Code review:

- Account.kt: collapse three identical backend-not-configured Failure
  constructions into one helper
- OnchainZapSendError: declare causeIsUserFacing on the enum so the
  sender owns which failures carry a human-readable cause, instead of
  the UI mapper hardcoding the list
- SendPaymentScreen: fee chip label is now a single format resource
  instead of manual string concatenation

amethyst/src/main/java/com/vitorpamplona/amethyst/model/Account.kt

@@ -892,6 +892,13 @@ class Account(
         return zapRequest
     }
 
+    private fun onchainBackendNotConfigured() =
+        OnchainZapSendResult.Failure(
+            OnchainZapSendStage.LOADING_UTXOS,
+            OnchainZapSendError.BACKEND_NOT_CONFIGURED,
+            ONCHAIN_BACKEND_NOT_CONFIGURED,
+        )
+
     /**
      * Send a NIP-BC onchain zap: build a Bitcoin transaction paying the recipient's
      * derived Taproot address, sign it, broadcast it, and publish the kind:8333
@@ -907,11 +914,7 @@ class Account(
     ): OnchainZapSendResult {
         val backend =
             cache.onchainBackend
-                ?: return OnchainZapSendResult.Failure(
-                    OnchainZapSendStage.LOADING_UTXOS,
-                    OnchainZapSendError.BACKEND_NOT_CONFIGURED,
-                    ONCHAIN_BACKEND_NOT_CONFIGURED,
-                )
+                ?: return onchainBackendNotConfigured()
         return OnchainZapSender.send(
             backend = backend,
             signer = signer,
@@ -936,11 +939,7 @@ class Account(
     ): OnchainZapSendResult {
         val backend =
             cache.onchainBackend
-                ?: return OnchainZapSendResult.Failure(
-                    OnchainZapSendStage.LOADING_UTXOS,
-                    OnchainZapSendError.BACKEND_NOT_CONFIGURED,
-                    ONCHAIN_BACKEND_NOT_CONFIGURED,
-                )
+                ?: return onchainBackendNotConfigured()
         return OnchainZapSender.sendToAddress(
             backend = backend,
             signer = signer,
@@ -964,11 +963,7 @@ class Account(
     ): OnchainZapSendResult {
         val backend =
             cache.onchainBackend
-                ?: return OnchainZapSendResult.Failure(
-                    OnchainZapSendStage.LOADING_UTXOS,
-                    OnchainZapSendError.BACKEND_NOT_CONFIGURED,
-                    ONCHAIN_BACKEND_NOT_CONFIGURED,
-                )
+                ?: return onchainBackendNotConfigured()
         return OnchainZapSender.sendSplit(
             backend = backend,
             signer = signer,

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/profile/payment/SendPaymentScreen.kt

@@ -749,8 +749,12 @@ private fun OnchainFeeSection(
                     label = {
                         Text(
                             if (rate != null) {
-                                "${stringRes(tier.labelRes)} · " +
-                                    stringRes(R.string.onchain_send_fee_rate_eta, "%.1f".format(rate), stringRes(tier.etaLabelRes))
+                                stringRes(
+                                    R.string.onchain_send_fee_tier_label_rate_eta,
+                                    stringRes(tier.labelRes),
+                                    "%.1f".format(rate),
+                                    stringRes(tier.etaLabelRes),
+                                )
                             } else {
                                 stringRes(tier.labelRes)
                             },

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/wallet/OnchainZapErrorStrings.kt

@@ -49,19 +49,11 @@ fun OnchainZapSendResult.Failure.userMessage(context: Context): String {
 }
 
 /**
- * Untranslated diagnostic detail worth showing under the localized headline:
- * build/sign/dust failures carry a crafted, specific reason (e.g. insufficient
- * funds, signer tampering, which share is below dust). Other stages only carry
- * low-level exception text, which we keep out of the UI.
+ * Untranslated diagnostic detail worth showing under the localized headline.
+ * Which errors carry a human-readable cause is a fact about the sender,
+ * declared on [OnchainZapSendError.causeIsUserFacing].
  */
-fun OnchainZapSendResult.Failure.technicalDetail(): String? =
-    when (error) {
-        OnchainZapSendError.BUILD_FAILED,
-        OnchainZapSendError.SIGN_FAILED,
-        OnchainZapSendError.RECIPIENT_BELOW_DUST,
-        -> cause?.message
-        else -> null
-    }
+fun OnchainZapSendResult.Failure.technicalDetail(): String? = if (error.causeIsUserFacing) cause?.message else null
 
 @StringRes
 private fun OnchainZapSendError.messageRes(): Int =

amethyst/src/main/res/values/strings.xml

@@ -2240,6 +2240,7 @@
     <string name="onchain_send_fee_eta_normal">~30 min</string>
     <string name="onchain_send_fee_eta_fast">~10 min</string>
     <string name="onchain_send_fee_rate_eta">%1$s sat/vB · %2$s</string>
+    <string name="onchain_send_fee_tier_label_rate_eta">%1$s · %2$s sat/vB · %3$s</string>
     <string name="onchain_send_loading_fees">Loading fee estimates…</string>
     <string name="onchain_send_button_amount">Send %1$s sats</string>
     <plurals name="onchain_send_button_amount_split">

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/onchain/OnchainZapSender.kt

@@ -61,27 +61,35 @@ enum class OnchainZapSendStage {
  * [OnchainZapSendResult.Failure.message] stays available for logs, tests, and
  * the CLI.
  */
-enum class OnchainZapSendError {
+enum class OnchainZapSendError(
+    /**
+     * Whether [OnchainZapSendResult.Failure.cause] carries a message crafted
+     * for humans (insufficient funds, signer tampering, which share is below
+     * dust) as opposed to low-level exception text. UIs may surface the cause
+     * message as a diagnostic detail when this is true.
+     */
+    val causeIsUserFacing: Boolean,
+) {
     /** No [OnchainBackend] is configured for the account. */
-    BACKEND_NOT_CONFIGURED,
+    BACKEND_NOT_CONFIGURED(causeIsUserFacing = false),
 
     /** The chain backend could not return the sender's UTXOs. */
-    LOAD_UTXOS_FAILED,
+    LOAD_UTXOS_FAILED(causeIsUserFacing = false),
 
     /** Coin selection / PSBT assembly failed (e.g. insufficient funds). */
-    BUILD_FAILED,
+    BUILD_FAILED(causeIsUserFacing = true),
 
     /** A recipient's split share is below the dust threshold. */
-    RECIPIENT_BELOW_DUST,
+    RECIPIENT_BELOW_DUST(causeIsUserFacing = true),
 
     /** Signing, signature verification, or finalization failed. */
-    SIGN_FAILED,
+    SIGN_FAILED(causeIsUserFacing = true),
 
     /** The signed transaction could not be broadcast. */
-    BROADCAST_FAILED,
+    BROADCAST_FAILED(causeIsUserFacing = false),
 
     /** The payment broadcast, but a kind:8333 receipt could not be published. */
-    RECEIPT_PUBLISH_FAILED,
+    RECEIPT_PUBLISH_FAILED(causeIsUserFacing = false),
 }
 
 /** Outcome of an [OnchainZapSender.send] attempt. */