feat(spv): verify Merkle proofs progressively, gate spendability on it

Balance/history now render as soon as tx downloads finish instead of blocking on every historical Merkle proof, critical for mobile where proof-checking can take much longer than the download itself. Proofs continue to be checked in the background and each tx's Verified flag catches up progressively; header ranges are now fetched in batches (blockchain.block.headers) instead of one call per header to keep this fast over high-latency links. Coin selection (UtxoSpendability.IsSpendable) refuses to spend a UTXO until its Merkle proof is actually checked, regardless of confirmation count, so a server that fabricates a confirmed balance can get it displayed early but never spent before the forgery is caught. The disk cache only ever persists the fully-verified end state of a sync. UI surfaces the new PendingVerificationSats/SpendableSats split with a "verifying..." badge, and the sync save now runs off the UI thread to avoid freezing on slower hardware.
2026-07-17 19:41:31 +02:00
parent feb765663c
commit 3460e53b4f
16 changed files with 487 additions and 82 deletions
@@ -41,6 +41,20 @@ It cannot (given correct Merkle verification):
 - Fabricate a confirmed transaction with a valid Merkle proof
 - Forge a payment to a wrong address

+**Progressive verification (mobile-friendly sync).** On a wallet with many historical
+transactions, `WalletSynchronizer` no longer blocks the whole sync on every Merkle proof:
+balance and history are shown as soon as transaction downloads finish (`PartialResult`,
+`Core/Spv/WalletSynchronizer.cs`), while proofs continue to be checked in the background and
+each transaction's `Verified` flag catches up progressively. This means the UI can display a
+server-reported balance/history that includes not-yet-verified entries — clearly marked with a
+"verifying…" badge and a separate non-spendable total (`PendingVerificationSats`). The
+security-critical invariant this depends on: coin selection (`TransactionFactory`, gated by
+`Wallet/UtxoSpendability.IsSpendable`) refuses to spend a UTXO whose `Verified` flag isn't true,
+regardless of confirmations — so a server that fabricates a fake confirmed balance can get it
+*displayed* early, but never *spent*, before the forged Merkle proof is caught and the sync
+fails outright. The disk cache (`SyncCache`) only ever persists the fully-verified end state of
+a sync, never a partial one, so no unverified data survives a restart.
+
 ---

 ## Key and seed management
@@ -395,6 +395,8 @@ public sealed class Loc
        ["msg.pending"]           = ["in attesa di conferma",                                             "pending confirmation",         "pendiente de confirmación",            "en attente de confirmation",               "aguardando confirmação",                   "ausstehende Bestätigung"],
        ["msg.notspendable"]      = ["non ancora spendibile",                                             "not yet spendable",            "aún no gastable",                      "pas encore dépensable",                    "ainda não gastável",                       "noch nicht verwendbar"],
        ["msg.immature"]          = ["in maturazione",                                                    "maturing",                      "en maduración",                        "en maturation",                            "em maturação",                             "in Reifung"],
+        ["msg.verifying"]         = ["in verifica SPV",                                                   "SPV-verifying",                 "en verificación SPV",                  "en cours de vérification SPV",             "em verificação SPV",                       "SPV-Prüfung läuft"],
+        ["history.unverified"]    = ["in verifica…",                                                      "verifying…",                    "verificando…",                         "vérification…",                            "verificando…",                             "wird geprüft…"],
        ["msg.settings.saved"]    = ["Impostazioni salvate.",                                             "Settings saved.",              "Configuración guardada.",              "Paramètres enregistrés.",                  "Configurações salvas.",                    "Einstellungen gespeichert."],
        ["msg.certreset"]         = [
            "Certificati SSL azzerati: riprova la connessione.",
@@ -28,6 +28,9 @@ public partial class MainWindowViewModel
    [ObservableProperty]
    private string immatureText = "";

+    [ObservableProperty]
+    private string verifyingText = "";
+
    [ObservableProperty]
    private string networkInfo = "";

@@ -253,6 +256,7 @@ public partial class MainWindowViewModel
            BalanceText = $"0.00000000 {Profile.CoinUnit}";
            UnconfirmedText = "";
            ImmatureText = "";
+            VerifyingText = "";
            ReceiveAddress = _account.GetReceiveAddress(0).ToString();
            History.Clear();
            Addresses.Clear();
@@ -265,7 +269,11 @@ public partial class MainWindowViewModel
                    $"m/{_doc!.AccountPath}/0/{i}"));
            return;
        }
-        BalanceText = Fmt(cache.ConfirmedSats - cache.ImmatureSats);
+        // Not "Confirmed - Immature - PendingVerification": those two can overlap (an
+        // immature coinbase can also be unverified), which double-subtracts and can go
+        // negative. SpendableSats is computed directly from the same IsSpendable gate
+        // coin selection uses, so it's always correct.
+        BalanceText = Fmt(cache.SpendableSats);
        var pending = cache.History.Where(t => t.Height <= 0).Sum(t => t.DeltaSats);
        UnconfirmedText = pending != 0
            ? $"{Loc.Tr("msg.pending")}: {(pending > 0 ? "+" : "")}{Fmt(pending)} — {Loc.Tr("msg.notspendable")}"
@@ -273,13 +281,20 @@ public partial class MainWindowViewModel
        ImmatureText = cache.ImmatureSats != 0
            ? $"{Loc.Tr("msg.immature")}: {Fmt(cache.ImmatureSats)} — {Loc.Tr("msg.notspendable")}"
            : "";
+        // Progressive verification (§7.4): funds confirmed by the server but whose Merkle
+        // proof background verification hasn't reached yet — same "not spendable" treatment
+        // as immature/pending, distinct wording so it doesn't read as a maturity/confirmation problem.
+        VerifyingText = cache.PendingVerificationSats != 0
+            ? $"{Loc.Tr("msg.verifying")}: {Fmt(cache.PendingVerificationSats)} — {Loc.Tr("msg.notspendable")}"
+            : "";
        ReceiveAddress = _account.GetReceiveAddress(cache.NextReceiveIndex).ToString();
        History.Clear();
        foreach (var tx in cache.History)
            History.Add(new HistoryRow(
                tx.Height > 0 ? tx.Height.ToString() : "mempool",
                (tx.DeltaSats >= 0 ? "+" : "") + Fmt(tx.DeltaSats, withLabel: false),
-                tx.Txid));
+                tx.Txid,
+                tx.Verified));

