Show ramp crypto amounts at the asset's own decimal precision

[j0ntz]

CHANGELOG

Does this branch warrant an entry to the CHANGELOG?

• Yes
• No

Dependencies

none

Requirements

If you have made any visual changes to the GUI. Make sure you have:

• Tested on iOS device
• Tested on Android device
• Tested on small-screen device (iPod Touch)
• Tested on large-screen device (tablet)

Description

Asana: https://app.asana.com/0/1215088146871429/1217660195990065

MoonPay told us it raises the decimal precision of its buy flow on
September 1, 2026. BTC goes from 5 to 8 decimals, ETH from 6 to 9 decimals, and
SOL from 3 to 9 decimals, while stablecoins stay at 2. MoonPay asked integrators to confirm they treat crypto
amounts as variable-precision decimals rather than assuming a fixed count per
asset. The sell flow is unaffected.

Audit of the live buy path (src/plugins/ramps/moonpay/, which is what the Buy
tab routes to; the legacy amountQuotePlugin sits behind pluginListBuyOld):

• Parsing and maths are already variable-precision. The quote amount is a JSON
  number rendered back through Number.toString() and then handled by
  biggystring, which is arbitrary precision and accepts exponent notation.
• Nothing in the buy path stores or reconciles the amount at a fixed scale, and
  the one quote-vs-delivered comparison normalises both sides through the same
  number parse, so it assumes no decimal count.
• One place did hardcode a decimal count: RampCreateScene rendered the crypto
  amount field with maxDecimals={6}, which FilledTextInput applies as
  toFixed(value, 0, 6), a truncation. Every MoonPay asset currently quotes at
  6 decimals or fewer, so the cap has never bitten; after September 1 it would
  silently drop 2 digits of BTC and 3 of ETH and SOL.

This derives the cap from the selected asset's own denomination instead, bounded
at 9 decimals. That gives BTC 8, ETH 9, SOL 9 and USDC 6, matching MoonPay's new
per-asset maximums. The bound matters for assets with more precision than a
quote can carry: when the user types a fiat amount the field divides by a float
exchange rate, whose rounding noise shows up past about 9 decimals, so rendering
ETH's full 18 would surface digits that are not real.

Verified on the iOS simulator with a before and after pair on the same input,
attached below. verify-repo.sh passes (eslint, jest, changelog).

---

Note

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[j0ntz]

📸 Test evidence

before truncated at 6 decimals

after btc shows 8 decimals

after eth shows 9 decimals

Captured by the agent's in-app test run (build-and-test).

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