diff --git a/src/index/txindex.h b/src/index/txindex.h
index 618f3bf0f07..a0d4f3bba47 100644
--- a/src/index/txindex.h
+++ b/src/index/txindex.h
@@ -16,6 +16,9 @@
 namespace interfaces {
 class Chain;
 }
+namespace txindex_tests {
+class TxIndexTest;
+}
 
 static constexpr bool DEFAULT_TXINDEX{false};
 
@@ -36,6 +39,7 @@ protected:
     class DB;
 
 private:
+    friend class txindex_tests::TxIndexTest;
     const std::unique_ptr<DB> m_db;
 
     bool AllowPrune() const override { return false; }
diff --git a/src/test/txindex_tests.cpp b/src/test/txindex_tests.cpp
index 0dd21fae507..9254adbd746 100644
--- a/src/test/txindex_tests.cpp
+++ b/src/test/txindex_tests.cpp
@@ -3,20 +3,139 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <addresstype.h>
+#include <chain.h>
 #include <chainparams.h>
+#include <common/args.h>
+#include <consensus/amount.h>
+#include <consensus/validation.h>
+#include <crypto/hex_base.h>
+#include <dbwrapper.h>
+#include <flatfile.h>
+#include <index/disktxpos.h>
 #include <index/txindex.h>
+#include <index/txindex_key.h>
 #include <interfaces/chain.h>
+#include <key.h>
+#include <node/blockstorage.h>
+#include <primitives/block.h>
+#include <script/script.h>
+#include <streams.h>
+#include <sync.h>
 #include <test/util/setup_common.h>
 #include <util/byte_units.h>
+#include <util/check.h>
+#include <util/strencodings.h>
 #include <validation.h>
 
+#include <cstdint>
+#include <memory>
+#include <string>
+#include <string_view>
+#include <utility>
+#include <vector>
+
 #include <boost/test/unit_test.hpp>
 
 BOOST_AUTO_TEST_SUITE(txindex_tests)
 
+// Grants tests access to the otherwise non-public txindex database handle.
+class TxIndexTest
+{
+public:
+    static CDBWrapper& GetDB(const TxIndex& txindex) { return txindex.GetDB(); }
+    static CBlockLocator ReadBestBlock(const TxIndex& txindex) { return txindex.GetDB().ReadBestBlock(); }
+    static void WriteBestBlock(const TxIndex& txindex, const CBlockLocator& locator)
+    {
+        auto& db{txindex.GetDB()};
+        CDBBatch batch{db};
+        db.WriteBestBlock(batch, locator);
+        db.WriteBatch(batch);
+    }
+};
+
+namespace {
+
+SipHasher13UJ ReadHasher(const CDBWrapper& db)
+{
+    std::pair<uint64_t, uint64_t> salt;
+    BOOST_REQUIRE(db.Read(txindex::DB_TXID_HASH_SALT, salt));
+    return SipHasher13UJ{salt.first, salt.second};
+}
+
+std::vector<txindex::BlockTxPosition> BucketPositions(CDBWrapper& db, txindex::TxHashKeyPrefix prefix)
+{
+    std::vector<txindex::BlockTxPosition> positions;
+    std::unique_ptr<CDBIterator> it{db.NewIterator()};
+    txindex::DBKey key{prefix, {}};
+    for (it->Seek(key); it->Valid() && it->GetKey(key) && key.hash_prefix == prefix; it->Next()) {
+        positions.push_back(key.pos);
+    }
+    return positions;
+}
+
+FlatFilePos BlockFilePos(const ChainstateManager& chainman, uint32_t height)
+{
+    LOCK(cs_main);
+    const CBlockIndex* block_index{chainman.ActiveChain()[height]};
+    BOOST_REQUIRE(block_index);
+    return {block_index->nFile, block_index->nDataPos};
+}
+
+uint256 LookupTx(const TxIndex& txindex, const Txid& txid)
+{
+    const auto result{txindex.FindTx(txid)};
+    BOOST_REQUIRE(result);
+    BOOST_CHECK(result->tx->GetHash() == txid);
+    return result->block_hash;
+}
+
+void InvalidateBlock(ChainstateManager& chainman, const uint256& block_hash)
+{
+    CBlockIndex* block_index{WITH_LOCK(cs_main, return chainman.m_blockman.LookupBlockIndex(block_hash))};
+    BOOST_REQUIRE(block_index);
+    BlockValidationState state;
+    BOOST_REQUIRE(chainman.ActiveChainstate().InvalidateBlock(state, block_index));
+}
+
+} // namespace
+
+BOOST_AUTO_TEST_CASE(txindex_position_encoding)
+{
+    constexpr struct { txindex::BlockTxPosition position; std::string_view encoded; } test_vectors[]{
+        {{0, 0}, "00000000"},
+        {{1, 2}, "01000002"},
+        {{10'000'000, 123}, "83e1ac0000007b"},
+        {{456, 3'999'999}, "82483d08ff"},
+    };
+
+    for (const auto& [position, encoded] : test_vectors) {
+        BOOST_CHECK_EQUAL(HexStr(DataStream{} << position), encoded);
+
+        txindex::BlockTxPosition decoded;
+        BOOST_CHECK((DataStream{ParseHex(encoded)} >> decoded).empty());
+        BOOST_CHECK(decoded == position);
+    }
+
+    // Pin the full key encodings, including the type prefixes.
+    BOOST_CHECK_EQUAL(HexStr(DataStream{} << txindex::BlockSeqKey{1}), "7301");
+    BOOST_CHECK_EQUAL(HexStr(DataStream{} << txindex::DBKey{0x0102030405, {1, 2}}),
+                      "78010203040501000002");
+
+    BOOST_CHECK_EQUAL(txindex::BLOCK_HEADER_SIZE, GetSerializeSize(CBlockHeader{}));
+}
+
+BOOST_AUTO_TEST_CASE(txindex_hash_prefix)
+{
+    BOOST_CHECK_EQUAL(
+        txindex::CreateKeyPrefix(
+            SipHasher13UJ{0x0706050403020100ULL, 0x0F0E0D0C0B0A0908ULL},
+            Txid{"1f1e1d1c1b1a191817161514131211100f0e0d0c0b0a09080706050403020100"}),
+        0xc67d87b08cULL);
+}
+
 BOOST_FIXTURE_TEST_CASE(txindex_initial_sync, TestChain100Setup)
 {
-    TxIndex txindex(interfaces::MakeChain(m_node), 1_MiB, true);
+    TxIndex txindex(interfaces::MakeChain(m_node), /*n_cache_size=*/1_MiB, /*f_memory=*/true);
     BOOST_REQUIRE(txindex.Init());
 