        Addresses.Clear();
        foreach (var a in cache.Addresses)
@@ -273,32 +273,34 @@ public partial class MainWindowViewModel
                    _doc.Cache?.NextChangeIndex ?? 0,
                    net);
                _synchronizer.Progress += msg => Dispatcher.UIThread.Post(() => StatusMessage = msg);
+                // Progressive verification (§7.4): the wallet becomes usable as soon as
+                // transaction downloads finish, without waiting for every historical Merkle
+                // proof — critical on mobile, where verifying thousands of proofs can take far
+                // longer than the download itself. Never persisted to disk on its own (only the
+                // final, fully-verified snapshot below is saved); coin selection still refuses
+                // any UTXO whose proof isn't checked yet (CachedUtxo.Verified), so showing this
+                // early can't be exploited to spend a server-fabricated balance.
+                _synchronizer.PartialResult += r => Dispatcher.UIThread.Post(() => ApplyPartialResult(r));
            }

            do
            {
                _resyncRequested = false;
-                var result = await _synchronizer.SyncOnceAsync(ct);
+                // Off the UI thread: sync does heavy CPU work (LINQ over thousands of
+                // cached txs/UTXOs) and blocking JSON/disk I/O between awaits, negligible
+                // on desktop but enough to freeze the UI (ANR) on slower mobile hardware.
+                var (result, rawHex, verifiedAt, blockHeaders) = await Task.Run(async () =>
+                {
+                    var r = await _synchronizer.SyncOnceAsync(ct);
+                    var caches = _synchronizer.ExportCaches(PalladiumNetworks.For(_account.Profile.Kind));
+                    return (r, caches.RawTxHex, caches.VerifiedAt, caches.BlockHeaders);
+                }, ct);
                _lastTransactions = result.Transactions;

-                var (rawHex, verifiedAt, blockHeaders) = _synchronizer.ExportCaches(
-                    PalladiumNetworks.For(_account.Profile.Kind));
-                _doc.Cache = new SyncCache
-                {
-                    TipHeight = result.TipHeight,
-                    ConfirmedSats = result.ConfirmedSats,
-                    UnconfirmedSats = result.UnconfirmedSats,
-                    ImmatureSats = result.ImmatureSats,
-                    NextReceiveIndex = result.NextReceiveIndex,
-                    NextChangeIndex = result.NextChangeIndex,
-                    History = [.. result.History],
-                    Utxos = [.. result.Utxos],
-                    Addresses = [.. result.AddressRows],
-                    RawTxHex = rawHex,
-                    VerifiedAt = verifiedAt,
-                    BlockHeaders = blockHeaders,
-                };
-                WalletStore.Save(_doc, _walletPath!, _password);
+                var cache = new SyncCache { RawTxHex = rawHex, VerifiedAt = verifiedAt, BlockHeaders = blockHeaders };
+                FillDisplayFields(cache, result);
+                _doc.Cache = cache;
+                await WalletStore.SaveAsync(_doc, _walletPath!, _password);
                ApplyCache(_doc.Cache);
                _syncFailed = false;
                StatusMessage = $"{Loc.Tr("msg.synced")}: {Loc.Tr("msg.height")} {result.TipHeight}, " +
@@ -345,6 +347,44 @@ public partial class MainWindowViewModel
            _ = ConnectAndSync();
    }

+    /// <summary>
+    /// Applies a provisional (not-yet-fully-verified) snapshot fired mid-sync: updates the
+    /// in-memory display cache only — never persisted to disk on its own, so an interrupted
+    /// sync can't leave behind a cache file whose VerifiedAt/RawTxHex/BlockHeaders (needed to
+    /// resume without re-downloading) were never written.
+    /// </summary>
+    private void ApplyPartialResult(SyncResult result)
+    {
+        if (_doc is null)
+            return;
+        var previous = _doc.Cache;
+        var cache = new SyncCache
+        {
+            RawTxHex = previous?.RawTxHex,
+            VerifiedAt = previous?.VerifiedAt,
+            BlockHeaders = previous?.BlockHeaders,
+        };
+        FillDisplayFields(cache, result);
+        _doc.Cache = cache;
+        _lastTransactions = result.Transactions;
+        ApplyCache(cache);
+    }
+
+    private static void FillDisplayFields(SyncCache cache, SyncResult result)
+    {
+        cache.TipHeight = result.TipHeight;
+        cache.ConfirmedSats = result.ConfirmedSats;
+        cache.UnconfirmedSats = result.UnconfirmedSats;
+        cache.ImmatureSats = result.ImmatureSats;
+        cache.PendingVerificationSats = result.PendingVerificationSats;
+        cache.SpendableSats = result.SpendableSats;
+        cache.NextReceiveIndex = result.NextReceiveIndex;
+        cache.NextChangeIndex = result.NextChangeIndex;
+        cache.History = [.. result.History];
+        cache.Utxos = [.. result.Utxos];
+        cache.Addresses = [.. result.AddressRows];
+    }
+
    /// <summary>
    /// Peer discovery. Always clickable: if the wallet is already connected, it reuses
    /// that connection; otherwise it opens a short-lived connection to a candidate server
@@ -17,7 +17,7 @@ using PalladiumWallet.Core.Wallet;
 namespace PalladiumWallet.App.ViewModels;

 /// <summary>Transaction history row for the view.</summary>
-public sealed record HistoryRow(string Conferma, string Importo, string Txid);
+public sealed record HistoryRow(string Conferma, string Importo, string Txid, bool Verified = true);

 /// <summary>Address view row with pre-computed keys and derivation path.</summary>
 public sealed record AddressRow(
@@ -102,7 +102,8 @@ public sealed class TransactionDetailsViewModel
    {
        if (d.Confirmations <= 0)
            return loc["tx.status.mempool"];
-        return $"{d.Confirmations} {loc["tx.status.confirmations"]} ({loc["tx.status.block"]} {d.Height})";
+        var status = $"{d.Confirmations} {loc["tx.status.confirmations"]} ({loc["tx.status.block"]} {d.Height})";
+        return d.Verified ? status : $"{status} — {loc["history.unverified"]}";
    }

