fix(samples/frontend): floor default quote amount to corridor minimum

[pengying]

Corridor minimums vary widely (e.g. USD->EUR ~$23, USD->BRL ~$12, USD->MXN
~$1), so a fixed default still failed with AMOUNT_OUT_OF_RANGE on higher-min
corridors. The quote step now looks up the corridor's minSendingAmount via
the exchange-rates endpoint and floors the default amount to it (+5% buffer
for FX drift). maxSendingAmount is intentionally not used as it is unreliable
for some corridors in this environment.

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

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[pengying]

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• fix(samples/frontend): floor default quote amount to corridor minimum #605 👈 (View in Graphite)
• fix(samples/frontend): scale default quote amount by source currency decimals #604
• fix(samples/kotlin): select crypto network when funding a quote from USDC #603
• feat(samples/kotlin): add exchange-rates lookup flow #602
• feat(samples/kotlin): add USD → USDC wallet payout flow #593
• fix(samples/kotlin): send customer email for USDB embedded wallets #590
• feat(samples/kotlin): support GRID_API_BASE_URL override #589
• main

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[greptile-apps]

Greptile Summary

The CreateQuote step now pre-fetches corridor metadata from the exchange-rates endpoint to determine the minSendingAmount, then floors the default lockedCurrencyAmount to 105 % of that minimum before populating the JSON editor.

• A new useEffect fetches corridor minimums on sourceCurrency/destCurrency change, with a correct cancellation guard to avoid stale-closure races.
• The body-building useEffect takes Math.max(defaultAmount, Math.ceil(minSendingAmount * 1.05)), ensuring the pre-filled amount clears every corridor's minimum with a small FX-drift buffer.
• When the exchange-rates fetch fails the component silently falls back to the un-floored default — no error state is surfaced in the UI before the user submits.

Confidence Score: 4/5

Safe to merge for a sample app; the corridor-floor logic is correct and the cancellation pattern prevents stale-state bugs.

The change is self-contained and the arithmetic is consistent (minor units throughout). The one gap is that a failed exchange-rates fetch resets to the un-floored default silently — the user only discovers this from the subsequent quote error. For a developer sample this is low-risk, but it leaves the new behaviour invisible on network failures.

samples/frontend/src/steps/CreateQuote.tsx — specifically the .catch() handler that silently discards the fetch error.

Important Files Changed

Filename	Overview
samples/frontend/src/steps/CreateQuote.tsx	Adds a useEffect that pre-fetches the corridor's minSendingAmount via the exchange-rates endpoint and floors lockedCurrencyAmount to 105 % of that minimum. Cancellation pattern is correct. Silent failure on fetch error leaves the body at the un-floored default with no visual indication.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant User
    participant CreateQuote
    participant ExchangeRatesAPI
    participant QuotesAPI

    User->>CreateQuote: Select sourceCurrency / destCurrency
    CreateQuote->>CreateQuote: setMinSendingAmount(null) → body uses defaultAmount
    CreateQuote->>ExchangeRatesAPI: "GET /api/exchange-rates?sourceCurrency=...&destinationCurrency=...&sendingAmount=probe"
    alt Fetch succeeds
        ExchangeRatesAPI-->>CreateQuote: "{ data: [{ minSendingAmount: N, ... }] }"
        CreateQuote->>CreateQuote: setMinSendingAmount(max(all mins))
        CreateQuote->>CreateQuote: "lockedCurrencyAmount = max(defaultAmount, ceil(min * 1.05))"
    else Fetch fails / cancelled
        ExchangeRatesAPI-->>CreateQuote: error
        CreateQuote->>CreateQuote: setMinSendingAmount(null) → body stays at defaultAmount (silent)
    end
    User->>CreateQuote: Click "Create Quote"
    CreateQuote->>QuotesAPI: "POST /api/quotes { lockedCurrencyAmount, ... }"
    QuotesAPI-->>CreateQuote: Quote response or AMOUNT_OUT_OF_RANGE

Loading

%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
sequenceDiagram
    participant User
    participant CreateQuote
    participant ExchangeRatesAPI
    participant QuotesAPI

    User->>CreateQuote: Select sourceCurrency / destCurrency
    CreateQuote->>CreateQuote: setMinSendingAmount(null) → body uses defaultAmount
    CreateQuote->>ExchangeRatesAPI: "GET /api/exchange-rates?sourceCurrency=...&destinationCurrency=...&sendingAmount=probe"
    alt Fetch succeeds
        ExchangeRatesAPI-->>CreateQuote: "{ data: [{ minSendingAmount: N, ... }] }"
        CreateQuote->>CreateQuote: setMinSendingAmount(max(all mins))
        CreateQuote->>CreateQuote: "lockedCurrencyAmount = max(defaultAmount, ceil(min * 1.05))"
    else Fetch fails / cancelled
        ExchangeRatesAPI-->>CreateQuote: error
        CreateQuote->>CreateQuote: setMinSendingAmount(null) → body stays at defaultAmount (silent)
    end
    User->>CreateQuote: Click "Create Quote"
    CreateQuote->>QuotesAPI: "POST /api/quotes { lockedCurrencyAmount, ... }"
    QuotesAPI-->>CreateQuote: Quote response or AMOUNT_OUT_OF_RANGE

Loading

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
samples/frontend/src/steps/CreateQuote.tsx:53
**Silent min-fetch failure leaves body at the un-floored default**

When the exchange-rates request fails (transient network error, API error, non-`ok` response), the `.catch()` handler silently resets `minSendingAmount` to `null`. The body `useEffect` then fires with `flooredToMin = 0`, so `lockedCurrencyAmount` falls back to `defaultAmount` — potentially below the corridor minimum. There is no in-UI indication of this, so a developer would only discover the fallback by seeing the `AMOUNT_OUT_OF_RANGE` error after submitting the quote. Surfacing the error (e.g. via `setError`) or at least displaying a warning in the form header would make the failure visible before submission.

_{Reviews (1): Last reviewed commit: "fix(samples/frontend): floor default quo..." | Re-trigger Greptile}

[greptile-apps]

Silent min-fetch failure leaves body at the un-floored default

When the exchange-rates request fails (transient network error, API error, non-ok response), the .catch() handler silently resets minSendingAmount to null. The body useEffect then fires with flooredToMin = 0, so lockedCurrencyAmount falls back to defaultAmount — potentially below the corridor minimum. There is no in-UI indication of this, so a developer would only discover the fallback by seeing the AMOUNT_OUT_OF_RANGE error after submitting the quote. Surfacing the error (e.g. via setError) or at least displaying a warning in the form header would make the failure visible before submission.

Prompt To Fix With AI

This is a comment left during a code review.
Path: samples/frontend/src/steps/CreateQuote.tsx
Line: 53

Comment:
**Silent min-fetch failure leaves body at the un-floored default**

When the exchange-rates request fails (transient network error, API error, non-`ok` response), the `.catch()` handler silently resets `minSendingAmount` to `null`. The body `useEffect` then fires with `flooredToMin = 0`, so `lockedCurrencyAmount` falls back to `defaultAmount` — potentially below the corridor minimum. There is no in-UI indication of this, so a developer would only discover the fallback by seeing the `AMOUNT_OUT_OF_RANGE` error after submitting the quote. Surfacing the error (e.g. via `setError`) or at least displaying a warning in the form header would make the failure visible before submission.

How can I resolve this? If you propose a fix, please make it concise.