diff --git a/src/NATS.Server/JetStream/Cluster/JetStreamMetaGroup.cs b/src/NATS.Server/JetStream/Cluster/JetStreamMetaGroup.cs
index eddb230..fd6d82f 100644
--- a/src/NATS.Server/JetStream/Cluster/JetStreamMetaGroup.cs
+++ b/src/NATS.Server/JetStream/Cluster/JetStreamMetaGroup.cs
@@ -264,6 +264,101 @@ public sealed class JetStreamMetaGroup
         return Task.CompletedTask;
     }
 
+    // ---------------------------------------------------------------
+    // Validated assignment processing
+    // Go reference: jetstream_cluster.go:4541-5925
+    // ---------------------------------------------------------------
+
+    /// <summary>
+    /// Validates and processes a stream assignment.
+    /// Returns false if the assignment is invalid (empty name, null group).
+    /// Idempotent: duplicate assignments for the same stream name are accepted.
+    /// Go reference: jetstream_cluster.go:4541 processStreamAssignment.
+    /// </summary>
+    public bool ProcessStreamAssignment(StreamAssignment sa)
+    {
+        if (string.IsNullOrEmpty(sa.StreamName) || sa.Group == null)
+            return false;
+
+        AddStreamAssignment(sa);
+        return true;
+    }
+
+    /// <summary>
+    /// Updates an existing stream assignment's configuration.
+    /// Returns false if the stream does not exist.
+    /// Go reference: jetstream_cluster.go processUpdateStreamAssignment.
+    /// </summary>
+    public bool ProcessUpdateStreamAssignment(StreamAssignment sa)
+    {
+        if (!_assignments.TryGetValue(sa.StreamName, out var existing))
+            return false;
+
+        // Update the config while preserving consumers and other state
+        var updated = new StreamAssignment
+        {
+            StreamName = sa.StreamName,
+            Group = sa.Group,
+            ConfigJson = sa.ConfigJson,
+            Created = existing.Created,
+            SyncSubject = existing.SyncSubject,
+        };
+        // Copy consumers from old to new
+        foreach (var (name, ca) in existing.Consumers)
+            updated.Consumers[name] = ca;
+
+        _assignments[sa.StreamName] = updated;
+        return true;
+    }
+
+    /// <summary>
+    /// Removes a stream and all its consumers.
+    /// Returns false if stream didn't exist. Returns true if removed.
+    /// Go reference: jetstream_cluster.go processStreamRemoval.
+    /// </summary>
+    public bool ProcessStreamRemoval(string streamName)
+    {
+        if (!_assignments.ContainsKey(streamName))
+            return false;
+
+        RemoveStreamAssignment(streamName);
+        return true;
+    }
+
+    /// <summary>
+    /// Validates and processes a consumer assignment.
+    /// Returns false if the parent stream does not exist or consumer name is empty.
+    /// Go reference: jetstream_cluster.go:5300 processConsumerAssignment.
+    /// </summary>
+    public bool ProcessConsumerAssignment(ConsumerAssignment ca)
+    {
+        if (string.IsNullOrEmpty(ca.ConsumerName) || string.IsNullOrEmpty(ca.StreamName))
+            return false;
+
+        if (!_assignments.ContainsKey(ca.StreamName))
+            return false;
+
+        AddConsumerAssignment(ca.StreamName, ca);
+        return true;
+    }
+
+    /// <summary>
+    /// Removes a consumer assignment.
+    /// Returns false if stream or consumer doesn't exist.
+    /// Go reference: jetstream_cluster.go processConsumerRemoval.
+    /// </summary>
+    public bool ProcessConsumerRemoval(string streamName, string consumerName)
+    {
+        if (!_assignments.TryGetValue(streamName, out var sa))
+            return false;
+
+        if (!sa.Consumers.ContainsKey(consumerName))
+            return false;
+
+        RemoveConsumerAssignment(streamName, consumerName);
+        return true;
+    }
+
     // ---------------------------------------------------------------
     // Monitor-facing mutation methods
     // Called by JetStreamClusterMonitor when processing committed RAFT entries.
diff --git a/tests/NATS.Server.Tests/JetStream/Cluster/JetStreamAssignmentProcessingTests.cs b/tests/NATS.Server.Tests/JetStream/Cluster/JetStreamAssignmentProcessingTests.cs
new file mode 100644
index 0000000..5ed8cf9
--- /dev/null
+++ b/tests/NATS.Server.Tests/JetStream/Cluster/JetStreamAssignmentProcessingTests.cs
@@ -0,0 +1,169 @@
+using NATS.Server.JetStream.Cluster;
+
+namespace NATS.Server.Tests.JetStream.Cluster;
+
+/// <summary>
+/// Tests for validated stream/consumer assignment processing.
+/// Go reference: jetstream_cluster.go:4541-5925.
