diff --git a/js/algorithm.js b/js/algorithm.js
index 9a64ad3..589d039 100644
--- a/js/algorithm.js
+++ b/js/algorithm.js
@@ -78,16 +78,26 @@ function scoreCandidate(employee, isoDate, allHistory, randomFn) {
 
 // Returns { assignments: [{ date, employeeId, manual: false }], skippedDates: [] }
 // slotsPerDay controls how many top-scored candidates are picked per day (default 1).
-export function generatePlan(workdays, employees, history, randomFn = Math.random, slotsPerDay = 1) {
-  const monthAssignments = [];
+export function generatePlan(workdays, employees, history, randomFn = Math.random, slotsPerDay = 1, preAssigned = []) {
+  // preAssigned entries (e.g. manual picks for this month) are kept fixed: they
+  // occupy slots, count toward eligibility, and are never double-booked. Only
+  // the remaining slots per day are auto-filled. Returns only the newly
+  // generated assignments (not the preAssigned ones).
+  const monthAssignments = [...preAssigned];
+  const generated = [];
   const skippedDates = [];
 
   for (const date of workdays) {
+    const taken = monthAssignments.filter(a => a.date === date);
+    const needed = slotsPerDay - taken.length;
+    if (needed <= 0) continue; // already fully staffed (e.g. by manual entries)
+
+    const takenIds = new Set(taken.map(a => a.employeeId));
     const candidates = employees.filter(e =>
-      isEligible(e, date, monthAssignments, history)
+      !takenIds.has(e.id) && isEligible(e, date, monthAssignments, history)
     );
 
-    if (candidates.length < slotsPerDay) {
+    if (candidates.length < needed) {
       skippedDates.push(date);
       continue;
     }
@@ -96,10 +106,12 @@ export function generatePlan(workdays, employees, history, randomFn = Math.rando
       .map(e => ({ employee: e, score: scoreCandidate(e, date, history, randomFn) }))
       .sort((a, b) => b.score - a.score);
 
-    for (let i = 0; i < slotsPerDay; i++) {
-      monthAssignments.push({ date, employeeId: scored[i].employee.id, manual: false });
+    for (let i = 0; i < needed; i++) {
+      const entry = { date, employeeId: scored[i].employee.id, manual: false };
+      monthAssignments.push(entry);
+      generated.push(entry);
     }
   }
 
-  return { assignments: monthAssignments, skippedDates };
+  return { assignments: generated, skippedDates };
 }
diff --git a/js/calendar.js b/js/calendar.js
index 6b67b65..8450bcb 100644
--- a/js/calendar.js
+++ b/js/calendar.js
@@ -123,12 +123,15 @@ export function renderCalendar() {
     const workdays = getWorkdays(currentYear, currentMonth,
       state.calendar.holidays, state.calendar.companyClosures);
     const prefix = currentYear + '-' + String(currentMonth).padStart(2, '0');
+    // Preserve manual picks for this month; only auto-entries get regenerated.
+    const manualEntries = state.history.filter(h => h.date.startsWith(prefix) && h.manual);
     const historyWithoutMonth = state.history.filter(h => !h.date.startsWith(prefix));
-    // Reset month entries from history so dedup-by-(date,employeeId) doesn't keep stale rows
-    state.history = historyWithoutMonth;
-    const result = generatePlan(workdays, state.employees, historyWithoutMonth, Math.random, state.settings.moderatorsPerDay);
-    currentPlan = result.assignments;
-    addHistoryEntries(state, currentPlan);
+    // Drop stale auto-entries for the month, keep history + manual picks
+    state.history = historyWithoutMonth.concat(manualEntries);
+    const result = generatePlan(workdays, state.employees, historyWithoutMonth,
+      Math.random, state.settings.moderatorsPerDay, manualEntries);
+    addHistoryEntries(state, result.assignments);
+    currentPlan = manualEntries.concat(result.assignments);
     saveState();
     renderSkippedWarning(warnBox, result.skippedDates);
     renderGrid(wrapper, exportBtn);
diff --git a/tests/algorithm.test.mjs b/tests/algorithm.test.mjs
index 8b9c957..0e37250 100644
--- a/tests/algorithm.test.mjs
+++ b/tests/algorithm.test.mjs
@@ -187,4 +187,31 @@ function emp(id, constraints = {}) {
   assert.deepEqual(result.skippedDates, ['2026-04-02'], 'Thu in skippedDates');
 }
 
+// preAssigned: manual pick occupies a slot, only remaining slots auto-filled, no double-book
+{
+  const employees = [emp('alice'), emp('bob'), emp('clara')];
+  const manual = [{ date: '2026-04-01', employeeId: 'alice', manual: true }];
+  const result = generatePlan(['2026-04-01'], employees, [], fixed, 2, manual);
+  assert.equal(result.assignments.length, 1, 'slotsPerDay=2 with 1 manual → 1 auto entry');
+  assert.notEqual(result.assignments[0].employeeId, 'alice', 'manual employee not double-booked');
+  assert.equal(result.assignments[0].manual, false, 'auto entry has manual:false');
+}
+
+// preAssigned: day fully staffed by manual entries → nothing generated, not skipped
+{
+  const employees = [emp('alice'), emp('bob')];
+  const manual = [{ date: '2026-04-01', employeeId: 'alice', manual: true }];
+  const result = generatePlan(['2026-04-01'], employees, [], fixed, 1, manual);
+  assert.equal(result.assignments.length, 0, 'fully manual day → no auto entries');
+  assert.deepEqual(result.skippedDates, [], 'fully manual day not skipped');
+}
+
+// preAssigned counts toward maxPerMonth eligibility
+{
+  const employees = [emp('alice', { maxPerMonth: 1 }), emp('bob'), emp('clara')];
+  const manual = [{ date: '2026-04-01', employeeId: 'alice', manual: true }];
+  const result = generatePlan(['2026-04-02'], employees, [], fixed, 1, manual);
+  assert.notEqual(result.assignments[0].employeeId, 'alice', 'manual entry counts toward maxPerMonth');
+}
+
 console.log('All algorithm tests passed.');