diff --git a/src/main/java/com/ffii/fpsms/modules/stock/service/StockTransferRecordService.kt b/src/main/java/com/ffii/fpsms/modules/stock/service/StockTransferRecordService.kt
index 36dd5aa..c67d38b 100644
--- a/src/main/java/com/ffii/fpsms/modules/stock/service/StockTransferRecordService.kt
+++ b/src/main/java/com/ffii/fpsms/modules/stock/service/StockTransferRecordService.kt
@@ -13,8 +13,6 @@ import org.springframework.stereotype.Service
 import org.springframework.transaction.annotation.Transactional
 import com.ffii.fpsms.modules.stock.entity.InventoryLotLineRepository
 import com.ffii.fpsms.modules.stock.entity.enum.InventoryLotLineStatus
-import org.springframework.http.HttpStatus
-import org.springframework.web.server.ResponseStatusException
 
 
 @Service
@@ -30,6 +28,8 @@ open class StockTransferRecordService(
 ) {
     private enum class TransferStockInMode {
         MERGED_EXISTING_LOT,
+        /** Merged into smallest-id AVAILABLE line among several with same expiry (same lot key at target). */
+        MERGED_EXISTING_LOT_AMBIGUOUS,
         CREATED_NEW_LOT
     }
 
@@ -49,12 +49,17 @@ open class StockTransferRecordService(
             name = "Stock Transfer Record",
             code = when (stockInMode) {
                 TransferStockInMode.MERGED_EXISTING_LOT -> "MERGED_EXISTING_LOT"
+                TransferStockInMode.MERGED_EXISTING_LOT_AMBIGUOUS -> "MERGED_EXISTING_LOT_AMBIGUOUS"
                 TransferStockInMode.CREATED_NEW_LOT -> "CREATED_NEW_LOT"
             },
             type = "success",
             message = when (stockInMode) {
-                TransferStockInMode.MERGED_EXISTING_LOT -> "Stock transfer completed successfully (merged into existing target lot)"
-                TransferStockInMode.CREATED_NEW_LOT -> "Stock transfer completed successfully (created new target lot)"
+                TransferStockInMode.MERGED_EXISTING_LOT ->
+                    "Stock transfer completed successfully (merged into existing target lot)"
+                TransferStockInMode.MERGED_EXISTING_LOT_AMBIGUOUS ->
+                    "已併入較早建立的批次（同批號有多筆可用）"
+                TransferStockInMode.CREATED_NEW_LOT ->
+                    "Stock transfer completed successfully (created new target lot)"
             },
             errorPosition = null
         )
@@ -120,20 +125,29 @@ open class StockTransferRecordService(
             status = InventoryLotLineStatus.AVAILABLE,
         ).filter { it.id != inventoryLotLine.id }
 
+        // Same expiry as source required to merge; repository orders by ill.id ASC — first = earliest-created line
+        val sameExpiryMergeable = mergeCandidates.filter {
+            val targetExpiry = it.inventoryLot?.expiryDate
+            targetExpiry != null && targetExpiry == expiryDate
+        }
+
         return when {
             mergeCandidates.isEmpty() ->
                 stockInLineService.createStockIn(stockInRequest) to TransferStockInMode.CREATED_NEW_LOT
-            mergeCandidates.size == 1 ->
+            sameExpiryMergeable.isEmpty() ->
+                stockInLineService.createStockIn(stockInRequest) to TransferStockInMode.CREATED_NEW_LOT
+            sameExpiryMergeable.size == 1 ->
                 stockInLineService.createStockInForExistingInventoryLotLine(
                     stockInRequest,
-                    mergeCandidates.single(),
+                    sameExpiryMergeable.single(),
                 ) to TransferStockInMode.MERGED_EXISTING_LOT
-            else ->
-                throw ResponseStatusException(
-                    HttpStatus.BAD_REQUEST,
-                    "目標倉在同一品項與批號下有多筆可用的庫存批號行，無法自動併批；請檢查資料。" +
-                        " (Multiple AVAILABLE inventory lot lines for same warehouse, item, lotNo.)",
-                )
+            else -> {
+                val targetLine = sameExpiryMergeable.first()
+                stockInLineService.createStockInForExistingInventoryLotLine(
+                    stockInRequest,
+                    targetLine,
+                ) to TransferStockInMode.MERGED_EXISTING_LOT_AMBIGUOUS
+            }
         }
     }