[Bug]: CJIT at suggested minimum yields unspendable channel while app reports 354 sats received

[piotr-iohk]

What happened?

Fresh mainnet wallet (after wipe). Receive → spending balance setup (CJIT), amount = the app's suggested minimum 2000 sats. Paid the invoice from another Bitkit.

Blocktank quoted a total fee of 1835 sats (network 1056 + service 779) and opened a 0-conf channel of 671 850 sats with push_msat: 165000 — i.e. exactly 2000 − 1835 = 165 sats pushed to us. That is below the 354-sat channel reserve and below the 354-sat dust limit, so ldk-node reports outbound_capacity_msat = 0 and ClaimableOnChannelClose.amount_satoshis = 0. The 165 sats are neither spendable nor claimable on close.

The app then shows three different numbers for the same channel:

Screen	Shows	Reality
Receive sheet + activity item	Received +354	165 pushed
Lightning Connections → spending balance / Connection 1	↑354	0 spendable
Home → Spending	0	correct

The 354 comes from ChannelDetails.amountOnClose = outboundCapacity + unspendablePunishmentReserve (0 + 354), which assumes we always hold at least the reserve. It coincidentally equals the reserve, not anything the user received.

Root causes (both in app code, both shared with iOS):

1. Minimum amount ignores the channel reserve. BlocktankRepo.refreshMinCjitSats: minCjitSats = ceil(feeSat × 1.1 / 1000) × 1000. This only guarantees ~10 % of the fee is left after fees (~180 sats at today's ~1.8k fee), which is below the 354-sat reserve/dust. Whether the user ends up with a usable channel depends on where Blocktank's fee lands at that moment — the same 2000-sat CJIT on iOS an hour earlier happened to leave >354 and was spendable. The estimate also uses getDefaultLspBalance(clientBalance = 0) while the real order uses amount + lspBalance, so the actual fee (1835) can exceed the fee the minimum was derived from (≤ 1818).
2. amountOnClose overstates when balance < reserve (ext/ChannelDetails.kt, used by ActivityRepo.insertActivityFromCjit, NotifyChannelReadyHandler, WakeNodeWorker, Lightning Connections). There is already a // TODO: use channelDetails.claimableOnCloseSats.

Expected behavior

• The suggested minimum CJIT amount guarantees a usable result: amount − feeSat ≥ channel reserve (354) + a sane margin (e.g. ≥ 1000 sats net), or the flow refuses/warns when amount − fee would be unspendable.
• The received sheet, activity item and Lightning Connections show the real amount: invoiceSat − feeSat (165) for the CJIT receive, and claimableOnCloseSats / outbound for balances — consistent with Home (0 spendable).

Steps to Reproduce

1. New mainnet wallet, no channels.
2. Receive → enter an amount → choose the spending-balance (CJIT) option at the suggested minimum amount (2000 sats today).
3. Note the confirmation screen shows amount − fee (165 sats here; Blocktank total fee 1835).
4. Pay the invoice from another wallet.
5. Receive sheet / activity show +354; Lightning Connections shows ↑354; Home shows Spending 0; nothing can be sent.

Deterministic whenever Blocktank's fee is within 354 sats of the suggested minimum (minCjitSats − feeSat ≤ 354).

Logs / Screenshots / Recordings

Logcat excerpt (UTC), channel 18ca78cf…, LSP 02a37103… (Blocktank-LND4), CJIT 41b441a1-998b-45ee-a378-70b4ed8e5ba7:

07:24:29 DEBUG [BlocktankRepo.kt:251] Updated minCjitSats to: 2000
07:24:49 TRACE Received message OpenChannel(… funding_satoshis: 671850, dust_limit_satoshis: 354, … push_msat: 165000, channel_reserve_satoshis: 354 })
07:24:49 TRACE Handling event OpenChannelRequest { … funding_satoshis: 671850, channel_negotiation_type: PushMsat(165000), … }
07:24:50 INFO  Accepting inbound 0conf Anchor channel of 671850sats from trusted peer 02a37103…
07:24:50 TRACE Building commitment transaction number 281474976710655 … for us … including to_remote output with value 668436   ← no to_local output
07:24:50 INFO  Channel 18ca78cf… ready to be used with funding_txo aecb6598…:0
07:24:51 VERBOSE Balances in ldk-node: {"totalLightningBalanceSats":0,"lightningBalances":[{"type":"…ClaimableOnChannelClose","channelId":"18ca78cf…","amountSatoshis":0,…}]}
07:24:56 INFO  Synced 1 payments successfully   (CJIT activity inserted with amountOnClose = 0 + 354)
07:27:18 INFO  channel support summary: channelId='18ca78cf…', ready='true', usable='false', outboundMsat='0', inboundMsat='664307000'

Full excerpt (122 lines) staged as

2026-09-15-cjit-minimum-unspendable-logcat-excerpt.txt

Bitkit Version

2.5.0 (mainnet, to.bitkit)

Device / OS

Samsung Galaxy S22 (SM-S901B), Android 16 (One UI 8)

Reproducibility

Sometimes (<50%)

Additional context

• Mainnet, new wallet right after a wipe, Lightning / CJIT.
• No HTLC is involved — Blocktank delivers CJIT via push_msat, so the LSP behaved as quoted; both problems are on the app side.
• Also affects iOS (same logic): BlocktankViewModel.swift:455 (ceil(Double(fees.feeSat) * 1.1 / 1000) * 1000) and ViewModels/Extensions/ChannelDetails.swift amountOnClose (outboundCapacityMsat / 1000 + unspendablePunishmentReserve), used by TransferViewModel for the CJIT receive. The same 2000-sat CJIT on iOS was spendable only because the fee at that time left more than 354 sats.
• Related: [Bug]: CJIT received sheet & activity item have wrong amount #626 (closed — CJIT sheet/activity wrong amount; amountOnClose was the fix and still overstates in this edge case), LN Balance not correct #615 (open — channel balance ≠ home balance).
• Not a 2.5.0 regression; both formulas pre-date it. Money-losing at the app's own suggested minimum on mainnet, though.

[piotr-iohk]

Follow-up: the pushed sats are not lost, they are invisible below the dust limit and reappear all at once.

Sent three more payments into the same channel from another wallet: 1, 100, 90 sats.

After	Our side (sats)	Home → Spending	ldk-node
CJIT push	165	0	outboundMsat=0, claimable 0
+1	166	0	outboundMsat=0, claimable 0
+100	266	0	outboundMsat=0, claimable 0
+90	356	356	outboundMsat=2000, claimable 356

Below 354 sats the commitment transaction has no to_local output, so LDK reports 0 claimable. At 356 the output appears and the whole balance shows up (maxSendLightningSats = 2, i.e. 356 − 354 reserve).

Logcat (UTC):

08:13:24 Received message UpdateAddHTLC(… amount_msat: 1000 …)
08:13:40 LDK event fired: PaymentReceived amountMsat=100000
08:14:09 channel support summary: outboundMsat='0', inboundMsat='664206000'
08:15:56 Received message UpdateAddHTLC(… amount_msat: 90000 …)
08:15:56 …including to_remote output with value 668245
08:15:56 …including to_local output with value 356          ← first to_local output for this channel
08:15:57 Balances in state: "totalLightningSats":356,"maxSendLightningSats":2
08:16:44 channel support summary: outboundMsat='2000', inboundMsat='664116000'

Activity list now reads Received +354 / +1 / +100 / +90 (= 545) against a 356 balance; the first entry should read 165 (invoiceSat − feeSat).