Send: Lightning ‘Available/Max’ doesn't account for routing fees, causes `Failed to send the given payment`

[piotr-iohk]

Android’s “Available” on the Lightning amount screen (SendAmountScreen.kt) shows maxSendLightningSats without reserving anything for routing fees, but the confirm screen does estimate routing fees (estimateLightningRoutingFeesIfNeeded()). That mismatch can make “Max”/available misleading (especially for amountless invoices), and it’s exactly why iOS can show 8 997 while Android shows 9 000, in case of total spending balance = 10 000.

Expected: available/max invoice amount ≈ maxSendLightningSats - estimatedRoutingFee (optionally with a small buffer) iOS uses additional 2 sats buffer -> https://github.com/synonymdev/bitkit-ios/blob/master/Bitkit/Views/Wallets/Send/SendAmountView.swift#L264
Actual: shows raw maxSendLightningSats; applying max available funds ends with error
Impact: user can pick an amount that fails or is confusing; cross-platform inconsistency

See iOS left/Android right (both having 10 000 spending balance and trying to send max available to each other; Android fails):
⚠️ additionally we see that sending over 50% of lightning funds triggers a 50% balance warning sheet on iOS, which is not triggered on Android. iOS is consistent with Bitkit RN in that, so Android should also display this 50% balance warning.

Screen.Recording.2025-12-18.at.12.22.41.mov

[jvsena42]

About the 50% warning, the android uses the total balance (onchain + lighting) while the iOS is using only unchain/lightning balance. I'll check on RN repo what is the behavior

[jvsena42]

About the 50% warning, the android uses the total balance (onchain + lighting) while the iOS is using only unchain/lightning balance. I'll check on RN repo what is the behavior

Indeed, the RN uses the individual balance

		// amount > 50% of total balance
		if (usesLightning) {
			// If lightning tx use lightning balance.
			if (amount > lightningBalance.localBalance / 2) {
				warnings.push('dialog2');
			}
		} else {
			// If on-chain tx use on-chain balance.
			if (amount > onChainBalance / 2) {
				warnings.push('dialog2');
			}
		}