    private string Signed(long sats)
@@ -317,6 +317,9 @@
                    <TextBlock Text="{Binding ImmatureText}" Foreground="#FCD34D"
                               TextWrapping="Wrap"
                               IsVisible="{Binding ImmatureText, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+                    <TextBlock Text="{Binding VerifyingText}" Foreground="#FCD34D"
+                               TextWrapping="Wrap"
+                               IsVisible="{Binding VerifyingText, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
                    <TextBlock Text="{Binding NetworkInfo}" Classes="on-hero" FontSize="12"
                               Margin="0,2,0,0"/>
                </StackPanel>
@@ -373,13 +376,16 @@
                                <DataTemplate x:DataType="vm:HistoryRow">
                                    <Panel Cursor="Hand">
                                        <!-- Desktop: 3 fixed columns -->
-                                        <Grid ColumnDefinitions="90,160,*"
+                                        <Grid ColumnDefinitions="90,160,*,Auto"
                                              IsVisible="{Binding $parent[UserControl].((vm:MainWindowViewModel)DataContext).IsDesktop}">
                                            <TextBlock Grid.Column="0" Text="{Binding Conferma}" Foreground="{DynamicResource TextSecondaryBrush}"/>
                                            <TextBlock Grid.Column="1" Text="{Binding Importo}" FontFamily="monospace"/>
                                            <TextBlock Grid.Column="2" Text="{Binding Txid}"
                                                       FontFamily="monospace" FontSize="12"
                                                       TextTrimming="CharacterEllipsis"/>
+                                            <TextBlock Grid.Column="3" Text="{Binding $parent[UserControl].((vm:MainWindowViewModel)DataContext).Loc[history.unverified]}"
+                                                       Foreground="#FCD34D" FontSize="11" Margin="6,0,0,0"
+                                                       IsVisible="{Binding !Verified}"/>
                                        </Grid>
                                        <!-- Mobile: vertical card -->
                                        <StackPanel Spacing="2"
@@ -389,6 +395,9 @@
                                            <TextBlock Text="{Binding Conferma}" Foreground="{DynamicResource TextSecondaryBrush}" FontSize="11"/>
                                            <TextBlock Text="{Binding Txid}" FontFamily="monospace" FontSize="11"
                                                       TextTrimming="CharacterEllipsis"/>
+                                            <TextBlock Text="{Binding $parent[UserControl].((vm:MainWindowViewModel)DataContext).Loc[history.unverified]}"
+                                                       Foreground="#FCD34D" FontSize="11"
+                                                       IsVisible="{Binding !Verified}"/>
                                        </StackPanel>
                                    </Panel>
                                </DataTemplate>
@@ -104,8 +104,9 @@ static int Info(string[] o)
    Console.WriteLine($"xpub:     {doc.AccountXpub}");
    if (doc.Cache is { } cache)
    {
-        Console.WriteLine($"balance:  {CoinAmount.Format(cache.ConfirmedSats - cache.ImmatureSats, account.Profile.CoinUnit)} spendable"
+        Console.WriteLine($"balance:  {CoinAmount.Format(cache.SpendableSats, account.Profile.CoinUnit)} spendable"
            + (cache.ImmatureSats != 0 ? $" + {CoinAmount.Format(cache.ImmatureSats)} maturing (not spendable)" : "")
+            + (cache.PendingVerificationSats != 0 ? $" + {CoinAmount.Format(cache.PendingVerificationSats)} awaiting SPV verification (not spendable)" : "")
            + (cache.UnconfirmedSats != 0 ? $" + {CoinAmount.Format(cache.UnconfirmedSats)} pending confirmation (not spendable)." : ""));
        Console.WriteLine($"sync:     height {cache.TipHeight}, {cache.History.Count} transactions");
        Console.WriteLine($"receive:  {account.GetReceiveAddress(cache.NextReceiveIndex)}");
@@ -149,6 +150,8 @@ static async Task<int> Sync(string[] o)
        ConfirmedSats = result.ConfirmedSats,
        UnconfirmedSats = result.UnconfirmedSats,
        ImmatureSats = result.ImmatureSats,
+        PendingVerificationSats = result.PendingVerificationSats,
+        SpendableSats = result.SpendableSats,
        NextReceiveIndex = result.NextReceiveIndex,
        NextChangeIndex = result.NextChangeIndex,
        History = [.. result.History],
@@ -160,8 +163,9 @@ static async Task<int> Sync(string[] o)
    };
    WalletStore.Save(doc, path, Opt(o, "--password"));

-    Console.WriteLine($"Balance: {CoinAmount.Format(result.ConfirmedSats - result.ImmatureSats, account.Profile.CoinUnit)} spendable"
+    Console.WriteLine($"Balance: {CoinAmount.Format(result.SpendableSats, account.Profile.CoinUnit)} spendable"
        + (result.ImmatureSats != 0 ? $" + {CoinAmount.Format(result.ImmatureSats)} maturing (not spendable)" : "")
+        + (result.PendingVerificationSats != 0 ? $" + {CoinAmount.Format(result.PendingVerificationSats)} awaiting SPV verification (not spendable)" : "")
        + (result.UnconfirmedSats != 0 ? $" + {CoinAmount.Format(result.UnconfirmedSats)} pending confirmation (not spendable)" : ""));
    Console.WriteLine($"History ({result.History.Count}):");
    foreach (var tx in result.History)
@@ -11,6 +11,9 @@ public readonly record struct UnspentItem(string TxHash, int TxPos, long ValueSa
 /// <summary>Merkle proof (blockchain.transaction.get_merkle).</summary>
 public sealed record MerkleProofResponse(int BlockHeight, int Pos, IReadOnlyList<string> Merkle);

+/// <summary>Range of headers (blockchain.block.headers): Hex is Count concatenated 80-byte headers.</summary>
+public readonly record struct HeaderRangeResponse(int Count, string Hex);
+
 /// <summary>Chain tip notified by blockchain.headers.subscribe.</summary>
 public readonly record struct ChainTip(int Height, string HeaderHex);

@@ -82,6 +85,23 @@ public static class ElectrumApi
        return r.GetString()!;
    }

+    /// <summary>
+    /// Range fetch (blockchain.block.headers): one RPC returns up to <paramref name="count"/>
+    /// concatenated 80-byte headers starting at <paramref name="startHeight"/>. Used to anchor
+    /// a tx height to a hardcoded checkpoint (§7.3) without one round-trip per header — critical
+    /// on high-latency links (mobile) where thousands of serial single-header calls stall sync.
+    /// The server may return fewer than requested (see <see cref="HeaderRangeResponse.Count"/>);
+    /// callers must loop until the full range is covered.
+    /// </summary>
+    public static async Task<HeaderRangeResponse> GetBlockHeadersAsync(this ElectrumClient c,
+        int startHeight, int count, CancellationToken ct = default)
+    {
+        var r = await c.RequestAsync("blockchain.block.headers", ct, startHeight, count);
+        return new HeaderRangeResponse(
+            r.GetProperty("count").GetInt32(),
+            r.GetProperty("hex").GetString()!);
+    }
+
    public static async Task<string> BroadcastAsync(this ElectrumClient c, string rawTxHex,
        CancellationToken ct = default)
    {
@@ -29,6 +29,23 @@ public sealed class SyncResult
    /// <see cref="ConfirmedSats"/>.
    /// </summary>
    public required long ImmatureSats { get; init; }
+
+    /// <summary>
+    /// Confirmed and past its threshold, but not yet spendable because its Merkle proof
+    /// hasn't been checked yet (§7.4 progressive verification catching up in the background).
+    /// Subset of <see cref="ConfirmedSats"/> — NOT disjoint from <see cref="ImmatureSats"/>
+    /// (an immature coinbase can also be unverified), so never subtract both from
+    /// <see cref="ConfirmedSats"/> to get a spendable total — use <see cref="SpendableSats"/>.
+    /// </summary>
+    public required long PendingVerificationSats { get; init; }
+
+    /// <summary>
+    /// Sum of UTXOs that actually pass <see cref="Wallet.UtxoSpendability.IsSpendable"/> right
+    /// now — the true spendable balance. Computed directly from the same gate coin selection
+    /// uses, rather than by subtracting <see cref="ImmatureSats"/>/<see cref="PendingVerificationSats"/>
+    /// from <see cref="ConfirmedSats"/>, since those two can overlap.
+    /// </summary>
+    public required long SpendableSats { get; init; }
    public required int NextReceiveIndex { get; init; }
    public required int NextChangeIndex { get; init; }
    public required IReadOnlyList<CachedTx> History { get; init; }
@@ -46,14 +63,37 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
    /// <summary>Human-readable progress (for CLI and GUI status bar).</summary>
    public event Action<string>? Progress;

+    /// <summary>
+    /// Fires with a fresh, self-consistent snapshot as soon as transaction downloads finish
+    /// (Merkle proofs may still be pending — see <see cref="CachedTx.Verified"/>/
+    /// <see cref="CachedUtxo.Verified"/>) and again periodically as background verification
+    /// progresses (§7.4). The wallet is usable after the first firing instead of waiting for
+    /// every historical proof to be checked; <see cref="SyncOnceAsync"/>'s returned Task still
+    /// only completes once verification is fully done, for callers that need the final state.
+    /// </summary>
+    public event Action<SyncResult>? PartialResult;
+
    private readonly ConcurrentDictionary<string, Transaction> _txCache = new();
-    private readonly Dictionary<string, int> _verifiedAtHeight = [];
+
+    // Concurrent: written incrementally by individual merkle-verification tasks as they
+    // complete (§7.4 progressive verification), not just once after they all finish.
+    private readonly ConcurrentDictionary<string, int> _verifiedAtHeight = new();
    private readonly ConcurrentDictionary<int, Task<string>> _headerFetches = new();

    // checkpoint height -> highest height already proven to hash-chain back to it
    // (in-memory only: cheap to recompute from _headerFetches, no need to persist).
    private readonly ConcurrentDictionary<int, int> _anchoredUpTo = new();

+    // Serializes header-range downloads: concurrent AnchorToCheckpointAsync calls (one per tx
+    // being verified) would otherwise race on overlapping ranges and issue duplicate range
+    // requests. Fetches are network-bound and few (batches of up to 2016 headers), so
+    // serializing them costs nothing that matters.
+    private readonly SemaphoreSlim _headerRangeLock = new(1, 1);
+
+    // Max headers requested per blockchain.block.headers call. The server may return fewer
+    // (its own configured cap) — FetchHeaderRangeAsync loops on the actual count returned.
+    private const int HeaderBatchSize = 2016;
+
    // Indices known from the previous sync: used by ScanChainAsync for incremental
    // discovery — already-used addresses are fetched in a single burst instead of
    // sequential batches, reducing round-trips from O(used/gapLimit) to O(1).
@@ -162,7 +202,15 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
        foreach (var item in historyByAddress.Values.SelectMany(h => h))
            txHeights[item.TxHash] = item.Height;

-        // 4+5. Download missing transactions and verify Merkle proofs in parallel.
+        // 4. Download missing transactions — needed to compute amounts/UTXOs locally.
+        // Merkle-proof verification (5) is deliberately NOT awaited together with this: on a
+        // wallet with thousands of transactions, downloads finish in seconds while proofs can
+        // take much longer over a high-latency link, and the wallet has everything it needs to
+        // show balance/history the moment downloads are done. Verification then continues in
+        // the background (§7.4 progressive verification), firing PartialResult as proofs land,
+        // while coin selection stays locked out of any UTXO until its own proof is checked
+        // (CachedUtxo.Verified, enforced in UtxoSpendability.IsSpendable) — a malicious server
+        // cannot get a fabricated balance spent just because it was shown early.
        var network  = PalladiumNetworks.For(account.Profile.Kind);
        var missing  = txHeights.Keys.Where(txid => !_txCache.ContainsKey(txid)).ToList();
        var toVerify = txHeights
@@ -171,46 +219,81 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
            .ToList();

        if (missing.Count > 0 || toVerify.Count > 0)
-        {
            Progress?.Invoke($"downloading {missing.Count} txs, verifying {toVerify.Count} proofs…");
-            var dlDone   = 0;
-            var merkDone = 0;

-            var dlTasks = missing.Select(txid => RetryOnBusyAsync(async () =>
-            {
-                var raw = await client.GetTransactionAsync(txid, ct);
-                _txCache[txid] = Transaction.Parse(raw, network);
-                var n = Interlocked.Increment(ref dlDone);
-                if (n % 50 == 0 || n == missing.Count)
-                    Progress?.Invoke($"tx {n}/{missing.Count}, proofs {merkDone}/{toVerify.Count}…");
-            }, ct));
+        var dlDone = 0;
+        await Task.WhenAll(missing.Select(txid => RetryOnBusyAsync(async () =>
+        {
+            var raw = await client.GetTransactionAsync(txid, ct);
+            _txCache[txid] = Transaction.Parse(raw, network);
+            var n = Interlocked.Increment(ref dlDone);
+            if (n % 50 == 0 || n == missing.Count)
+                Progress?.Invoke($"tx {n}/{missing.Count}, proofs 0/{toVerify.Count}…");
+        }, ct)));

-            var merkTasks = toVerify.Select(kv => RetryOnBusyAsync(async () =>
-            {
-                var (txid, height) = kv;
-                var proofTask  = client.GetMerkleAsync(txid, height, ct);
-                var headerTask = _headerFetches.GetOrAdd(height,
-                    h => client.GetBlockHeaderAsync(h, ct));
-                var proof  = await proofTask;
-                var header = BlockHeaderInfo.Parse(await headerTask);
-                await AnchorToCheckpointAsync(height, ct);
-                if (!MerkleProof.Verify(
-                        uint256.Parse(txid), proof.Pos,
-                        proof.Merkle.Select(uint256.Parse), header.MerkleRoot))
-                    throw new SpvVerificationException(
-                        $"Invalid Merkle proof for {txid} (block {height}): server is not trustworthy.");
-                var n = Interlocked.Increment(ref merkDone);
-                if (n % 50 == 0 || n == toVerify.Count)
-                    Progress?.Invoke($"tx {dlDone}/{missing.Count}, proofs {n}/{toVerify.Count}…");
-            }, ct));
+        SyncResult BuildSnapshot() =>
+            BuildResult(tip.Height, tracked, historyByAddress, txHeights, nextReceive, nextChange);

-            await Task.WhenAll(dlTasks.Concat(merkTasks));
-            foreach (var (txid, height) in toVerify)
-                _verifiedAtHeight[txid] = height;
-        }
+        PartialResult?.Invoke(BuildSnapshot());

+        var merkDone = 0;
+        var merkTasks = toVerify.Select(kv => RetryOnBusyAsync(async () =>
+        {
+            var (txid, height) = kv;
+            var proofTask = client.GetMerkleAsync(txid, height, ct);
+            // Anchor first: on a checkpointed height this fills _headerFetches[height]
+            // via the batched range fetch (§7.3), so the header lookup below is a cache
+            // hit instead of a second individual blockchain.block.header RPC per tx —
+            // halves round-trips for this stage on mainnet, where it matters most on
+            // high-latency mobile links. Falls back to an individual fetch when no
+            // checkpoint covers this height (testnet/regtest today).
+            await AnchorToCheckpointAsync(height, ct);
+            var headerHex = await _headerFetches.GetOrAdd(height, h => client.GetBlockHeaderAsync(h, ct));
+            var header    = BlockHeaderInfo.Parse(headerHex);
+            var proof     = await proofTask;
+            if (!MerkleProof.Verify(
+                    uint256.Parse(txid), proof.Pos,
+                    proof.Merkle.Select(uint256.Parse), header.MerkleRoot))
+                throw new SpvVerificationException(
+                    $"Invalid Merkle proof for {txid} (block {height}): server is not trustworthy.");
+            _verifiedAtHeight[txid] = height;
+            var n = Interlocked.Increment(ref merkDone);
+            if (n % 50 == 0 || n == toVerify.Count)
+                Progress?.Invoke($"tx {missing.Count}/{missing.Count}, proofs {n}/{toVerify.Count}…");
+            if (n % PartialResultBatchSize == 0)
+                PartialResult?.Invoke(BuildSnapshot());
+        }, ct)).ToList();
+
+        if (merkTasks.Count > 0)
+            await Task.WhenAll(merkTasks);
+
+        return BuildSnapshot();
+    }
+
+    // Rebuilding the full snapshot (UTXOs/history/address rows) is O(wallet size); firing it on
+    // every single verified proof would make the background verification phase itself O(n²) for
+    // a wallet with thousands of transactions. Batching keeps "verified" badges catching up
+    // visibly without that cost.
+    private const int PartialResultBatchSize = 200;
+
+    private bool IsTxVerified(string txid, int height) =>
+        height <= 0 || (_verifiedAtHeight.TryGetValue(txid, out var vh) && vh == height);
+
+    /// <summary>
+    /// Assembles a <see cref="SyncResult"/> from the current state of <see cref="_txCache"/> and
+    /// <see cref="_verifiedAtHeight"/>. Callable multiple times per sync (§7.4): once as soon as
+    /// transaction downloads finish (proofs still pending), and again as verification progresses,
+    /// each time reflecting whichever transactions have been proof-checked so far.
+    /// </summary>
+    private SyncResult BuildResult(
+        int tipHeight,
+        List<TrackedAddress> tracked,
+        Dictionary<string, IReadOnlyList<HistoryItem>> historyByAddress,
+        Dictionary<string, int> txHeights,
+        int nextReceive,
+        int nextChange)
+    {
        var transactions = txHeights.Keys.ToDictionary(txid => txid, txid => _txCache[txid]);
-        var verified     = txHeights.ToDictionary(kv => kv.Key, kv => kv.Value > 0);

        // 6. Local UTXO reconstruction.
        var byScript = tracked.ToDictionary(t => t.ScriptPubKey, t => t);
@@ -222,6 +305,8 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
        var utxos = new List<CachedUtxo>();
        foreach (var (txid, tx) in transactions)
        {
+            var height = txHeights[txid];
+            var verifiedTx = IsTxVerified(txid, height);
            for (var vout = 0; vout < tx.Outputs.Count; vout++)
            {
                var output = tx.Outputs[vout];
@@ -237,8 +322,9 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
                    Address       = addr.Address.ToString(),
                    IsChange      = addr.IsChange,
                    AddressIndex  = addr.Index,
-                    Height        = txHeights[txid],
+                    Height        = height,
                    IsCoinbase    = tx.IsCoinBase,
+                    Verified      = verifiedTx,
                });
            }
        }
@@ -247,6 +333,7 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
        var history = new List<CachedTx>();
        foreach (var (txid, tx) in transactions)
        {
+            var height = txHeights[txid];
            var received = tx.Outputs
                .Where(o => byScript.ContainsKey(o.ScriptPubKey))
                .Sum(o => o.Value.Satoshi);
@@ -257,9 +344,9 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
            history.Add(new CachedTx
            {
                Txid      = txid,
-                Height    = txHeights[txid],
+                Height    = height,
                DeltaSats = received - sentSats,
-                Verified  = verified[txid],
+                Verified  = IsTxVerified(txid, height),
            });
        }
        history.Sort((a, b) =>
@@ -286,12 +373,14 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl

        return new SyncResult
        {
-            TipHeight        = tip.Height,
+            TipHeight        = tipHeight,
            ConfirmedSats    = utxos.Where(u => u.Height > 0).Sum(u => u.ValueSats),
            UnconfirmedSats  = utxos.Where(u => u.Height <= 0).Sum(u => u.ValueSats),
            ImmatureSats     = utxos.Where(u =>
-                u.Height > 0 && u.Confirmations(tip.Height) < u.RequiredConfirmations(account.Profile))
+                u.Height > 0 && u.Confirmations(tipHeight) < u.RequiredConfirmations(account.Profile))
                .Sum(u => u.ValueSats),
+            PendingVerificationSats = utxos.Where(u => u.Height > 0 && !u.Verified).Sum(u => u.ValueSats),
+            SpendableSats    = utxos.Where(u => u.IsSpendable(account.Profile, tipHeight)).Sum(u => u.ValueSats),
            NextReceiveIndex = nextReceive,
            NextChangeIndex  = nextChange,
            History          = history,
@@ -325,9 +414,11 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
        if (_anchoredUpTo.TryGetValue(cp.Height, out var anchoredTo) && anchoredTo >= height)
            return;

-        var headers = await Task.WhenAll(Enumerable.Range(cp.Height, height - cp.Height + 1)
-            .Select(async h => BlockHeaderInfo.Parse(
-                await _headerFetches.GetOrAdd(h, hh => client.GetBlockHeaderAsync(hh, ct)))));
+        await FetchHeaderRangeAsync(cp.Height, height, ct);
+
+        var headers = Enumerable.Range(cp.Height, height - cp.Height + 1)
+            .Select(h => BlockHeaderInfo.Parse(_headerFetches[h].Result))
+            .ToArray();

        if (!headers[0].MatchesCheckpoint(cp))
            throw new SpvVerificationException(
@@ -341,6 +432,41 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
        _anchoredUpTo.AddOrUpdate(cp.Height, height, (_, existing) => Math.Max(existing, height));
    }

+    /// <summary>
+    /// Ensures every height in [<paramref name="fromHeight"/>, <paramref name="toHeightInclusive"/>]
+    /// is present in <see cref="_headerFetches"/>, downloading gaps with
+    /// blockchain.block.headers (§7.3) instead of one blockchain.block.header call per height.
+    /// </summary>
+    private async Task FetchHeaderRangeAsync(int fromHeight, int toHeightInclusive, CancellationToken ct)
+    {
+        await _headerRangeLock.WaitAsync(ct);
+        try
+        {
+            var h = fromHeight;
+            while (h <= toHeightInclusive)
+            {
+                if (_headerFetches.ContainsKey(h)) { h++; continue; }
+
+                var requested = Math.Min(HeaderBatchSize, toHeightInclusive - h + 1);
+                var range = await client.GetBlockHeadersAsync(h, requested, ct);
+                if (range.Count == 0)
+                    throw new SpvVerificationException(
+                        $"Server returned no headers starting at height {h}: server is not trustworthy.");
+
+                for (var i = 0; i < range.Count; i++)
+                {
+                    var headerHex = range.Hex.Substring(i * BlockHeaderInfo.Size * 2, BlockHeaderInfo.Size * 2);
+                    _headerFetches.TryAdd(h + i, Task.FromResult(headerHex));
+                }
+                h += range.Count;
+            }
+        }
+        finally
+        {
+            _headerRangeLock.Release();
+        }
+    }
+
    /// <summary>
    /// Scans one chain (receiving or change).
    ///
@@ -414,7 +540,12 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
        return (firstUnused, tracked, history);
    }

-    private static async Task RetryOnBusyAsync(Func<Task> op, CancellationToken ct)
+    // Counts "server busy" retries across the whole sync: surfaced via Progress so a slow
+    // sync can be diagnosed as server-side throttling (exponential backoff eating the time)
+    // rather than guessed at from wall-clock numbers alone.
+    private int _busyRetries;
+
+    private async Task RetryOnBusyAsync(Func<Task> op, CancellationToken ct)
    {
        var delay = 200;
        for (var attempt = 0; ; attempt++)
@@ -423,13 +554,14 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
            catch (ElectrumServerException ex)
                when (IsBusy(ex) && attempt < 7)
            {
+                ReportBusyRetry(delay);
                await Task.Delay(delay, ct);
                delay = Math.Min(delay * 2, 5_000);
            }
        }
    }

-    private static async Task<T> RetryOnBusyAsync<T>(Func<Task<T>> op, CancellationToken ct)
+    private async Task<T> RetryOnBusyAsync<T>(Func<Task<T>> op, CancellationToken ct)
    {
        var delay = 200;
        for (var attempt = 0; ; attempt++)
@@ -438,12 +570,20 @@ public sealed class WalletSynchronizer(IWalletAccount account, ElectrumClient cl
            catch (ElectrumServerException ex)
                when (IsBusy(ex) && attempt < 7)
            {
+                ReportBusyRetry(delay);
                await Task.Delay(delay, ct);
                delay = Math.Min(delay * 2, 5_000);
            }
        }
    }

+    private void ReportBusyRetry(int delayMs)
+    {
+        var n = Interlocked.Increment(ref _busyRetries);
+        if (n == 1 || n % 20 == 0)
+            Progress?.Invoke($"server busy, retry #{n} (waiting {delayMs}ms)…");
+    }
+
    private static bool IsBusy(ElectrumServerException ex) =>
        ex.Message.Contains("-102") ||
        ex.Message.Contains("-101") ||
@@ -98,6 +98,16 @@ public sealed class SyncCache

    /// <summary>Confirmed but not yet spendable (coinbase immature or under min confirmations). Subset of ConfirmedSats.</summary>
    public long ImmatureSats { get; set; }
+
+    /// <summary>
+    /// Confirmed, past its threshold, but its Merkle proof isn't checked yet. Subset of
+    /// ConfirmedSats — NOT disjoint from ImmatureSats, so never subtract both from
+    /// ConfirmedSats to get a spendable total; use SpendableSats instead.
+    /// </summary>
+    public long PendingVerificationSats { get; set; }
+
+    /// <summary>The actual spendable balance: sum of UTXOs passing UtxoSpendability.IsSpendable.</summary>
+    public long SpendableSats { get; set; }
    public int NextReceiveIndex { get; set; }
    public int NextChangeIndex { get; set; }
    public List<CachedTx> History { get; set; } = [];
@@ -141,6 +151,13 @@ public sealed class CachedTx
    public required string Txid { get; set; }
    public int Height { get; set; }
    public long DeltaSats { get; set; }
+
+    /// <summary>
+    /// True once this tx's Merkle proof has actually been checked against a
+    /// checkpoint-anchored header (not merely "confirmed" — a confirmed tx can still
+    /// be pending its own proof while background verification catches up). Always
+    /// false for unconfirmed (mempool) entries, which have no proof to check yet.
+    /// </summary>
    public bool Verified { get; set; }
 }

@@ -155,4 +172,11 @@ public sealed class CachedUtxo
    public int Height { get; set; }
    public bool IsCoinbase { get; set; }
    public bool Frozen { get; set; }
+
+    /// <summary>
+    /// True once the owning tx's Merkle proof has been checked (see <see cref="CachedTx.Verified"/>).
+    /// Gates spendability in <see cref="Wallet.UtxoSpendability.IsSpendable"/>: a server can report a
+    /// fake confirmed UTXO before its proof is checked, so coin selection must never touch it early.
+    /// </summary>
+    public bool Verified { get; set; }
 }
@@ -37,4 +37,12 @@ public static class WalletStore
        File.WriteAllText(tmp, content);
        File.Move(tmp, path, overwrite: true);
    }
+
+    /// <summary>
+    /// Same as <see cref="Save"/> but with the JSON serialization and disk write off the
+    /// calling thread — for callers on a UI thread saving a large cache (thousands of cached
+    /// transactions/headers), where the synchronous version would block the UI.
+    /// </summary>
+    public static Task SaveAsync(WalletDocument doc, string path, string? password = null) =>
+        Task.Run(() => Save(doc, path, password));
 }
@@ -82,6 +82,13 @@ public sealed class TransactionFactory(IWalletAccount account)
                reasons.Append($"{underConf.Count} output(s) need {profile.MinConfirmations} confirmations ({best} so far). ");
            }

+            var unverified = utxos.Where(u =>
+                !u.Frozen && u.Height > 0 && !u.Verified &&
+                u.Confirmations(tipHeight) >= u.RequiredConfirmations(profile)).ToList();
+            if (unverified.Count > 0)
+                reasons.Append($"{unverified.Count} output(s) confirmed but still awaiting Merkle-proof verification " +
+                    $"({CoinAmount.Format(unverified.Sum(u => u.ValueSats))}). ");
+
            throw new WalletSpendException(reasons.Length > 0
                ? $"No spendable UTXOs: {reasons.ToString().TrimEnd()}"
                : "No spendable UTXOs selected.");
@@ -18,7 +18,13 @@ public static class UtxoSpendability
    public static int Confirmations(this CachedUtxo utxo, int tipHeight) =>
        utxo.Height <= 0 ? 0 : tipHeight - utxo.Height + 1;

-    /// <summary>True when the UTXO has met its confirmation threshold and is not frozen.</summary>
+    /// <summary>
+    /// True when the UTXO has met its confirmation threshold, is not frozen, and its Merkle
+    /// proof has actually been checked. Without the <see cref="CachedUtxo.Verified"/> gate a
+    /// malicious server could report a fake confirmed UTXO and have it spent before background
+    /// verification ever caught the forgery.
+    /// </summary>
    public static bool IsSpendable(this CachedUtxo utxo, ChainProfile profile, int tipHeight) =>
-        !utxo.Frozen && utxo.Height > 0 && utxo.Confirmations(tipHeight) >= utxo.RequiredConfirmations(profile);
+        !utxo.Frozen && utxo.Height > 0 && utxo.Verified
+        && utxo.Confirmations(tipHeight) >= utxo.RequiredConfirmations(profile);
 }
@@ -101,6 +101,15 @@ public class WalletSynchronizerTests
                Headers.TryGetValue(p[0].GetInt32(), out var hex)
                    ? hex
                    : throw new FakeElectrumError(-32600, "no such block"));
+            server.Handle("blockchain.block.headers", p =>
+            {
+                var start = p[0].GetInt32();
+                var count = p[1].GetInt32();
+                var hexes = new List<string>();
+                for (var h = start; hexes.Count < count && Headers.TryGetValue(h, out var hex); h++)
+                    hexes.Add(hex);
+                return new { count = hexes.Count, hex = string.Concat(hexes) };
+            });
        }
    }

@@ -232,7 +241,10 @@ public class WalletSynchronizerTests
        Assert.Equal(0, result.ConfirmedSats);
        Assert.Equal(250_000, result.UnconfirmedSats);
        var entry = Assert.Single(result.History);
-        Assert.False(entry.Verified);
+        // Verified means "its Merkle proof was checked" — a mempool tx has no proof to
+        // check yet, so it's vacuously true; "not confirmed" is signalled by Height <= 0,
+        // not by Verified (no merkle/header RPCs happen, asserted below).
+        Assert.True(entry.Verified);
        Assert.Equal(0, server.CallCount("blockchain.transaction.get_merkle"));
        Assert.Equal(0, server.CallCount("blockchain.block.header"));
    }
@@ -318,8 +330,11 @@ public class WalletSynchronizerTests
        var result = await new WalletSynchronizer(checkpointAccount, client).SyncOnceAsync();

        Assert.Equal(1_000_000, result.ConfirmedSats);
-        // 100..105 inclusive = 6 headers fetched to walk the chain back to the checkpoint.
-        Assert.Equal(6, server.CallCount("blockchain.block.header"));
+        // Anchoring runs before the header lookup, so the range call (blockchain.block.headers)
+        // covering 100..105 back to the checkpoint already includes the tx's own header —
+        // no separate single-header call needed.
+        Assert.Equal(0, server.CallCount("blockchain.block.header"));
+        Assert.Equal(1, server.CallCount("blockchain.block.headers"));
    }

    [Fact]
@@ -352,12 +367,13 @@ public class WalletSynchronizerTests
        await sync.SyncOnceAsync(); // walks and memoizes the anchor up to 105

        // 103 <= the memoized 105: anchoring must early-return without re-walking,
-        // so the header call count stays at the 6 of the first walk (103 is cached).
+        // so the header/range call counts stay at those of the first walk (103 is cached).
        scenario.Register(tx2, 103, checkpointAccount.GetReceiveAddress(1));
        var result = await sync.SyncOnceAsync();

        Assert.Equal(1_500_000, result.ConfirmedSats);
-        Assert.Equal(6, server.CallCount("blockchain.block.header"));
+        Assert.Equal(0, server.CallCount("blockchain.block.header"));
+        Assert.Equal(1, server.CallCount("blockchain.block.headers"));
    }

