diff --git a/SeaHaven.Services/Helpers/WorkOrderNumberRules.cs b/SeaHaven.Services/Helpers/WorkOrderNumberRules.cs
new file mode 100644
index 0000000..5ceb0ba
--- /dev/null
+++ b/SeaHaven.Services/Helpers/WorkOrderNumberRules.cs
@@ -0,0 +1,29 @@
+using System.Globalization;
+
+namespace SeaHaven.Services.Helpers
+{
+    public static class WorkOrderNumberRules
+    {
+        private const int MaximumLength = 11;
+        private const int MinimumProvisionalDigits = 5;
+
+        public static bool IsProvisional(string? woNumber)
+            => !string.IsNullOrWhiteSpace(woNumber)
+               && woNumber.Trim().StartsWith("SH", StringComparison.OrdinalIgnoreCase);
+
+        public static string FormatProvisional(long seed)
+        {
+            if (seed < 1)
+                throw new ArgumentOutOfRangeException(nameof(seed), "Provisional WO number seed must be positive.");
+
+            var digits = seed.ToString(CultureInfo.InvariantCulture)
+                .PadLeft(MinimumProvisionalDigits, '0');
+            var provisional = $"SH{digits}";
+
+            if (provisional.Length > MaximumLength)
+                throw new ArgumentOutOfRangeException(nameof(seed), "Provisional WO number exceeds the supported length.");
+
+            return provisional;
+        }
+    }
+}
diff --git a/SeaHaven.Services/Implementation/WorkOrderBoardCreateService.cs b/SeaHaven.Services/Implementation/WorkOrderBoardCreateService.cs
index 161e212..a5df677 100644
--- a/SeaHaven.Services/Implementation/WorkOrderBoardCreateService.cs
+++ b/SeaHaven.Services/Implementation/WorkOrderBoardCreateService.cs
@@ -284,9 +284,19 @@ namespace SeaHaven.Services.Implementation
                 return normalized;
             }
 
+            var maxId = await _mutationData.GetMaxWorkOrderIdAsync(CancellationToken.None);
             for (var attempt = 0; attempt < 20; attempt++)
             {
-                var candidate = await GenerateNextSequentialWoNumberAsync();
+                string candidate;
+                try
+                {
+                    candidate = WorkOrderNumberRules.FormatProvisional((long)maxId + 1 + attempt);
+                }
+                catch (ArgumentOutOfRangeException)
+                {
+                    break;
+                }
+
                 if (!await _boardDataService.InternalWoNumberExistsAsync(candidate, 0))
                     return candidate;
             }
@@ -294,15 +304,6 @@ namespace SeaHaven.Services.Implementation
             throw new WorkOrderBoardValidationException("WoNumberGenerationFailed", "Could not generate a unique WO number.");
         }
 
-        private async Task<string> GenerateNextSequentialWoNumberAsync()
-        {
-            var maxId = await _mutationData.GetMaxWorkOrderIdAsync(CancellationToken.None);
-            var seed = Math.Max(maxId + 1, 1);
-            if (!WorkOrderNumberNormalizer.TryNormalize(seed.ToString(), out var normalized, out _))
-                normalized = seed.ToString().PadLeft(11, '0');
-            return normalized;
-        }
-
         private async Task StageChangesAsync(
             int workOrderId,
             IEnumerable<WorkOrderBoardFieldMutations.BoardFieldChange> changes,
diff --git a/SeaHaven.Services/Implementation/WorkOrderBoardUpdateService.cs b/SeaHaven.Services/Implementation/WorkOrderBoardUpdateService.cs
index d0ea7f7..341bf63 100644
--- a/SeaHaven.Services/Implementation/WorkOrderBoardUpdateService.cs
+++ b/SeaHaven.Services/Implementation/WorkOrderBoardUpdateService.cs
@@ -362,6 +362,13 @@ namespace SeaHaven.Services.Implementation
             if (parsed == null)
                 throw new WorkOrderBoardValidationException("InvalidValue", $"Invalid lifecycleStatus: {value}");
 
