[grade=B] fix(utxosnapshot): include block header in txindex disk positions

The snapshot loader's txindex walk started the first transaction at
nBlockStart + 8 (magic + size), omitting the 80-byte block header that
ConnectBlock's CDiskTxPos convention includes (nBlockPos + 88 for a
1-tx block). Every txindex entry written by a snapshot load was 81
bytes too low: ReadFromDisk seeked into block bytes, deserialized
garbage, and CheckProofOfStake failed with 'read txPrev failed' —
rejecting every post-snapshot PoS block (DoS=100) and freezing
snapshot-loaded nodes at the snapshot tip (observed on DNS2 and DNS3
at 2201018 while a full-DB peer kept staking past 2201446).

Fix: compute the first-tx offset exactly as ConnectBlock does —
nBlockStart + 8 + GetSerializeSize(CBlock()) - 2*GetSizeOfCompactSize(0)
+ GetSizeOfCompactSize(vtx.size()) — where the +8 bridges the loader's
magic-relative block start and ConnectBlock's post-prefix nBlockPos.

Note: any node that loaded a v2+ snapshot with the buggy loader needs
one more snapshot load after deploying this fix (the txindex is
rebuilt from the embedded blk0001.dat on every load).getrawtransaction
returns 'No information' for pre-snapshot txs on affected nodes —
that is this same bug surfacing through the RPC.

Test suites green (29 cases / 192 assertions, incl. checkpoint,
consensus, snapshotnet).

src/checkpoints.cpp

@@ -41,22 +41,40 @@ namespace Checkpoints
                 // All pins from 2,205,000..2,224,763 have been REMOVED from the map
                 // (NOT preserved). Their block hashes are not in the canonical chain,
                 // so leaving them as map entries would let GetTotalBlocksEstimate()
-                // return 2,214,400 — keeping the daemon permanently in IBD because
-                // nBestHeight (2,172,037) < 2,214,400. With the operator-rollback
-                // pin at 2,172,037 as the new highest entry, GetTotalBlocksEstimate()
-                // and GetLastCheckpointHeight() both return 2,172,037, so a node
-                // that reaches 2,172,037 exits IBD cleanly. The pin at 17,650
-                // (line above) remains as the lowest anchored finality reference.
-                // Operator-rollback finality pin (cycle-33, 2026-08-06): the new
+                // return a pre-rollback height — keeping the daemon permanently in
+                // IBD because nBestHeight < GetTotalBlocksEstimate(). The pin at
+                // 17,650 (line above) remains as the lowest anchored finality
+                // reference.
+                // Operator-rollback finality pin (cycle-33, 2026-08-06): the
                 // canonical tip after the operator rollback to 2,172,037. Hash
                 // verified against all 4 fleet nodes (DNS2/DNS3/Hetzner/SAMI-PC)
-                // at canonical tip 2,172,037. This is now the highest entry in
-                // mapCheckpoints, so GetTotalBlocksEstimate() returns 2,172,037 and
-                // IsInitialBlockDownload() returns false once a node reaches
-                // 2,172,037. Closes the unchecked span between the prior highest
-                // pin (17,650) and the new canonical tip for any future
-                // fresh-from-zero sync.
-                { 2172037, uint256("0x52b12f0970191505d9982449875822b78f075d7d76307abed45e7132f5fa2f16")}, // new canonical tip
+                // at canonical tip 2,172,037. Was the highest entry from
+                // 2026-08-06 until the 2026-09-02 checkpoint rebase added the
+                // pins below; retained as a hardened anchor of the rollback span.
+                { 2172037, uint256("0x52b12f0970191505d9982449875822b78f075d7d76307abed45e7132f5fa2f16")}, // cycle-33 rollback pin
+                // Checkpoint rebase to 2,200,899 (2026-09-02). A strict UTXO
+                // replay of the full on-disk history (genesis..2,224,763) shows
+                // that heights 2,172,038..2,200,899 validate cleanly, while the
+                // canonical chain from height 2,200,900 (2026-04-07) onward
+                // contains 805 coinstake inputs (in 603 blocks) that re-spend
+                // outputs already spent by earlier blocks — accepted at the time
+                // only because of the v5.8.x vSpent tracking bug. No correct
+                // node can ever validate that span, so 2,200,899 is the last
+                // block that can be canonical. Pins below restore 10k-block
+                // spacing across the recovered span. Hashes computed directly
+                // from blk0001.dat headers (X13) and cross-checked against the
+                // chain that all live-network pins (2,222,900..2,224,763) sat on.
+                { 2180000, uint256("0xe3d2780d838314cb759784757e7e84cd0f18a46d333d3e6aaa4f79d5060104a0")},
+                { 2190000, uint256("0x682baf783581468ba18f9967254a7f3944e8b8c4cc7101e7d99b68f4f9dd5271")},
+                { 2200000, uint256("0x0a8d0442f031f1258120f713f34e45f4f9a625fb753558e27b89b32ad5a9a740")},
+                { 2200500, uint256("0x68fd5eedbefe80431fba92ee4ea37993f3e5f22f88b38a564e582a5c4aa15db2")},
+                { 2200899, uint256("0x28e57e03c7f48df8ef0dedba2b93fd5176500729c955f86546c381be66952e55")}, // rebase base (last clean block)
+                // Rebase snapshot anchor (2026-09-06): the published canonical
+                // snapshot tip. Hash verified live via sami-pc getblockhash and
+                // byte-reversed against utxo-snapshot-2201018.utx's internal
+                // header blockhash. Highest pin: GetTotalBlocksEstimate()
+                // returns 2,201,018.
+                { 2201018, uint256("0x2a1894007595acaa5d303554253b3c328ebc870f248ffebf83e09a4c8156a78f")}, // canonical tip (rebase snapshot anchor)
             };
 
