diff --git a/sas/mocks/sasjs/services/dcMockUtils.js b/sas/mocks/sasjs/services/dcMockUtils.js
index 15aed55..a286297 100644
--- a/sas/mocks/sasjs/services/dcMockUtils.js
+++ b/sas/mocks/sasjs/services/dcMockUtils.js
@@ -460,6 +460,60 @@ function sasVarsEntry(col) {
     return { format: fmt, label: col.label || col.name, length: String(col.length), type: col.type === 'N' ? 'num' : 'char' }
 }
 
+/**
+ * Assigns the retained key (RK) to staged rows for a table registered with
+ * rk_underlying, mirroring %mp_retainedkey as %mpe_loader invokes it: any RK
+ * column in the input is dropped, and every row's RK is derived from its
+ * business key - the value that key already holds on the base table, or the
+ * next key value for a new one. Business keys are matched the way SAS compares
+ * them: case-sensitive, trailing blanks ignored.
+ *
+ * @param {Object} opts - { rows, tableReg, baseData }
+ *   tableReg - the table's MPE_TABLES row (needs rk_underlying and buskey)
+ *   baseData - { columns, rows } of the base table, for the key lookup
+ * @returns {String} the RK column name when the assignment ran, else ''
+ */
+function mpeAssignRetainedKey(opts) {
+    const { rows, tableReg, baseData } = opts
+    const rkUnderlying = tableReg && tableReg.rk_underlying
+        ? String(tableReg.rk_underlying).trim()
+        : ''
+    const rkCol = tableReg && tableReg.buskey ? String(tableReg.buskey).trim() : ''
+    if (!rkUnderlying || !rkCol || !rows || rows.length === 0) return ''
+
+    const base = baseData || { columns: [], rows: [] }
+    const submitted = Object.keys(rows[0] || {}).find(
+        (k) => k.toUpperCase() === rkCol.toUpperCase()
+    )
+    const rkName = submitted ||
+        (base.columns.find((c) => c.name.toUpperCase() === rkCol.toUpperCase()) || {}).name ||
+        rkCol
+    const keys = rkUnderlying.split(/\s+/)
+    const keyValue = (value) => {
+        if (value === null || value === undefined) return ''
+        return typeof value === 'number' ? String(value) : String(value).replace(/\s+$/, '')
+    }
+    const businessKey = (row) => keys.map((k) => keyValue(colVal(row, k))).join('|')
+
+    const baseByKey = new Map()
+    let nextRk = 0
+    for (const row of base.rows) {
+        const rk = colVal(row, rkCol)
+        baseByKey.set(businessKey(row), rk)
+        const num = Number(rk)
+        if (isFinite(num) && num > nextRk) nextRk = num
+    }
+
+    for (const row of rows) {
+        Object.keys(row).forEach((k) => {
+            if (k.toUpperCase() === rkCol.toUpperCase()) delete row[k]
+        })
+        const existing = baseByKey.get(businessKey(row))
+        row[rkName] = existing === undefined || existing === null ? ++nextRk : existing
+    }
+    return rkName
+}
+
 /**
  * Stages a submission: writes staged.json under staging/<tableId>/, copies the
  * session audit items, and inserts the MPE_SUBMIT row. Shared by the
diff --git a/sas/mocks/sasjs/services/editors/loadfile.js b/sas/mocks/sasjs/services/editors/loadfile.js
index 0c03468..5449672 100644
--- a/sas/mocks/sasjs/services/editors/loadfile.js
+++ b/sas/mocks/sasjs/services/editors/loadfile.js
@@ -147,6 +147,12 @@ if (!msg) {
                     if (tableEntry.var_txfrom) computedCols.push(tableEntry.var_txfrom.toUpperCase())
                     if (tableEntry.var_txto) computedCols.push(tableEntry.var_txto.toUpperCase())
                     if (tableEntry.var_processed) computedCols.push(tableEntry.var_processed.toUpperCase())
+                    // loadfile.sas drops the RK from the comparison for a table
+                    // with rk_underlying (the loader derives it), so an upload
+                    // for one legitimately omits it.
+                    if (tableEntry.rk_underlying && tableEntry.buskey) {
+                        computedCols.push(String(tableEntry.buskey).toUpperCase())
+                    }
                 }
             }
             const baseCols = schema.map((c) => c.name.toUpperCase())
@@ -191,6 +197,23 @@ if (!msg) {
         }
 
