[Bug]: Self-payment on Lightning shows misleading "Insufficient Spending" error

[jvsena42]

Summary

When a user pastes/scans a Lightning invoice (or BIP21 unified URI containing a lightning= param) that was created by their own node, the Send flow shows a misleading "Insufficient Spending Balance — ₿ X more needed to pay this Lightning invoice" toast instead of explaining that self-payments are not possible.

LDK rejects the payment internally with Cannot generate a route to ourselves, but that error is only logged — the UI never surfaces it.

Evidence (from a user support log)

Session log bitkit_2026-04-16_15-38-43.log:

• :3203 — user's own node creates invoice lnbc1p57zpxp… (hash 986b2247…)
• :7710 — user pastes bitcoin:bc1qv2fjjp…?lightning=lnbc1p57zpxp… (the same invoice)
• :7715 — ERROR ldk_node::payment::bolt11: Failed to find route for fee estimation: "Cannot generate a route to ourselves"
• :7717 — estimateRoutingFeesForAmount error [LdkError='LDK Node error: Failed to find a route for fee estimation.']
• :18506 / :18521 — same pattern on a second attempt

The user then typed an amount above maxSendLightningSats (6131 sats vs. total 6485) and saw other__pay_insufficient_spending_amount_description, which is technically correct for that amount but hides the real blocker: even sending exactly maxSend would fail because the invoice's destination is their own node.

Expected behavior

As soon as the scanned/pasted Lightning invoice is decoded, compare the invoice's payee pubkey against the local node id. If they match, block the flow with a dedicated error (e.g. "You can't pay an invoice created by your own wallet").

Proposed implementation

1. Early validation on scan — in AppViewModel.validateLightningInvoice (app/src/main/java/to/bitkit/viewmodels/AppViewModel.kt:904):
   • Compare invoice.payeePubkey with lightningRepo.getNodeId().
   • If equal, call showAddressValidationError with a new string resource (e.g. other__pay_self_invoice_title / other__pay_self_invoice_description) and return early — before the insufficient-spending check.
2. Same guard for unified BIP21 — in validateOnChainAddress where extractViableLightningInvoice pulls the embedded BOLT11 (AppViewModel.kt:951), apply the same self-payment check before switching payMethod to SendMethod.LIGHTNING.
3. Same guard on the amount-entered path — onScanLightning (AppViewModel.kt:1431) and the unified-invoice branch in onScan* (AppViewModel.kt:1396 area) for cases where the decoded invoice carries an amount or the user enters one later.
4. Surface the LDK failure too — in estimateMaxAmountRoutingFee (AppViewModel.kt:2141) and estimateLightningRoutingFeesIfNeeded (AppViewModel.kt:2120), if the underlying error is Cannot generate a route to ourselves, fire the same self-payment toast instead of silently zeroing the estimate.
5. Add a new string resource in app/src/main/res/values/strings.xml (alphabetical order) — English copy only; keep localization keys aligned with existing other__pay_* conventions.

Files to touch

• app/src/main/java/to/bitkit/viewmodels/AppViewModel.kt (:904, :931, :1431, :2120, :2141)
• app/src/main/java/to/bitkit/repositories/LightningRepo.kt — confirm getNodeId() is callable from this path (:1221)
• app/src/main/res/values/strings.xml

Verification

1. In a dev build, receive any amount via Lightning so a channel exists.
2. Open Receive, copy the BIP21 unified QR (bitcoin:…?lightning=…).
3. Paste it into the Send flow.
4. Expected: the dedicated "self-payment" toast appears immediately on paste/scan — no "Insufficient Spending" toast, no amount entry screen.
5. Repeat with just the raw BOLT11 (no bitcoin: prefix) — same toast.
6. Verify a genuine external invoice still works end-to-end.

Related

Diagnosed from support ticket — user reported "everytime I try it says there isn't enough btc to fill this invoice but I have enough" while repeatedly pasting their own wallet's Receive URI into Send.