diff --git a/src/NATS.Server/JetStream/Cluster/ClusterAssignmentTypes.cs b/src/NATS.Server/JetStream/Cluster/ClusterAssignmentTypes.cs
index 9650d83..b227cdc 100644
--- a/src/NATS.Server/JetStream/Cluster/ClusterAssignmentTypes.cs
+++ b/src/NATS.Server/JetStream/Cluster/ClusterAssignmentTypes.cs
@@ -14,6 +14,18 @@ public sealed class RaftGroup
     public string Cluster { get; set; } = string.Empty;
     public string Preferred { get; set; } = string.Empty;
 
+    /// <summary>
+    /// Optional desired replica count for this group.
+    /// When set, ProcessAddPeer uses this to detect under-replication.
+    /// Go reference: jetstream_cluster.go:2284 sa.missingPeers() — len(Peers) &lt; Config.Replicas.
+    /// </summary>
+    public int DesiredReplicas { get; set; }
+
+    /// <summary>
+    /// True when <see cref="DesiredReplicas"/> has been explicitly set (non-zero).
+    /// </summary>
+    public bool HasDesiredReplicas => DesiredReplicas > 0;
+
     public int QuorumSize => (Peers.Count / 2) + 1;
     public bool HasQuorum(int ackCount) => ackCount >= QuorumSize;
 }
diff --git a/tests/NATS.Server.Tests/JetStream/Cluster/PeerManagementTests.cs b/tests/NATS.Server.Tests/JetStream/Cluster/PeerManagementTests.cs
new file mode 100644
index 0000000..ce570f9
--- /dev/null
+++ b/tests/NATS.Server.Tests/JetStream/Cluster/PeerManagementTests.cs
@@ -0,0 +1,433 @@
+// Go parity: golang/nats-server/server/jetstream_cluster.go:2290-2439
+// Covers: ProcessAddPeer, ProcessRemovePeer, RemovePeerFromStream, RemapStreamAssignment —
+// peer-driven stream reassignment in the JetStreamMetaGroup.
+using NATS.Server.JetStream.Cluster;
+using NATS.Server.JetStream.Models;
+
+namespace NATS.Server.Tests.JetStream.Cluster;
+
+/// <summary>
+/// Tests for JetStreamMetaGroup peer management and stream reassignment.
+/// Go reference: jetstream_cluster.go:2290-2439 (processAddPeer, processRemovePeer,
+/// removePeerFromStreamLocked, remapStreamAssignment).
+/// </summary>
+public class PeerManagementTests
+{
+    // ---------------------------------------------------------------
+    // ProcessAddPeer — peer registration
+    // Go reference: jetstream_cluster.go:2290 processAddPeer
+    // ---------------------------------------------------------------
+
+    [Fact]
+    public void ProcessAddPeer_registers_new_peer()
+    {
+        // Go reference: jetstream_cluster.go:2290 processAddPeer — peer is tracked
+        var meta = new JetStreamMetaGroup(3);
+
+        meta.ProcessAddPeer("peer-1");
+
+        meta.GetKnownPeers().ShouldContain("peer-1");
+    }
+
+    [Fact]
+    public void ProcessAddPeer_registers_multiple_peers_independently()
+    {
+        // Go reference: jetstream_cluster.go:2290 — each peer is independently tracked
+        var meta = new JetStreamMetaGroup(3);
+
+        meta.ProcessAddPeer("peer-1");
+        meta.ProcessAddPeer("peer-2");
+        meta.ProcessAddPeer("peer-3");
+
+        var known = meta.GetKnownPeers();
+        known.Count.ShouldBe(3);
+        known.ShouldContain("peer-1");
+        known.ShouldContain("peer-2");
+        known.ShouldContain("peer-3");
+    }
+
+    [Fact]
+    public void ProcessAddPeer_duplicate_add_is_idempotent()
+    {
+        // AddKnownPeer uses a HashSet so duplicates do not inflate the count.
+        var meta = new JetStreamMetaGroup(3);
+
+        meta.ProcessAddPeer("peer-1");
+        meta.ProcessAddPeer("peer-1");
+
+        meta.GetKnownPeers().Count.ShouldBe(1);
+    }
+
+    // ---------------------------------------------------------------
+    // ProcessAddPeer — under-replication detection
+    // Go reference: jetstream_cluster.go:2311-2339 missingPeers + peer append
+    // ---------------------------------------------------------------
+
+    [Fact]
+    public void ProcessAddPeer_triggers_rereplication_of_underreplicated_stream()
+    {
+        // Go reference: jetstream_cluster.go:2315 sa.missingPeers() — adds new peer to group
+        var meta = new JetStreamMetaGroup(3); // leader by default (selfIndex == leaderIndex == 1)
+
+        // Stream assigned with 2 peers but DesiredReplicas == 3 → under-replicated
+        var group = new RaftGroup
+        {
+            Name = "orders-rg",
+            Peers = ["peer-1", "peer-2"],
+            DesiredReplicas = 3,
+        };
+        var sa = new StreamAssignment { StreamName = "ORDERS", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        meta.ProcessAddPeer("peer-3");
+
+        var updated = meta.GetStreamAssignment("ORDERS")!;
+        updated.Group.Peers.ShouldContain("peer-3");
+        updated.Group.Peers.Count.ShouldBe(3);
+    }
+
+    [Fact]
+    public void ProcessAddPeer_does_not_add_peer_to_fully_replicated_stream()
+    {
+        // Go reference: jetstream_cluster.go:2315 missingPeers() returns false when at desired count
+        var meta = new JetStreamMetaGroup(3);
+
+        var group = new RaftGroup
+        {
+            Name = "events-rg",
+            Peers = ["peer-1", "peer-2", "peer-3"],
+            DesiredReplicas = 3,
+        };
+        var sa = new StreamAssignment { StreamName = "EVENTS", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        meta.ProcessAddPeer("peer-4");
+
+        var updated = meta.GetStreamAssignment("EVENTS")!;
+        updated.Group.Peers.Count.ShouldBe(3);
+        updated.Group.Peers.ShouldNotContain("peer-4");
+    }
+
+    [Fact]
+    public void ProcessAddPeer_does_not_add_peer_already_in_group()
+    {
+        // Peer already a member — should not be added twice.
+        var meta = new JetStreamMetaGroup(3);
+
+        var group = new RaftGroup
+        {
+            Name = "logs-rg",
+            Peers = ["peer-1"],
+            DesiredReplicas = 2,
+        };
+        var sa = new StreamAssignment { StreamName = "LOGS", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        meta.ProcessAddPeer("peer-1");
+
+        var updated = meta.GetStreamAssignment("LOGS")!;
+        updated.Group.Peers.Count.ShouldBe(1);
+    }
+
+    [Fact]
+    public void ProcessAddPeer_non_leader_does_not_modify_assignments()
+    {
+        // Go reference: jetstream_cluster.go:2301 — only leader triggers re-assignment
+        var meta = new JetStreamMetaGroup(3, selfIndex: 2); // not leader
+
+        var group = new RaftGroup
+        {
+            Name = "rg",
+            Peers = ["peer-1"],
+            DesiredReplicas = 3,
+        };
+        var sa = new StreamAssignment { StreamName = "S", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        meta.ProcessAddPeer("peer-2");
+
+        // Peer is registered but stream is not modified since not leader.
+        meta.GetKnownPeers().ShouldContain("peer-2");
+        meta.GetStreamAssignment("S")!.Group.Peers.Count.ShouldBe(1);
+    }
+
+    // ---------------------------------------------------------------
+    // ProcessRemovePeer — stream reassignment
+    // Go reference: jetstream_cluster.go:2342 processRemovePeer
+    // ---------------------------------------------------------------
+
+    [Fact]
+    public void ProcessRemovePeer_reassigns_streams_away_from_peer()
+    {
+        // Go reference: jetstream_cluster.go:2385-2392 — streams with removed peer get remapped
+        var meta = new JetStreamMetaGroup(3);
+
+        // Register three peers
+        meta.AddKnownPeer("peer-1");
+        meta.AddKnownPeer("peer-2");
+        meta.AddKnownPeer("peer-3");
+
+        var group = new RaftGroup
+        {
+            Name = "rg",
+            Peers = ["peer-1", "peer-2"],
+        };
+        var sa = new StreamAssignment { StreamName = "ORDERS", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        meta.ProcessRemovePeer("peer-1");
+
+        var updated = meta.GetStreamAssignment("ORDERS")!;
+        updated.Group.Peers.ShouldNotContain("peer-1");
+    }
+
+    [Fact]
+    public void ProcessRemovePeer_removes_peer_from_known_peers()
+    {
+        // Go reference: jetstream_cluster.go:2342 — peer is de-registered
+        var meta = new JetStreamMetaGroup(3);
+        meta.AddKnownPeer("peer-1");
+
+        meta.ProcessRemovePeer("peer-1");
+
+        meta.GetKnownPeers().ShouldNotContain("peer-1");
+    }
+
+    [Fact]
+    public void ProcessRemovePeer_unknown_peer_is_noop()
+    {
+        // Go reference: jetstream_cluster.go:2342 — no crash when peer not known
+        var meta = new JetStreamMetaGroup(3);