+/// </summary>
+public class JetStreamAssignmentProcessingTests
+{
+    [Fact]
+    public void ProcessStreamAssignment_validates_config()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        var sa = new StreamAssignment
+        {
+            StreamName = "valid-stream",
+            Group = new RaftGroup { Name = "rg-1", Peers = ["n1", "n2", "n3"] },
+            ConfigJson = """{"subjects":["test.>"]}""",
+        };
+
+        meta.ProcessStreamAssignment(sa).ShouldBeTrue();
+        meta.StreamCount.ShouldBe(1);
+    }
+
+    [Fact]
+    public void ProcessStreamAssignment_rejects_empty_name()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        var sa = CreateStreamAssignment("", "rg-1");
+        meta.ProcessStreamAssignment(sa).ShouldBeFalse();
+        meta.StreamCount.ShouldBe(0);
+    }
+
+    [Fact]
+    public void ProcessUpdateStreamAssignment_applies_config_change()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        meta.ProcessStreamAssignment(CreateStreamAssignment("updatable", "rg-u", """{"subjects":["old.>"]}"""));
+
+        var updated = CreateStreamAssignment("updatable", "rg-u", """{"subjects":["new.>"]}""");
+        meta.ProcessUpdateStreamAssignment(updated).ShouldBeTrue();
+
+        var assignment = meta.GetStreamAssignment("updatable");
+        assignment!.ConfigJson.ShouldContain("new.>");
+    }
+
+    [Fact]
+    public void ProcessUpdateStreamAssignment_returns_false_for_nonexistent()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        var sa = CreateStreamAssignment("ghost", "rg-g");
+        meta.ProcessUpdateStreamAssignment(sa).ShouldBeFalse();
+    }
+
+    [Fact]
+    public void ProcessConsumerAssignment_requires_existing_stream()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        var ca = new ConsumerAssignment
+        {
+            ConsumerName = "orphan-consumer",
+            StreamName = "nonexistent-stream",
+            Group = new RaftGroup { Name = "rg-c", Peers = ["n1", "n2", "n3"] },
+        };
+
+        meta.ProcessConsumerAssignment(ca).ShouldBeFalse();
+    }
+
+    [Fact]
+    public void ProcessConsumerAssignment_succeeds_with_existing_stream()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        meta.ProcessStreamAssignment(CreateStreamAssignment("s1", "rg-s1"));
+
+        var ca = new ConsumerAssignment
+        {
+            ConsumerName = "c1",
+            StreamName = "s1",
+            Group = new RaftGroup { Name = "rg-c1", Peers = ["n1", "n2", "n3"] },
+        };
+
+        meta.ProcessConsumerAssignment(ca).ShouldBeTrue();
+        meta.ConsumerCount.ShouldBe(1);
+    }
+
+    [Fact]
+    public void ProcessStreamRemoval_cascades_to_consumers()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        meta.ProcessStreamAssignment(CreateStreamAssignment("cascade", "rg-cas"));
+        meta.ProcessConsumerAssignment(new ConsumerAssignment
+        {
+            ConsumerName = "c1",
+            StreamName = "cascade",
+            Group = new RaftGroup { Name = "rg-c1", Peers = ["n1", "n2", "n3"] },
+        });
+
+        meta.ProcessStreamRemoval("cascade").ShouldBeTrue();
+        meta.StreamCount.ShouldBe(0);
+        meta.ConsumerCount.ShouldBe(0);
+    }
+
+    [Fact]
+    public void ProcessStreamRemoval_returns_false_for_nonexistent()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        meta.ProcessStreamRemoval("nope").ShouldBeFalse();
+    }
+
+    [Fact]
+    public void ProcessConsumerRemoval_returns_false_for_nonexistent_stream()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        meta.ProcessConsumerRemoval("ghost", "c1").ShouldBeFalse();
+    }
+
+    [Fact]
+    public void ProcessConsumerRemoval_returns_false_for_nonexistent_consumer()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        meta.ProcessStreamAssignment(CreateStreamAssignment("s1", "rg-s1"));
+        meta.ProcessConsumerRemoval("s1", "nope").ShouldBeFalse();
+    }
+
+    [Fact]
+    public void ProcessConsumerRemoval_succeeds()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        meta.ProcessStreamAssignment(CreateStreamAssignment("s1", "rg-s1"));
+        meta.ProcessConsumerAssignment(new ConsumerAssignment
+        {
+            ConsumerName = "c1",
+            StreamName = "s1",
+            Group = new RaftGroup { Name = "rg-c1", Peers = ["n1", "n2"] },
+        });
+
+        meta.ProcessConsumerRemoval("s1", "c1").ShouldBeTrue();
+        meta.ConsumerCount.ShouldBe(0);
+    }
+
+    [Fact]
+    public void ProcessUpdateStreamAssignment_preserves_consumers()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        meta.ProcessStreamAssignment(CreateStreamAssignment("s1", "rg-s1", """{"subjects":["old"]}"""));
+        meta.ProcessConsumerAssignment(new ConsumerAssignment
+        {
+            ConsumerName = "c1",
+            StreamName = "s1",
+            Group = new RaftGroup { Name = "rg-c1", Peers = ["n1", "n2"] },
+        });
+
+        var updated = CreateStreamAssignment("s1", "rg-s1", """{"subjects":["new"]}""");
+        meta.ProcessUpdateStreamAssignment(updated).ShouldBeTrue();
+
+        meta.ConsumerCount.ShouldBe(1);
+        meta.GetConsumerAssignment("s1", "c1").ShouldNotBeNull();
+    }
+
+    // Helper to create a StreamAssignment (StreamName is `required` so we must always provide it)
+    private static StreamAssignment CreateStreamAssignment(string name, string groupName, string config = "{}")
+        => new()
+        {
+            StreamName = name,
+            Group = new RaftGroup { Name = groupName, Peers = ["n1", "n2", "n3"] },
+            ConfigJson = config,
+        };
+}