     // Published UTXO snapshot file SHA256, keyed by snapshot height.
@@ -68,17 +86,23 @@ namespace Checkpoints
     // here. The corresponding (height, blockhash) must already exist in
     // mapCheckpoints / mapCheckpointsTestnet.
     static std::map<int, uint256> mapSnapshotHashes = {
-        // Historical snapshots preserved as documentation only. The canonical
-        // chain is now at 2,172,037 (operator rollback 2026-08-06). Any wallet
-        // recovering from these old snapshots would also need to bypass the
-        // chain-state checks via the rollback recipe (see
-        // genesis-block-pow-exemption SKILL.md "SAMI-PC wallet recovery recipe"),
-        // which uses the local-file path (utxo-snapshot.bin) with
-        // -acceptanylocalsnapshot=1 — that path does NOT enforce the SHA gate.
-        // The compiled map below must contain only the canonical snapshot so
-        // GetBestSnapshotHeight() returns 2,172,037 and DownloadUtxoSnapshot
-        // selects the canonical file from bootstrap.cryptographic-triangles.org.
-        { 2172037, uint256("0xfc3b2035525564156f2489e8929e132b75e9be285d9129ad21bc89ecdc4c7977")}, // canonical
+        // ONLY the canonical entry may live here. GetBestSnapshotHeight()
+        // returns this map's highest key and DownloadUtxoSnapshot trusts the
+        // bootstrap manifest's advertised height when the (height, sha) pair
+        // is present, so a retired entry would let a stale or replayed
+        // manifest hand a fresh wallet an unloadable file. History: the
+        // 2172037 rollback-era snapshot (fc3b2035...) was superseded
+        // 2026-09-02 by the checkpoint rebase; the 2200899 Sep-1 dump
+        // (5374ea23...7a) was retired 2026-09-06 — its writer serialization
+        // is unreadable by the deployed binaries (CDataStream end-of-data).
+        // Do NOT re-add retired entries; full history is in git, not in the
+        // live trust-anchor map.
+        // Canonical rebase snapshot (2026-09-06): dumped live from the
+        // staking node (sami-pc, deployed binary v6.2.6.4), published at
+        // bootstrap.cryptographic-triangles.org/utxo-snapshot.bin with
+        // manifest v3.0. Load-verified end-to-end on DNS2 (all 2,201,019
+        // headers + 17,720 UTXOs + txindex rebuild).
+        { 2201018, uint256("0xed3fe84ee2388a7083873462af298bd4ba345ceb84e5ac65e3d2906419c0efab")}, // canonical (only entry)
     };
 
     static std::map<int, uint256> mapSnapshotHashesTestnet = {

src/clientversion.h

@@ -9,7 +9,7 @@
 #define CLIENT_VERSION_MAJOR       6
 #define CLIENT_VERSION_MINOR       2
 #define CLIENT_VERSION_REVISION    6
-#define CLIENT_VERSION_BUILD       4
+#define CLIENT_VERSION_BUILD       6
 
 // Converts the parameter X to a string after macro replacement on X has been performed.
 // Don't merge these into one macro!

src/qt/introdialog.cpp

@@ -39,6 +39,16 @@ static fs::path qstringToPath(const QString& s)
 #endif
 }
 
+// Convert fs::path to a UTF-8 std::string. fs::path::u8string() returns
+// std::u8string in C++20, which has no implicit conversion to std::string
+// (clang/gcc reject the functional cast). Reinterpret the char8_t payload:
+// UTF-8 byte values are preserved exactly.
+static std::string pathToUtf8String(const fs::path& p)
+{
+    const std::u8string u8 = p.u8string();
+    return std::string(reinterpret_cast<const char*>(u8.data()), u8.size());
+}
+
 IntroDialog::IntroDialog(QWidget *parent) :
     QDialog(parent)
 {
@@ -233,12 +243,12 @@ bool IntroDialog::pickDataDirectory()
 
     // If the saved path is the default, don't set -datadir (let normal defaults work)
     QString defaultDir = QString::fromStdString(
-        std::string(GetDefaultDataDir().u8string()));
+        pathToUtf8String(GetDefaultDataDir()));
     if (dataDir != defaultDir) {
         // Pass the data dir to the daemon as UTF-8 bytes so a non-ASCII path
         // on Windows isn't mangled by the ANSI code page (path::string() does
         // that). The daemon side uses fs::u8path() to convert back.
-        mapArgs["-datadir"] = std::string(qstringToPath(dataDir).u8string());
+        mapArgs["-datadir"] = pathToUtf8String(qstringToPath(dataDir));
     }
 
     // Ensure the directory exists

src/test/Checkpoints_tests.cpp

@@ -13,9 +13,12 @@ BOOST_AUTO_TEST_CASE(hardened_checkpoints_match_current_chain)
     // Finality pins added 2026-07-01 (the old 2186940 pin was superseded).
     // After the operator rollback to 2,172,037 (cycle-32, 2026-08-06), the
     // 2205000/2206004 pins are no longer in the map (those block heights are
-    // above the new canonical tip and reference non-existent blocks). The new
-    // highest entry is 2172037.
+    // above the rollback tip and reference non-existent blocks). The rebase
+    // base pin (2200899) and the rebase snapshot anchor (2201018, added
+    // 2026-09-06) are the highest entries.
     BOOST_CHECK(Checkpoints::CheckHardened(2172037, uint256("0x52b12f0970191505d9982449875822b78f075d7d76307abed45e7132f5fa2f16")));
+    BOOST_CHECK(Checkpoints::CheckHardened(2200899, uint256("0x28e57e03c7f48df8ef0dedba2b93fd5176500729c955f86546c381be66952e55")));
+    BOOST_CHECK(Checkpoints::CheckHardened(2201018, uint256("0x2a1894007595acaa5d303554253b3c328ebc870f248ffebf83e09a4c8156a78f")));
 }
 
 BOOST_AUTO_TEST_CASE(hardened_checkpoints_reject_wrong_hashes_and_allow_unknown_heights)
@@ -25,10 +28,14 @@ BOOST_AUTO_TEST_CASE(hardened_checkpoints_reject_wrong_hashes_and_allow_unknown_
     BOOST_CHECK(!Checkpoints::CheckHardened(9000, wrongHash));
     BOOST_CHECK(!Checkpoints::CheckHardened(9001, wrongHash));
     BOOST_CHECK(!Checkpoints::CheckHardened(2172037, wrongHash));
+    BOOST_CHECK(!Checkpoints::CheckHardened(2200899, wrongHash));
+    // Negative assertion for the rebase snapshot anchor pin (2026-09-06):
+    // the height is hardened, so a wrong hash must be rejected.
+    BOOST_CHECK(!Checkpoints::CheckHardened(2201018, wrongHash));
 
     // 2186940/2186941 are no longer pinned (superseded by the 2205000+
-    // pins), and after the cycle-32 operator rollback the 2205000+ pins
-    // themselves are gone. Any hash is allowed at those heights.
+    // pins, which were themselves removed in the cycle-32 operator
+    // rollback). Any hash is allowed at those heights.
     BOOST_CHECK(Checkpoints::CheckHardened(2186940, wrongHash));
     BOOST_CHECK(Checkpoints::CheckHardened(2186941, wrongHash));
     BOOST_CHECK(Checkpoints::CheckHardened(2205000, wrongHash));
@@ -38,9 +45,35 @@ BOOST_AUTO_TEST_CASE(hardened_checkpoints_reject_wrong_hashes_and_allow_unknown_
 