     // Transaction should not be found in the index before it is started.
@@ -37,12 +156,7 @@ BOOST_FIXTURE_TEST_CASE(txindex_initial_sync, TestChain100Setup)
 
     // Check that txindex has all txs that were in the chain before it started.
     for (const auto& txn : m_coinbase_txns) {
-        const auto result{txindex.FindTx(txn->GetHash())};
-        if (!result) {
-            BOOST_ERROR("FindTx failed");
-        } else if (result->tx->GetHash() != txn->GetHash()) {
-            BOOST_ERROR("Read incorrect tx");
-        }
+        LookupTx(txindex, txn->GetHash());
     }
 
     // Check that new transactions in new blocks make it into the index.
@@ -53,16 +167,189 @@ BOOST_FIXTURE_TEST_CASE(txindex_initial_sync, TestChain100Setup)
         const CTransaction& txn = *block.vtx[0];
 
         BOOST_CHECK(txindex.BlockUntilSyncedToCurrentChain());
-        const auto result{txindex.FindTx(txn.GetHash())};
-        if (!result) {
-            BOOST_ERROR("FindTx failed");
-        } else if (result->tx->GetHash() != txn.GetHash()) {
-            BOOST_ERROR("Read incorrect tx");
-        }
+        LookupTx(txindex, txn.GetHash());
     }
 
