diff --git a/src/NATS.Server/JetStream/MirrorSource/SourceCoordinator.cs b/src/NATS.Server/JetStream/MirrorSource/SourceCoordinator.cs
index 0c8c669..da1be16 100644
--- a/src/NATS.Server/JetStream/MirrorSource/SourceCoordinator.cs
+++ b/src/NATS.Server/JetStream/MirrorSource/SourceCoordinator.cs
@@ -18,6 +18,13 @@ public sealed class SourceCoordinator
 
     public async Task OnOriginAppendAsync(StoredMessage message, CancellationToken ct)
     {
+        if (!string.IsNullOrWhiteSpace(_sourceConfig.SourceAccount)
+            && !string.IsNullOrWhiteSpace(message.Account)
+            && !string.Equals(_sourceConfig.SourceAccount, message.Account, StringComparison.Ordinal))
+        {
+            return;
+        }
+
         var subject = message.Subject;
         if (!string.IsNullOrWhiteSpace(_sourceConfig.SubjectTransformPrefix))
             subject = $"{_sourceConfig.SubjectTransformPrefix}{subject}";
diff --git a/src/NATS.Server/JetStream/Storage/StoredMessage.cs b/src/NATS.Server/JetStream/Storage/StoredMessage.cs
index b40f873..47d87b7 100644
--- a/src/NATS.Server/JetStream/Storage/StoredMessage.cs
+++ b/src/NATS.Server/JetStream/Storage/StoredMessage.cs
@@ -6,5 +6,6 @@ public sealed class StoredMessage
     public string Subject { get; init; } = string.Empty;
     public ReadOnlyMemory<byte> Payload { get; init; }
     public DateTime TimestampUtc { get; init; } = DateTime.UtcNow;
+    public string? Account { get; init; }
     public bool Redelivered { get; init; }
 }
diff --git a/tests/NATS.Server.Tests/JetStream/JetStreamMirrorSourceStrictRuntimeTests.cs b/tests/NATS.Server.Tests/JetStream/JetStreamMirrorSourceStrictRuntimeTests.cs
new file mode 100644
index 0000000..dd58e9a
--- /dev/null
+++ b/tests/NATS.Server.Tests/JetStream/JetStreamMirrorSourceStrictRuntimeTests.cs
@@ -0,0 +1,39 @@
+using NATS.Server.JetStream.MirrorSource;
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.Tests.JetStream;
+
+public class JetStreamMirrorSourceStrictRuntimeTests
+{
+    [Fact]
+    public async Task Mirror_source_transform_and_cross_account_filters_follow_runtime_contract()
+    {
+        var sourceTarget = new MemStore();
+        var source = new SourceCoordinator(sourceTarget, new StreamSourceConfig
+        {
+            Name = "SRC",
+            SubjectTransformPrefix = "agg.",
+            SourceAccount = "A",
+        });
+
+        await source.OnOriginAppendAsync(new StoredMessage
+        {
+            Sequence = 1,
+            Subject = "orders.created",
+            Payload = "ok"u8.ToArray(),
+            Account = "A",
+        }, default);
+        await source.OnOriginAppendAsync(new StoredMessage
+        {
+            Sequence = 2,
+            Subject = "orders.created",
+            Payload = "skip"u8.ToArray(),
+            Account = "B",
+        }, default);
+
+        var state = await sourceTarget.GetStateAsync(default);
+        state.Messages.ShouldBe((ulong)1);
+        (await sourceTarget.LoadAsync(1, default))!.Subject.ShouldBe("agg.orders.created");
+    }
+}