidity's decimals() returns a uint8, so anything outside that range is not
-// an answer this wallet can use.
-const MAX_DECIMALS = 255;
+// Solidity's decimals() returns a uint8, but ethers' formatUnits() and
+// parseUnits() refuse more than 80 decimal places ("invalid FixedNumber
+// decimals (too large)"). A scale of 81 to 255 can be neither displayed nor
+// encoded, so it is not an answer this wallet can use, the same as no answer.
+const MAX_DECIMALS = 80;
 
 const UNKNOWN_DISPLAYED_DECIMALS_MESSAGE =
     "The transfer was not sent, because the number of decimal places this" +
@@ -55,7 +57,7 @@ function mismatchMessage(displayed, onChain) {
 // A decimals value from any source as a number, or null if it is not one.
 // decimals() comes back from ethers as a bigint and the explorer's copy arrives
 // as a string, so both of those are accepted alongside a plain number; anything
-// fractional, negative, out of uint8 range, or of any other type at all is not.
+// fractional, negative, above MAX_DECIMALS, or of any other type at all is not.
 //
 // The types are enumerated rather than coerced because Number() is far too
 // willing: Number([]) is 0 and Number(true) is 1, so a coercing check would

tests/approvalAmount.test.js

@@ -184,6 +184,22 @@ describe("decodeCalldata amount", () => {
         expect(line).not.toMatch(/0\.0000/);
     });
 
+    // A token added by hand carries whatever its decimals() returned, and a
+    // uint8 reaches 255, but formatUnits() throws above 80. The throw left the
+    // call undecoded rather than refused
+    // (https://git.eeqj.de/sneak/AutistMask/issues/350).
+    test("a token reporting more than 80 decimals shows base units", () => {
+        state.trackedTokens = [
+            { address: NOVEL_TOKEN, symbol: "NOVEL", decimals: 81 },
+        ];
+        expect(
+            amountLine(transferData(FIVE_THOUSAND_AT_SIX), NOVEL_TOKEN),
+        ).toBe("5000000000 base units (decimals unknown)");
+        expect(amountLine(approveData(FIVE_THOUSAND_AT_SIX), NOVEL_TOKEN)).toBe(
+            "5000000000 base units (decimals unknown)",
+        );
+    });
+
     test("an unbounded allowance is still named, with or without a scale", () => {
         expect(amountLine(approveData(MAX_UINT256), NOVEL_TOKEN)).toBe(
             "Unlimited",

tests/transferAmount.test.js

@@ -4,7 +4,7 @@
 // contract at signing time, with nothing comparing the two, so a token whose
 // on-chain scale differed signed an amount that was never displayed.
 
-const { parseUnits } = require("ethers");
+const { formatUnits, parseUnits } = require("ethers");
 const {
     displayedDecimals,
     transferAmountUnits,
@@ -25,7 +25,17 @@ describe("displayedDecimals", () => {
         expect(displayedDecimals(MAX_DECIMALS)).toBe(MAX_DECIMALS);
     });
 
-    test("refuses anything that is not a uint8", () => {
+    // decimals() is a uint8, but formatUnits() and parseUnits() stop at 80
+    // places, so a larger scale cannot be shown or encoded
+    // (https://git.eeqj.de/sneak/AutistMask/issues/350).
+    test("accepts exactly the scales the formatter accepts", () => {
+        expect(() => formatUnits(1n, MAX_DECIMALS)).not.toThrow();
+        expect(() => parseUnits("1", MAX_DECIMALS)).not.toThrow();
+        expect(() => formatUnits(1n, MAX_DECIMALS + 1)).toThrow();
+        expect(() => parseUnits("1", MAX_DECIMALS + 1)).toThrow();
+    });
+
+    test("refuses anything that is not a uint8 the formatter accepts", () => {
         for (const bad of [
             null,
             undefined,

tests/uniswapUnknownDecimals.test.js

@@ -126,6 +126,19 @@ describe("a swap of a token outside the bundled list", () => {
     test("a bundled token on the other side still formats", () => {
         expect(swapDetail(data(), "Min. received").value).toBe("0.5000 WETH");
     });
+
+    // A token added by hand carries whatever its decimals() returned, and a
+    // uint8 reaches 255, but formatUnits() throws above 80. The throw left the
+    // whole swap undecoded rather than refused
+    // (https://git.eeqj.de/sneak/AutistMask/issues/350).
+    test("refuses to format when the token reports more than 80 decimals", () => {
+        state.trackedTokens = [
+            { address: NOVEL, symbol: "NOVEL", decimals: 81 },
+        ];
+        expect(swapDetail(data(), "Amount").value).toBe(
+            "1000000000 base units (decimals unknown)",
+        );
+    });
 });
 
 describe("the Min. received line takes the same rule", () => {