fix(exchange): reject sub-minimum native amounts in VerifiedFiatCalculator

apps/flipcash/core/src/main/res/values/strings.xml

@@ -679,6 +679,9 @@
     <string name="error_title_staleRates">Rate Unavailable</string>
     <string name="error_description_staleRates">Couldn\'t get a fresh rate. Please try again.</string>
 
+    <string name="error_title_amountTooSmall">Amount Too Small</string>
+    <string name="error_description_amountTooSmall">The amount you entered is too small to transfer\nPlease enter a larger amount</string>
+
     <string name="title_settings">Settings</string>
 
     <string name="title_withdrawUsdfAsUsdc">Withdraw as USDC</string>

apps/flipcash/features/cash/src/main/kotlin/com/flipcash/app/cash/internal/CashScreenViewModel.kt

@@ -316,11 +316,19 @@ internal class CashScreenViewModel @Inject constructor(
                     token = token,
                     balance = balance.underlyingTokenAmount,
                     rate = rate,
-                ).getOrElse {
+                ).getOrElse { error ->
                     dispatchEvent(Event.UpdateLoadingState(loading = false))
+                    val (title, message) = when (error) {
+                        is ComputeVerifiedFiatError.AmountBelowMinimum -> {
+                            R.string.error_title_amountTooSmall to R.string.error_description_amountTooSmall
+                        }
+                        else -> {
+                            R.string.error_title_staleRates to R.string.error_description_staleRates
+                        }
+                    }
                     BottomBarManager.showAlert(
-                        title = resources.getString(R.string.error_title_staleRates),
-                        message = resources.getString(R.string.error_description_staleRates),
+                        title = resources.getString(title),
+                        message = resources.getString(message),
                     )
                     return@onEach
                 }

services/opencode/src/main/kotlin/com/getcode/opencode/internal/exchange/RealVerifiedFiatCalculator.kt

@@ -114,6 +114,11 @@ internal class RealVerifiedFiatCalculator @Inject constructor(
         val underlyingTokenAmount = Fiat(quarks = quarks.toLong(), currencyCode = CurrencyCode.USD)
 
         val sellEstimate = Fiat.tokenBalance(quarks.toLong(), token, supply).convertingTo(rate)
+
+        if (!sellEstimate.hasDisplayableValue) {
+            return Result.failure(ComputeVerifiedFiatError.AmountBelowMinimum())
+        }
+
         val fx = sellEstimate.decimalValue.toBigDecimal().divideWithHighPrecision(units).toDouble()
 
         if (trace) {

services/opencode/src/main/kotlin/com/getcode/opencode/model/core/errors/Errors.kt

@@ -382,4 +382,5 @@ sealed class ComputeVerifiedFiatError(
     class StaleRate : ComputeVerifiedFiatError("Reserve state unavailable or stale")
     data class ComputationFailed(override val cause: Throwable? = null) :
         ComputeVerifiedFiatError(message = cause?.message, cause = cause)
+    class AmountBelowMinimum : ComputeVerifiedFiatError("Amount is too small to transfer")
 }

services/opencode/src/test/kotlin/com/getcode/opencode/internal/exchange/RealVerifiedFiatCalculatorTest.kt

@@ -396,6 +396,32 @@ class RealVerifiedFiatCalculatorTest {
 
     // endregion
 
+    // region sub-minimum amount
+
+    @Test
+    fun `returns AmountBelowMinimum when sell estimate has no displayable value`() = runTest {
+        // Simulates the Bugsnag scenario: a low-supply custom token where the
+        // bonding curve produces a native amount below the currency's smallest
+        // displayable unit (e.g. ₦0.003 rounds to ₦0.00 for NGN).
+        val supply = 1_000_000_000_000L
+        val token = bondingCurveToken(supply = supply)
+        stubVerifiedState(CurrencyCode.USD, testMint, supply)
+
+        // Use a sub-cent amount ($0.000001 = 1 quark) so the curve succeeds
+        // but the sell estimate is below the smallest displayable USD unit ($0.01).
+        val result = calculator.compute(
+            amount = Fiat(quarks = 1, currencyCode = CurrencyCode.USD),
+            token = token,
+            rate = Rate.oneToOne,
+            trace = false,
+        )
+
+        assertTrue(result.isFailure)
+        assertIs<ComputeVerifiedFiatError.AmountBelowMinimum>(result.exceptionOrNull())
+    }
+
+    // endregion
+
     // region helpers
 
     private fun usdfToken(): Token = MintMetadata(