Fail to send max / near max lightning payment

[piotr-iohk]

What happened?

When trying to send Lightning using Max available (or near max) in bitkit-android, payment fails with:

Failed to send the given payment

Observed amounts:

• Max available shown: 32760
• Fails for: 32760, 32759, 32758, 32757, 32756
• Succeeds for: 32755

I also verified with the probing tool:

• Probe succeeds for 32755
• Probe fails for amounts above 32755

So it seems to lack 5 sats... and that is when sending from bitkit-android to bitkit-ios

When trying to Phoenix it was the same - probe successful on 32755 sats.
On Blink it was different - probe successful on 32717 sats.

⚠️ Lightning Max available seems to be destination-dependent; different wallets can have different effective max send amounts. ⚠️

Expected behavior

Tapping Max available should produce a sendable amount, or the amount should be automatically adjusted so that send succeeds.

If full max cannot be sent due to routing fees/reserve/limits, UI should clearly cap or explain the effective max.

Steps to Reproduce

1. Open bitkit-android wallet with spending balance where Lightning Max available is 32760.
2. Start a Lightning send flow (invoice / LNURL-pay flow that uses Lightning send amount screen).
3. Tap Max available (or manually input 32756 to 32760).
4. Confirm send.
5. Observe error: Failed to send the given payment.
6. Retry with 32755 and observe success.

Logs / Screenshots / Recordings

bitkit_logs_1771415814981.zip

Screen_Recording_20260218_125534_Bitkit.mp4

Bitkit Version

2.0.2 (176)

Device / OS

Android 14

Reproducibility

Always

Additional context

• wallet has 2 channels,
• tried this sending to Bitkit, Phoenix, Blink - reproduced on all
• similar to Send: Lightning ‘Available/Max’ doesn't account for routing fees, causes Failed to send the given payment #533

[jvsena42]

Replace the custom fee calc method with the probing feature