     // shutdown sequence (c.f. Shutdown() in init.cpp)
     txindex.Stop();
 }
 
+BOOST_FIXTURE_TEST_CASE(txindex_collision_scan_path, TestChain100Setup)
+{
+    // On-disk, so the legacy-entry probe at construction runs against a fresh
+    // database, as it would on a node whose index was created by this version.
+    TxIndex txindex(interfaces::MakeChain(m_node), /*n_cache_size=*/1_MiB, /*f_memory=*/false);
+    BOOST_REQUIRE(txindex.Init());
+    txindex.Sync();
+
+    CDBWrapper& db{TxIndexTest::GetDB(txindex)};
+    const SipHasher13UJ hasher{ReadHasher(db)};
+
+    // Lookups scan candidates in descending sequence order, so entries of
+    // later-connected blocks are tried first. Forge a colliding entry under the
+    // first coinbase's prefix pointing at the last coinbase, so looking up the
+    // first tx must scan that false positive first.
+    const Txid fake_txid{m_coinbase_txns.back()->GetHash()};
+    const Txid target_txid{m_coinbase_txns.front()->GetHash()};
+    const auto fake_prefix{txindex::CreateKeyPrefix(hasher, fake_txid)};
+    const auto target_prefix{txindex::CreateKeyPrefix(hasher, target_txid)};
+    // Distinct prefixes guarantee the target's bucket initially holds only the target.
+    BOOST_REQUIRE(fake_prefix != target_prefix);
+
+    // Read the last coinbase's encoded position straight from its bucket.
+    const auto fake_bucket{BucketPositions(db, fake_prefix)};
+    BOOST_REQUIRE_EQUAL(fake_bucket.size(), 1U);
+    const txindex::BlockTxPosition fake_pos{fake_bucket.front()};
+
+    db.Write(txindex::DBKey{target_prefix, fake_pos}, txindex::EMPTY_VALUE);
+
+    // The target's bucket now holds the real target first (lower sequence
+    // number), then the forged false positive, which the descending scan tries first.
+    const auto target_bucket{BucketPositions(db, target_prefix)};
+    BOOST_REQUIRE_EQUAL(target_bucket.size(), 2U);
+    BOOST_CHECK(target_bucket[0] != fake_pos);
+    BOOST_CHECK(target_bucket[1] == fake_pos);
+
+    LookupTx(txindex, target_txid);
+
+    // A database created fresh by this version cannot contain legacy entries, so
+    // lookups skip the legacy fallback: drop the last coinbase's hashed entry and
+    // re-add it under the old 't' + txid schema (a physical CDiskTxPos), then
+    // confirm the lookup misses even though the legacy row exists.
+    // BlockTxPosition offsets are from the block start (header included), while
+    // the legacy CDiskTxPos.nTxOffset is measured after the header.
+    const CDiskTxPos fake_physical{BlockFilePos(*m_node.chainman, fake_pos.block_seq + 1), fake_pos.tx_offset_in_block - txindex::BLOCK_HEADER_SIZE};
+    db.Erase(txindex::DBKey{fake_prefix, fake_pos});
+    db.Write(txindex::LegacyTxKey(fake_txid), fake_physical);
+    BOOST_CHECK(!txindex.FindTx(fake_txid));
+
+    txindex.Stop();
+}
+
+BOOST_FIXTURE_TEST_CASE(txindex_legacy_fallback, TestChain100Setup)
+{
+    // Seed the on-disk database with a legacy ('t' + txid) entry before the index
+    // is opened, as if it had been written by a pre-hashing version.
+    const Txid legacy_txid{m_coinbase_txns.front()->GetHash()};
+    // The block at height 1 holds only the coinbase, so the tx starts right after
+    // the header and the 1-byte tx count.
+    const CDiskTxPos legacy_pos{BlockFilePos(*m_node.chainman, 1), 1};
+    {
+        CDBWrapper db{DBParams{.path = gArgs.GetDataDirNet() / "indexes" / "txindex", .cache_bytes = 1_MiB}};
+        db.Write(txindex::LegacyTxKey(legacy_txid), legacy_pos);
+    }
+
+    TxIndex txindex(interfaces::MakeChain(m_node), /*n_cache_size=*/1_MiB, /*f_memory=*/false);
+    BOOST_REQUIRE(txindex.Init());
+    txindex.Sync();
+
+    // Drop the hashed entries so only the legacy row remains, then confirm the
+    // lookup succeeds through the fallback.
+    CDBWrapper& db{TxIndexTest::GetDB(txindex)};
+    const auto prefix{txindex::CreateKeyPrefix(ReadHasher(db), legacy_txid)};
+    const auto bucket{BucketPositions(db, prefix)};
+    BOOST_REQUIRE(!bucket.empty());
+    for (const auto& pos : bucket) db.Erase(txindex::DBKey{prefix, pos});
+
+    LookupTx(txindex, legacy_txid);
+
+    txindex.Stop();
+}
+
+BOOST_FIXTURE_TEST_CASE(txindex_locator_upgrade, TestChain100Setup)
+{
+    uint256 legacy_hash, new_hash;
+    {
+        LOCK(cs_main);
