feat: read stablecoins from stable-tokens LD flag in useTokenFiatRates cp-8.3.0

[jpuri]

Description

On MM Pay UI read stablecoins from stable-tokens LD flag.

Changelog

CHANGELOG entry:

Related issues

Fixes: https://consensyssoftware.atlassian.net/browse/CONF-1671

Manual testing steps

NA

Screenshots/Recordings

NA

Pre-merge author checklist

• I've followed MetaMask Contributor Docs and MetaMask Mobile Coding Standards.
• I've completed the PR template to the best of my ability
• I've included tests if applicable
• I've documented my code using JSDoc format if applicable
• I've applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Performance checks (if applicable)

• I've tested on Android
  • Ideally on a mid-range device; emulator is acceptable
• I've tested with a power user scenario
  • Use these power-user SRPs to import wallets with many accounts and tokens
• I've instrumented key operations with Sentry traces for production performance metrics
  • See trace() for usage and addToken for an example

For performance guidelines and tooling, see the Performance Guide.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

---

Note

Medium Risk
Changes how USD fiat rates and MM Pay deposit prefills are computed; wrong flag or route data could mis-price tokens or prefill amounts, though defaults mirror prior behavior.

Overview
Stablecoin fiat pricing no longer uses a hardcoded per-chain address map in useTokenFiatRates. It now reads selectStablecoins from the remote stable-tokens LaunchDarkly flag (with the previous addresses as defaults and normalization for invalid flag shapes).

Deposit prefill (100% vs 50% of balance) switches from isStablecoin in token.ts to isRouteToken in relayFixedSpread, which treats a pay token as “stable” when it appears on any relay fixed-spread route as source or target. That helper is new; isStablecoin is removed from token.ts. Related hooks/tests are updated accordingly.

The stable-tokens flag is registered in the feature-flag registry (remote, not in prod by default).

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✅ All previously detected unit test flakiness issues in this PR have been fixed.

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[github-actions]

🔍 Smart E2E Test Selection

• Selected E2E tags: SmokeConfirmations, SmokeMoney, SmokeStake
• Selected Performance tags: None (no tests recommended)
• Risk Level: medium
• AI Confidence: 82%

click to see 🤖 AI reasoning details

E2E Test Selection:
The PR makes the following changes:

1. New selectStablecoins selector (feature-flag-driven): Moves stablecoin list from a hardcoded constant in useTokenFiatRates.ts to a remote feature flag (stable-tokens), with the same hardcoded values as defaults. This affects how token fiat rates are computed in confirmations — stablecoins get a 1:1 USD rate instead of market rate.

2. useTokenFiatRates.ts refactor: Now reads stablecoins from the Redux store via selectStablecoins instead of a local constant. The logic is functionally equivalent when the feature flag is not set (defaults match old hardcoded values), but the data source is now dynamic.

3. isStablecoin → isRouteToken rename/refactor: The isStablecoin function in token.ts is removed and replaced by isRouteToken in relayFixedSpread.ts. The new function uses addressesEqual for comparison (case-insensitive), which is more robust than the old string comparison. This affects useDepositPrefillAmount.ts which determines whether to prefill 100% (stablecoin) or 50% (non-stablecoin) of balance for deposits.

Tag selection rationale:

• SmokeConfirmations: useTokenFiatRates is used in pay-token-amount components and other confirmation hooks. The stablecoin detection change affects fiat rate display in transaction confirmations.
• SmokeMoney: useDepositPrefillAmount and useTransactionCustomAmount are directly used in MetaMask Card deposit flows. The prefill percentage logic (100% vs 50%) is critical for the deposit UX.
• SmokeStake: Lending deposit flows also use confirmation components and may be affected by the token fiat rate changes.

The feature flag registry change is a test infrastructure addition that registers the new stable-tokens flag — no functional risk on its own.

Performance Test Selection:
The changes are refactors of stablecoin detection logic (moving from hardcoded constants to a feature flag selector) and deposit prefill amount calculation. These are not performance-sensitive paths — they involve simple selector lookups and array comparisons. No performance test tags are warranted.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud