diff --git a/configs/coins/syscoin.json b/configs/coins/syscoin.json
index d0ca2aaae5..1d8af46612 100644
--- a/configs/coins/syscoin.json
+++ b/configs/coins/syscoin.json
@@ -65,7 +65,7 @@
     "internal_binding_template": ":{{.Ports.BlockbookInternal}}",
     "public_binding_template": ":{{.Ports.BlockbookPublic}}",
     "explorer_url": "",
-    "additional_params": "-dbcache=4294967296 -dbmaxopenfiles=500000 -workers=16 -chunk=1000",
+    "additional_params": "-dbcache=4294967296 -dbmaxopenfiles=500000 -workers=16 -chunk=1000 -extendedindex",
     "block_chain": {
       "parse": true,
       "mempool_workers": 8,
diff --git a/configs/coins/syscoin_testnet.json b/configs/coins/syscoin_testnet.json
index 77d186ac84..ee2a86946f 100644
--- a/configs/coins/syscoin_testnet.json
+++ b/configs/coins/syscoin_testnet.json
@@ -66,7 +66,7 @@
     "internal_binding_template": ":{{.Ports.BlockbookInternal}}",
     "public_binding_template": ":{{.Ports.BlockbookPublic}}",
     "explorer_url": "",
-    "additional_params": "-dbcache=4294967296 -dbmaxopenfiles=500000 -workers=16 -chunk=1000",
+    "additional_params": "-dbcache=4294967296 -dbmaxopenfiles=500000 -workers=16 -chunk=1000 -extendedindex",
     "block_chain": {
       "parse": true,
       "mempool_workers": 8,
diff --git a/docs/api-syscoin.md b/docs/api-syscoin.md
index 1871507051..98808e4e00 100644
--- a/docs/api-syscoin.md
+++ b/docs/api-syscoin.md
@@ -36,6 +36,24 @@ These fields appear on existing transaction endpoints, including
 `/api/v2/tx/{txid}` and address transaction lists. They are omitted for ordinary
 base coin transactions.
 
+### Spending transaction lookups
+
+The `syscoin` and `syscoin_testnet` package configurations enable
+`-extendedindex`. This stores the spending transaction ID, input index, and
+block height with each spent output during indexing. `/spending/{txid}/{vout}`
+can then read the spender directly instead of scanning the output address's
+transaction history. Transaction responses also include `spentTxId`,
+`spentIndex`, and `spentHeight` where applicable, without requiring
+`?spending=true`.
+
+Existing Blockbook databases created without `-extendedindex` must be rebuilt
+before using the updated service configuration. Changing the startup flag alone
+does not backfill historical records and fails with `ExtendedIndex setting does
+not match`. Build the new index in a separate database directory, retain the
+original index for rollback, and verify synchronization and spending lookups
+before switching public traffic. The Syscoin Core blockchain data does not need
+to be rebuilt.
+
 ## Address Responses
 
 Address responses retain the standard Blockbook `tokens` array for regular