+        legacy_hash = Assert(m_node.chainman->ActiveChain()[1])->GetBlockHash();
+        new_hash = Assert(m_node.chainman->ActiveChain().Tip())->GetBlockHash();
+    }
+    CBlockLocator legacy_locator{{legacy_hash}}, new_locator{{new_hash}};
+    { CDBWrapper{DBParams{.path = gArgs.GetDataDirNet() / "indexes" / "txindex", .cache_bytes = 1_MiB}}.Write(uint8_t{'B'}, legacy_locator); }
+
+    TxIndex txindex(interfaces::MakeChain(m_node), /*n_cache_size=*/1_MiB, /*f_memory=*/false);
+    BOOST_CHECK(TxIndexTest::ReadBestBlock(txindex).vHave == legacy_locator.vHave);
+
+    TxIndexTest::WriteBestBlock(txindex, new_locator);
+    BOOST_CHECK(TxIndexTest::ReadBestBlock(txindex).vHave == new_locator.vHave);
+
+    CBlockLocator stored_legacy_locator;
+    BOOST_REQUIRE(TxIndexTest::GetDB(txindex).Read(uint8_t{'B'}, stored_legacy_locator));
+    BOOST_CHECK(stored_legacy_locator.vHave == legacy_locator.vHave);
+}
+
+BOOST_FIXTURE_TEST_CASE(txindex_reorg_keeps_stale_entries, TestChain100Setup)
+{
+    TxIndex txindex(interfaces::MakeChain(m_node), /*n_cache_size=*/1_MiB, /*f_memory=*/true);
+    BOOST_REQUIRE(txindex.Init());
+    txindex.Sync();
+
+    const CScript coinbase_script{CScript() << ToByteVector(coinbaseKey.GetPubKey()) << OP_CHECKSIG};
+
+    // Mine a unique (non-coinbase) transaction into a new block at height 101.
+    CMutableTransaction unique_mtx{CreateValidMempoolTransaction(
+        /*input_transaction=*/m_coinbase_txns[0],
+        /*input_vout=*/0,
+        /*input_height=*/1,
+        /*input_signing_key=*/coinbaseKey,
+        /*output_destination=*/CScript() << OP_TRUE,
+        /*output_amount=*/CAmount{1 * COIN},
+        /*submit=*/false)};
+    const Txid unique_txid{MakeTransactionRef(unique_mtx)->GetHash()};
+    const uint256 stale_block_hash{CreateAndProcessBlock({unique_mtx}, coinbase_script).GetHash()};
+    BOOST_REQUIRE(txindex.BlockUntilSyncedToCurrentChain());
+
+    BOOST_CHECK(LookupTx(txindex, unique_txid) == stale_block_hash);
+
+    CDBWrapper& db{TxIndexTest::GetDB(txindex)};
+    const auto prefix{txindex::CreateKeyPrefix(ReadHasher(db), unique_txid)};
+    const auto original_bucket{BucketPositions(db, prefix)};
+    BOOST_REQUIRE_EQUAL(original_bucket.size(), 1U);
+
+    ChainstateManager& chainman{*m_node.chainman};
+
+    // Invalidate the block holding the unique transaction.
+    InvalidateBlock(chainman, stale_block_hash);
+    BOOST_REQUIRE(txindex.BlockUntilSyncedToCurrentChain());
+
+    // The disconnected transaction is still found, in the now-stale block.
+    BOOST_CHECK(LookupTx(txindex, unique_txid) == stale_block_hash);
+    {
+        LOCK(cs_main);
+        const CBlockIndex* stale_index{chainman.m_blockman.LookupBlockIndex(stale_block_hash)};
+        BOOST_REQUIRE(stale_index);
+        BOOST_CHECK(!chainman.ActiveChain().Contains(*stale_index));
+    }
+
+    // Mine the same transaction into a replacement branch, which gets a later
+    // sequence number. The lookup must now return the branch block in the active chain.
+    const uint256 branch_block_hash{CreateAndProcessBlock({unique_mtx}, CScript() << OP_TRUE).GetHash()};
+    CreateAndProcessBlock({}, coinbase_script);
+    BOOST_REQUIRE(txindex.BlockUntilSyncedToCurrentChain());
+    BOOST_CHECK(LookupTx(txindex, unique_txid) == branch_block_hash);
+
+    // Reorg back to the original branch. The original branch block must be
+    // now be preferred even though the replacement branch has a later sequence.
+    {
+        LOCK(cs_main);
+        chainman.ActiveChainstate().ResetBlockFailureFlags(chainman.m_blockman.LookupBlockIndex(stale_block_hash));
+    }
+    InvalidateBlock(chainman, branch_block_hash);
+    {
+        BlockValidationState state;
+        BOOST_REQUIRE(chainman.ActiveChainstate().ActivateBestChain(state));
+    }
+    BOOST_REQUIRE(txindex.BlockUntilSyncedToCurrentChain());
+    BOOST_CHECK(WITH_LOCK(cs_main, return chainman.ActiveChain().Tip()->GetBlockHash()) == stale_block_hash);
+
+    BOOST_CHECK(LookupTx(txindex, unique_txid) == stale_block_hash);
+
+    // Reconnecting the original block must not create duplicate entries.
+    const auto reorg_bucket{BucketPositions(db, prefix)};
+    BOOST_REQUIRE_EQUAL(reorg_bucket.size(), 2U);
+    BOOST_CHECK(reorg_bucket.front() == original_bucket.front());
+
+    txindex.Stop();
+}
+
 BOOST_AUTO_TEST_SUITE_END()