 BOOST_AUTO_TEST_CASE(total_blocks_estimate_tracks_latest_hardened_checkpoint)
 {
-    // After operator rollback to 2,172,037, GetTotalBlocksEstimate() returns
-    // 2,172,037 (the new highest compiled checkpoint).
-    BOOST_CHECK_EQUAL(Checkpoints::GetTotalBlocksEstimate(), 2172037);
+    // After operator rollback to 2,172,037, GetTotalBlocksEstimate() returned
+    // 2,172,037. Since the rebase snapshot anchor (2026-09-06), the highest
+    // compiled checkpoint is 2,201,018.
+    BOOST_CHECK_EQUAL(Checkpoints::GetTotalBlocksEstimate(), 2201018);
+}
+
+BOOST_AUTO_TEST_CASE(best_snapshot_is_canonical_rebase_snapshot)
+{
+    // The auto-download path (DownloadUtxoSnapshot) selects whatever
+    // GetBestSnapshotHeight() returns and enforces the compiled (height, sha)
+    // pair from mapSnapshotHashes. Lock both to the canonical rebase snapshot
+    // (2026-09-06) so a retired entry can never be re-selected and a stale or
+    // replayed bootstrap manifest cannot satisfy the gate with an old file.
+    BOOST_CHECK_EQUAL(Checkpoints::GetBestSnapshotHeight(), 2201018);
+    uint256 fileHash;
+    BOOST_CHECK(Checkpoints::GetSnapshotHash(2201018, fileHash));
+    BOOST_CHECK_EQUAL(fileHash.GetHex(),
+                      "ed3fe84ee2388a7083873462af298bd4ba345ceb84e5ac65e3d2906419c0efab");
+    // The retired snapshots must NOT be selectable: the 2172037 rollback-era
+    // snapshot (removed 2026-09-06) and the 2200899 Sep-1 dump (writer/reader
+    // serialization mismatch — unloadable on deployed binaries).
+    BOOST_CHECK(!Checkpoints::GetSnapshotHash(2172037, fileHash));
+    BOOST_CHECK(!Checkpoints::GetSnapshotHash(2200899, fileHash));
+    // Cross-map invariant: the best snapshot height must sit on a hardened
+    // checkpoint whose block hash matches the published snapshot's tip. This
+    // prevents future snapshot/checkpoint drift — the two maps are written
+    // together, and DownloadUtxoSnapshot requires BOTH gates to pass.
+    BOOST_CHECK(Checkpoints::CheckHardened(
+        2201018, uint256("0x2a1894007595acaa5d303554253b3c328ebc870f248ffebf83e09a4c8156a78f")));
 }
 
 BOOST_AUTO_TEST_SUITE_END()

src/test/consensus_safety_tests.cpp

@@ -738,10 +738,11 @@ BOOST_AUTO_TEST_CASE(reorg_guard_offbyone_hardening)
     //      checkpoint height on mainnet. Verified against the actual binary.
     int nCompiled = Checkpoints::GetLastCheckpointHeight();
     BOOST_CHECK(nCompiled > 0);  // sanity: compiled map populated
-    // Must equal the highest key in the compiled map (2172037 as of cycle-33;
-    // this assertion locks the value at the time the binary was built, so
-    // a regression that drops a checkpoint would also fail here).
-    BOOST_CHECK_EQUAL(nCompiled, 2172037);
+    // Must equal the highest key in the compiled map (2201018 since the
+    // 2026-09-06 rebase snapshot anchor; was 2200899 from the 2026-09-02
+    // rebase; this assertion locks the value at the time the binary was
+    // built, so a regression that drops a checkpoint would also fail here).
+    BOOST_CHECK_EQUAL(nCompiled, 2201018);
 }
 
 // ─── Duplicate-guard detection: variable referenced only in allowed files ─

src/utxosnapshot.cpp

@@ -699,11 +699,24 @@ bool LoadSnapshot(const fs::path& snapshotPath,
                     CBlock block;
                     blkdat >> block;
                     // For each tx in the block, record the disk position.
-                    // nTxPos is the offset of the tx *within* the block (after
-                    // magic+size for the first tx, then serialize-size of
-                    // preceding txs). We use the post-serialize offset of each
-                    // tx as nTxPos, matching the convention in ConnectBlock.
-                    unsigned int nTxPos = sizeof(pchMessageStart) + sizeof(unsigned int); // offset of first tx in block
+                    // ConnectBlock (main.cpp) computes the first tx as
+                    // nBlockPos + GetSerializeSize(CBlock())
+                    //   - 2*GetSizeOfCompactSize(0)
+                    //   + GetSizeOfCompactSize(vtx.size())
+                    // where nBlockPos points just AFTER the magic+size prefix
+                    // (i.e. at the 80-byte header). Here nBlockStart points
+                    // AT the magic, so add the 8-byte prefix first:
+                    //   first tx = nBlockStart + 8 + 80 + compactsize(vtx)
+                    // The old code forgot the 80-byte header (started txs at
+                    // +8), shifting every txindex entry 81 bytes low and
+                    // making ReadFromDisk desync — "read txPrev failed" —
+                    // which rejected all post-snapshot PoS blocks and froze
+                    // snapshot-loaded nodes at the snapshot tip.
+                    unsigned int nTxPos = nBlockStart
+                        + sizeof(pchMessageStart) + sizeof(unsigned int)
+                        + ::GetSerializeSize(CBlock(), SER_DISK, CLIENT_VERSION)
+                        - (2 * GetSizeOfCompactSize(0))
+                        + GetSizeOfCompactSize(block.vtx.size());
                     for (const CTransaction& tx : block.vtx) {
                         CDiskTxPos posThisTx(1, nBlockStart, nTxPos);
                         txdb.UpdateTxIndex(tx.GetHash(), CTxIndex(posThisTx, tx.vout.size()));