         tableId = makeTableId()
+        // ─── RK assignment (mpe_loader) ──────────────────────────────────────
+        // The uploaded file carries the business key and no RK (see the
+        // expected-columns check above), so the loader derives it before the
+        // rows are staged.
+        const finalTables = mpeLoadTableData('MPE_TABLES')
+        const finalReg =
+            finalTables && finalTables.rows
+                ? finalTables.rows.find((r) => r.libref === libref && r.dsn === dsn)
+                : null
+        const rkName = mpeAssignRetainedKey({
+            rows,
+            tableReg: finalReg,
+            baseData: makeTableLoader(libDataDir(libref))(dsn)
+        })
+        if (rkName && !headers.some((h) => h.toUpperCase() === rkName.toUpperCase())) {
+            headers.push(rkName)
+        }
         stageSubmission({
             dataDir: libDataDir(libref),
             stagingDir,
diff --git a/sas/mocks/sasjs/services/editors/stagedata.js b/sas/mocks/sasjs/services/editors/stagedata.js
index 0b0eb81..0566bf5 100644
--- a/sas/mocks/sasjs/services/editors/stagedata.js
+++ b/sas/mocks/sasjs/services/editors/stagedata.js
@@ -84,45 +84,21 @@ if (hookReg && hookReg.post_edit_hook) {
 // ─── RK assignment (mpe_loader) ──────────────────────────────────────────────
 // A table registered with rk_underlying is edited on its business key and the
 // RK column is withheld from the editor (see getdata.js), so the submitted rows
-// carry no RK. The loader derives it: %mpe_loader hands RK_UNDERLYING to
-// %bitemporal_dataloader, which keeps the RK the business key already holds and
-// mints the next value for a new key.
-
-const valueOf = (row, name) => {
-    if (row[name] !== undefined) return row[name]
-    const key = Object.keys(row).find((k) => k.toUpperCase() === name.toUpperCase())
-    return key === undefined ? undefined : row[key]
-}
+// carry no RK. The loader derives it, and MPE_SUBMIT records the staged width
+// - which includes the derived column.
 
 const finalTables = makeTableLoader(mpeDataDir)('MPE_TABLES')
 const finalReg =
     finalTables && finalTables.rows
         ? finalTables.rows.find((r) => r.libref === libref && r.dsn === dsn)
         : null
-const rkCols =
-    finalReg && finalReg.rk_underlying ? finalReg.rk_underlying.trim().split(/\s+/) : []
-const rkCol = finalReg && finalReg.buskey ? finalReg.buskey.trim() : ''
-const sendsRk =
-    rkCol !== '' && jsDataHeaders.some((h) => h.toUpperCase() === rkCol.toUpperCase())
-
-if (rkCols.length > 0 && rkCol !== '' && !sendsRk) {
-    const baseData = loadTableData(dsn) || { columns: [], rows: [] }
-    const rkName =
-        (baseData.columns.find((c) => c.name.toUpperCase() === rkCol.toUpperCase()) || {}).name ||
-        rkCol
-    const businessKey = (row) =>
-        rkCols.map((c) => String(valueOf(row, c) ?? '').trim().toUpperCase()).join('|')
-    const baseByKey = new Map()
-    let nextRk = 0
-    for (const row of baseData.rows) {
-        baseByKey.set(businessKey(row), valueOf(row, rkName))
-        const rk = Number(valueOf(row, rkName))
-        if (isFinite(rk) && rk > nextRk) nextRk = rk
-    }
-    for (const row of jsDataRows) {
-        const existing = baseByKey.get(businessKey(row))
-        row[rkName] = existing === undefined || existing === null ? ++nextRk : existing
-    }
+const rkName = mpeAssignRetainedKey({
+    rows: jsDataRows,
+    tableReg: finalReg,
+    baseData: loadTableData(dsn)
+})
+if (rkName && !jsDataHeaders.some((h) => h.toUpperCase() === rkName.toUpperCase())) {
+    jsDataHeaders.push(rkName)
 }
 
 // ─── Stage the data ──────────────────────────────────────────────────────────