diff --git a/src/ScadaLink.ConfigurationDatabase/Configurations/AuditLogEntityTypeConfiguration.cs b/src/ScadaLink.ConfigurationDatabase/Configurations/AuditLogEntityTypeConfiguration.cs
index ca914d3..9ad90e0 100644
--- a/src/ScadaLink.ConfigurationDatabase/Configurations/AuditLogEntityTypeConfiguration.cs
+++ b/src/ScadaLink.ConfigurationDatabase/Configurations/AuditLogEntityTypeConfiguration.cs
@@ -15,7 +15,15 @@ public class AuditLogEntityTypeConfiguration : IEntityTypeConfiguration<AuditEve
     {
         builder.ToTable("AuditLog");
 
-        builder.HasKey(e => e.EventId);
+        // Composite PK includes OccurredAtUtc — required by the monthly partition scheme
+        // (ps_AuditLog_Month) so the clustered key is partition-aligned. EventId still
+        // needs to be globally unique for InsertIfNotExistsAsync idempotency, so a
+        // separate unique index is declared on EventId alone.
+        builder.HasKey(e => new { e.EventId, e.OccurredAtUtc });
+
+        builder.HasIndex(e => e.EventId)
+            .IsUnique()
+            .HasDatabaseName("UX_AuditLog_EventId");
 
         // Enum-as-string columns: bounded varchar(32) ASCII.
         builder.Property(e => e.Channel)
diff --git a/tests/ScadaLink.ConfigurationDatabase.Tests/Configurations/AuditLogEntityTypeConfigurationTests.cs b/tests/ScadaLink.ConfigurationDatabase.Tests/Configurations/AuditLogEntityTypeConfigurationTests.cs
index b70ee0f..9427442 100644
--- a/tests/ScadaLink.ConfigurationDatabase.Tests/Configurations/AuditLogEntityTypeConfigurationTests.cs
+++ b/tests/ScadaLink.ConfigurationDatabase.Tests/Configurations/AuditLogEntityTypeConfigurationTests.cs
@@ -28,8 +28,11 @@ public class AuditLogEntityTypeConfigurationTests : IDisposable
     }
 
     [Fact]
-    public void Configure_MapsToAuditLogTable_WithEventIdAsPrimaryKey()
+    public void Configure_MapsToAuditLogTable_WithCompositePrimaryKey()
     {
+        // Composite PK {EventId, OccurredAtUtc} is required by the partitioned
+        // AuditLog table — the clustered key must include the partition column
+        // (OccurredAtUtc) so each row can be located in its partition (#23 Bundle C).
         var entity = _context.Model.FindEntityType(typeof(AuditEvent));
 
         Assert.NotNull(entity);
@@ -37,8 +40,28 @@ public class AuditLogEntityTypeConfigurationTests : IDisposable
 
         var pk = entity.FindPrimaryKey();
         Assert.NotNull(pk);
-        var pkProperty = Assert.Single(pk!.Properties);
-        Assert.Equal(nameof(AuditEvent.EventId), pkProperty.Name);
+
+        var pkPropertyNames = pk!.Properties.Select(p => p.Name).ToArray();
+        Assert.Equal(new[] { nameof(AuditEvent.EventId), nameof(AuditEvent.OccurredAtUtc) }, pkPropertyNames);
+    }
+
+    [Fact]
+    public void Configure_DeclaresUniqueIndex_OnEventIdAlone_ForIdempotencyLookups()
+    {
+        // EventId remains globally unique (the idempotency key for
+        // InsertIfNotExistsAsync, per M1-T8) via a dedicated unique index that
+        // is independent of the composite PK.
+        var entity = _context.Model.FindEntityType(typeof(AuditEvent));
+        Assert.NotNull(entity);
+
+        var eventIdIndex = entity!.GetIndexes()
+            .SingleOrDefault(i => i.GetDatabaseName() == "UX_AuditLog_EventId");
+
+        Assert.NotNull(eventIdIndex);
+        Assert.True(eventIdIndex!.IsUnique);
+
+        var indexedProperty = Assert.Single(eventIdIndex.Properties);
+        Assert.Equal(nameof(AuditEvent.EventId), indexedProperty.Name);
     }
 
     [Fact]
@@ -56,7 +79,7 @@ public class AuditLogEntityTypeConfigurationTests : IDisposable
     }
 
     [Fact]
-    public void Configure_FiveExpectedIndexes_WithCorrectNames()
+    public void Configure_ExpectedIndexes_WithCorrectNames()
     {
         var entity = _context.Model.FindEntityType(typeof(AuditEvent));
         Assert.NotNull(entity);
@@ -66,6 +89,8 @@ public class AuditLogEntityTypeConfigurationTests : IDisposable
             .OrderBy(n => n, StringComparer.Ordinal)
             .ToList();
 
+        // Five reconciliation/query indexes from alog.md §4, plus the EventId unique
+        // index introduced alongside the composite PK (Bundle C).
         var expected = new[]
         {
             "IX_AuditLog_Channel_Status_Occurred",
@@ -73,6 +98,7 @@ public class AuditLogEntityTypeConfigurationTests : IDisposable
             "IX_AuditLog_OccurredAtUtc",
             "IX_AuditLog_Site_Occurred",
             "IX_AuditLog_Target_Occurred",
+            "UX_AuditLog_EventId",
         };
 
         Assert.Equal(expected, indexNames);