+            if (parsed == LifecycleStatus.Completed && WorkOrderNumberRules.IsProvisional(workOrder.InternalWONumber))
+            {
+                throw new WorkOrderBoardValidationException(
+                    "ProvisionalWoNumber",
+                    "Replace the provisional SH work order number before marking the work order Completed.");
+            }
+
             var old = workOrder.LifecycleStatus?.ToString() ?? workOrder.Status;
             if (workOrder.LifecycleStatus == parsed)
                 return FieldChange.Unchanged(auditField);
diff --git a/SeaHavenIndustries.Tests/WorkOrderBoardCreateServiceTests.cs b/SeaHavenIndustries.Tests/WorkOrderBoardCreateServiceTests.cs
index a85376a..d3df3a9 100644
--- a/SeaHavenIndustries.Tests/WorkOrderBoardCreateServiceTests.cs
+++ b/SeaHavenIndustries.Tests/WorkOrderBoardCreateServiceTests.cs
@@ -44,8 +44,28 @@ public class WorkOrderBoardCreateServiceTests
         Assert.Equal(LifecycleStatus.Incomplete, result.LifecycleStatus);
         Assert.Equal("BK5", result.SiteCode);
         Assert.Equal(WorkOrderType.PM, result.WorkOrderType);
-        Assert.False(string.IsNullOrWhiteSpace(result.WoNumber));
-        Assert.Equal(11, result.WoNumber!.Length);
+        Assert.Equal("SH00001", result.WoNumber);
+    }
+
+    [Fact]
+    public async Task Create_BlankWoNumber_PreservesExistingNumericPlaceholderAndGeneratesShNumber()
+    {
+        var (context, service) = CreateSut();
+        context.workOrders.Add(new WorkOrder
+        {
+            InternalWONumber = "00000000123",
+            WorkerOrderNumber = "00000000123"
+        });
+        await context.SaveChangesAsync();
+
+        var result = await service.CreateAsync(new WorkOrderBoardCreateRequestDto
+        {
+            WorkOrderType = WorkOrderType.PM,
+            SiteCode = "BK5"
+        }, WorkOrderAccountTestHelpers.AccountUser(), "actor-1");
+
+        Assert.Equal("SH00002", result.WoNumber);
+        Assert.True(await context.workOrders.AnyAsync(w => w.InternalWONumber == "00000000123"));
     }
 
     [Fact]
diff --git a/SeaHavenIndustries.Tests/WorkOrderBoardUpdateServiceTests.cs b/SeaHavenIndustries.Tests/WorkOrderBoardUpdateServiceTests.cs
index 0c61a84..14fd7e8 100644
--- a/SeaHavenIndustries.Tests/WorkOrderBoardUpdateServiceTests.cs
+++ b/SeaHavenIndustries.Tests/WorkOrderBoardUpdateServiceTests.cs
@@ -153,6 +153,51 @@ public class WorkOrderBoardUpdateServiceTests
         Assert.False(afterStatus.IsPastDue);
     }
 
+    [Fact]
+    public async Task PatchField_ProvisionalWoNumber_BlocksCompletionUntilReplaced()
+    {
+        var (context, service) = CreateSut();
+        var wo = new WorkOrder
+        {
+            Id = 1,
+            InternalWONumber = "SH00001",
+            WorkerOrderNumber = "SH00001",
+            LifecycleStatus = LifecycleStatus.Scheduled,
+            ScheduledDate = DateTime.UtcNow.Date.AddDays(1),
+            RowVersion = new byte[] { 1, 0, 0, 0, 0, 0, 0, 1 }
+        };
+        context.workOrders.Add(wo);
+        await context.SaveChangesAsync();
+
+        var blocked = await Assert.ThrowsAsync<WorkOrderBoardValidationException>(() =>
+            service.PatchFieldAsync(1, new WorkOrderBoardPatchRequestDto
+            {
+                Field = WorkOrderBoardFieldNames.LifecycleStatus,
+                Value = "Complete",
+                WorkOrderVersion = ToVersion(wo)
+            }, "actor-1"));
+
+        Assert.Equal("ProvisionalWoNumber", blocked.Code);
+
+        var renumbered = await service.PatchFieldAsync(1, new WorkOrderBoardPatchRequestDto
+        {
+            Field = WorkOrderBoardFieldNames.WoNumber,
+            Value = "12345",
+            WorkOrderVersion = ToVersion(wo)
+        }, "actor-1");
+        Assert.Equal("00000012345", renumbered.WoNumber);
+
+        wo = await context.workOrders.SingleAsync(w => w.Id == 1);
+        var completed = await service.PatchFieldAsync(1, new WorkOrderBoardPatchRequestDto
+        {
+            Field = WorkOrderBoardFieldNames.LifecycleStatus,
+            Value = "Complete",
+            WorkOrderVersion = ToVersion(wo)
+        }, "actor-1");
+
+        Assert.Equal(LifecycleStatus.Completed, completed.LifecycleStatus);
+    }
+
     [Fact]
     public async Task PatchField_DueDateAlone_DoesNotClearPastDue()
     {