    [Fact]
@@ -426,6 +442,89 @@ public class WalletSynchronizerTests
        Assert.Equal(1, server.CallCount("blockchain.block.header"));
    }

+    // ---- verifica progressiva (§7.4) ----
+
+    [Fact]
+    public async Task PartialResult_arriva_prima_della_proof_poi_il_risultato_finale_e_completamente_verificato()
+    {
+        var account = Account();
+        var scenario = new Scenario();
+        scenario.Pay(account.GetReceiveAddress(0), 1_000_000, height: 100);
+
+        var (server, client) = await StartAsync(scenario);
+        await using var _ = server; await using var __ = client;
+
+        // Gate the Merkle-proof response so the download phase can complete (and fire
+        // PartialResult) well before verification does.
+        using var gate = new ManualResetEventSlim(false);
+        server.Handle("blockchain.transaction.get_merkle", p =>
+        {
+            gate.Wait();
+            return new { block_height = p[1].GetInt32(), pos = 0, merkle = Array.Empty<string>() };
+        });
+
+        var sync = new WalletSynchronizer(account, client);
+        SyncResult? partial = null;
+        var partialReceived = new TaskCompletionSource();
+        sync.PartialResult += r => { partial = r; partialReceived.TrySetResult(); };
+
+        var syncTask = sync.SyncOnceAsync();
+        await partialReceived.Task.WaitAsync(TimeSpan.FromSeconds(5));
+
+        Assert.NotNull(partial);
+        Assert.False(Assert.Single(partial!.History).Verified);
+        Assert.False(Assert.Single(partial.Utxos).Verified);
+        Assert.Equal(1_000_000, partial.PendingVerificationSats);
+        Assert.Equal(1_000_000, partial.ConfirmedSats); // reported confirmed, just not yet proof-checked
+
+        gate.Set();
+        var final = await syncTask;
+
+        Assert.True(Assert.Single(final.History).Verified);
+        Assert.True(Assert.Single(final.Utxos).Verified);
+        Assert.Equal(0, final.PendingVerificationSats);
+    }
+
+    [Fact]
+    public async Task Un_coinbase_immaturo_e_non_ancora_verificato_non_produce_un_saldo_spendibile_negativo()
+    {
+        // Regression: ImmatureSats and PendingVerificationSats can overlap (an immature
+        // coinbase can also be unverified) — "ConfirmedSats - ImmatureSats - PendingVerificationSats"
+        // double-subtracts that overlap and can go negative. SpendableSats must be computed
+        // directly from IsSpendable instead.
+        var account = Account();
+        var scenario = new Scenario { TipHeight = 200 };
+        // 11 confirmations at tip 200: far below CoinbaseMaturity+1 = 121 — immature.
+        scenario.Pay(account.GetReceiveAddress(0), 5_000_000_000, height: 190, coinbase: true);
+
+        var (server, client) = await StartAsync(scenario);
+        await using var _ = server; await using var __ = client;
+
+        using var gate = new ManualResetEventSlim(false);
+        server.Handle("blockchain.transaction.get_merkle", p =>
+        {
+            gate.Wait();
+            return new { block_height = p[1].GetInt32(), pos = 0, merkle = Array.Empty<string>() };
+        });
+
+        var sync = new WalletSynchronizer(account, client);
+        var partialReceived = new TaskCompletionSource();
+        SyncResult? partial = null;
+        sync.PartialResult += r => { partial = r; partialReceived.TrySetResult(); };
+
+        var syncTask = sync.SyncOnceAsync();
+        await partialReceived.Task.WaitAsync(TimeSpan.FromSeconds(5));
+
+        // Immature AND unverified at the same time: both non-spendable subsets are the
+        // full 5 PLM, but the actual spendable balance must be exactly zero, not negative.
+        Assert.Equal(5_000_000_000, partial!.ImmatureSats);
+        Assert.Equal(5_000_000_000, partial.PendingVerificationSats);
+        Assert.Equal(0, partial.SpendableSats);
+
+        gate.Set();
+        await syncTask;
+    }
+
    // ---- resilienza ----

    [Fact]
@@ -33,12 +33,28 @@ public class TransactionFactoryTests
            {
                Txid = txid, Vout = 0, ValueSats = sats,
                Address = account.GetReceiveAddress(0).ToString(),
-                IsChange = false, AddressIndex = 0, Height = 100,
+                IsChange = false, AddressIndex = 0, Height = 100, Verified = true,
            },
        };
        return (utxos, new Dictionary<string, Transaction> { [txid] = funding });
    }

+    [Fact]
+    public void Un_utxo_confermato_ma_non_ancora_verificato_non_e_spendibile()
+    {
+        // Confirmed by the server, but its Merkle proof hasn't been checked yet (progressive
+        // background verification, §7.4): must never be treated as spendable — otherwise a
+        // malicious server could get a fabricated balance spent before the forgery is caught.
+        var account = Account();
+        var (utxos, txs) = Fund(account, 1_000_000);
+        utxos[0].Verified = false;
+
+        var ex = Assert.Throws<WalletSpendException>(() => new TransactionFactory(account).Build(
+            utxos, txs, account.GetReceiveAddress(5), amountSats: 400_000,
+            feeRateSatPerVByte: 2, changeIndex: 0, tipHeight: 100));
+        Assert.Contains("awaiting Merkle-proof verification", ex.Message);
+    }
+
    [Fact]
    public void Una_spesa_firmata_verifica_e_paga_la_fee_attesa()
    {
@@ -169,7 +185,7 @@ public class TransactionFactoryTests
        {
            new() { Txid = txid, Vout = 0, ValueSats = 1_000_000,
                    Address = mainnetAccount.GetReceiveAddress(0).ToString(),
-                    IsChange = false, AddressIndex = 0, Height = 100, IsCoinbase = false },
+                    IsChange = false, AddressIndex = 0, Height = 100, IsCoinbase = false, Verified = true },
        };
        var txs = new Dictionary<string, Transaction> { [txid] = funding };

@@ -295,7 +311,7 @@ public class TransactionFactoryTests
            {
                Txid = txid, Vout = 0, ValueSats = 300_000,
                Address = account.GetReceiveAddress(i).ToString(),
-                IsChange = false, AddressIndex = i, Height = 100,
+                IsChange = false, AddressIndex = i, Height = 100, Verified = true,
            });
        }