Both the current and legacy JS clients convert token amounts through Number(bigint) and Math.trunc, and parse UI strings with parseFloat, so they lose precision and diverge from the on-chain conversion.

Problems

• Precision loss above 2^53: Number(amount) on a bigint silently loses precision for large balances / high-decimal mints.
• Trunc-before-divide zeroing: Math.trunc(scaledAmount) / decimalFactor rounds sub-unit amounts to 0 for a scaled-UI multiplier < 1 (amount=1, decimals=0, multiplier=0.5 → "0" instead of "0.5").
• Unchecked parseFloat: the ui→amount direction parses with parseFloat and no u64 range check.

Result: wallets/tools can display or sign incorrect amounts (off-chain, but users could act on it).

Affected code

• clients/js/src/amountToUiAmount.ts — Number(amount) + Math.trunc at :137,140 (interest-bearing) and :211,213 (scaled-UI); parseFloat / Math.trunc in the ui→amount path (:180,194).
• clients/js-legacy/src/actions/amountToUiAmount.ts — same pattern at :148,154,169,171,207; parseFloat at :94 and Math.trunc at :280.

Fix

Use bigint throughout, divide to display precision instead of truncating base units first (matching on-chain rounding order), and range-check parsed UI strings against u64.