+        var group = new RaftGroup { Name = "rg", Peers = ["peer-2", "peer-3"] };
+        var sa = new StreamAssignment { StreamName = "S", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        // Should not throw
+        meta.ProcessRemovePeer("peer-99");
+
+        // Stream unaffected
+        meta.GetStreamAssignment("S")!.Group.Peers.ShouldContain("peer-2");
+        meta.GetStreamAssignment("S")!.Group.Peers.ShouldContain("peer-3");
+    }
+
+    [Fact]
+    public void ProcessRemovePeer_non_leader_only_deregisters_peer()
+    {
+        // Go reference: jetstream_cluster.go:2378 — non-leader skips re-assignment
+        var meta = new JetStreamMetaGroup(3, selfIndex: 2);
+        meta.AddKnownPeer("peer-1");
+        meta.AddKnownPeer("peer-2");
+
+        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2"] };
+        var sa = new StreamAssignment { StreamName = "S", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        meta.ProcessRemovePeer("peer-1");
+
+        // Peer removed from known set
+        meta.GetKnownPeers().ShouldNotContain("peer-1");
+
+        // Stream assignments are NOT modified by a non-leader
+        meta.GetStreamAssignment("S")!.Group.Peers.ShouldContain("peer-1");
+    }
+
+    // ---------------------------------------------------------------
+    // RemovePeerFromStream
+    // Go reference: jetstream_cluster.go:2403 removePeerFromStreamLocked
+    // ---------------------------------------------------------------
+
+    [Fact]
+    public void RemovePeerFromStream_removes_peer_from_group()
+    {
+        // Go reference: jetstream_cluster.go:2404 — peer is removed from stream group
+        var meta = new JetStreamMetaGroup(3);
+        meta.AddKnownPeer("peer-1");
+        meta.AddKnownPeer("peer-2");
+        meta.AddKnownPeer("peer-3");
+
+        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2", "peer-3"] };
+        var sa = new StreamAssignment { StreamName = "EVENTS", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        meta.RemovePeerFromStream("EVENTS", "peer-2");
+
+        var updated = meta.GetStreamAssignment("EVENTS")!;
+        updated.Group.Peers.ShouldNotContain("peer-2");
+    }
+
+    [Fact]
+    public void RemovePeerFromStream_returns_false_for_nonexistent_stream()
+    {
+        // RemovePeerFromStream silently returns false when stream not found.
+        var meta = new JetStreamMetaGroup(3);
+
+        var result = meta.RemovePeerFromStream("GHOST", "peer-1");
+
+        result.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void RemovePeerFromStream_returns_false_when_peer_not_in_group()
+    {
+        // Peer not a member of the stream's group.
+        var meta = new JetStreamMetaGroup(3);
+
+        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2"] };
+        var sa = new StreamAssignment { StreamName = "S", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        var result = meta.RemovePeerFromStream("S", "peer-99");
+
+        result.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void RemovePeerFromStream_replaces_peer_when_replacement_available()
+    {
+        // Go reference: jetstream_cluster.go:7088-7094 — replacement peer picked from available pool
+        var meta = new JetStreamMetaGroup(3);
+        meta.AddKnownPeer("peer-1");
+        meta.AddKnownPeer("peer-2");
+        meta.AddKnownPeer("peer-3"); // replacement candidate
+
+        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2"] };
+        var sa = new StreamAssignment { StreamName = "ORDERS", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        var result = meta.RemovePeerFromStream("ORDERS", "peer-1");
+
+        result.ShouldBeTrue();
+        var updated = meta.GetStreamAssignment("ORDERS")!;
+        updated.Group.Peers.ShouldNotContain("peer-1");
+        updated.Group.Peers.Count.ShouldBe(2);
+        updated.Group.Peers.ShouldContain("peer-3");
+    }
+
+    [Fact]
+    public void RemovePeerFromStream_shrinks_group_when_no_replacement_available()
+    {
+        // Go reference: jetstream_cluster.go:7102-7110 — R>1 bare removal fallback
+        var meta = new JetStreamMetaGroup(3);
+        // Only peer-1 and peer-2 are known; peer-1 is in the group; no replacement
+        meta.AddKnownPeer("peer-1");
+        meta.AddKnownPeer("peer-2");
+
+        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2"] };
+        var sa = new StreamAssignment { StreamName = "LOGS", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        var result = meta.RemovePeerFromStream("LOGS", "peer-1");
+
+        // No replacement found → group shrinks
+        result.ShouldBeFalse();
+        var updated = meta.GetStreamAssignment("LOGS")!;
+        updated.Group.Peers.ShouldNotContain("peer-1");
+        updated.Group.Peers.Count.ShouldBe(1);
+    }
+
+    // ---------------------------------------------------------------
+    // RemapStreamAssignment
+    // Go reference: jetstream_cluster.go:7077 remapStreamAssignment
+    // ---------------------------------------------------------------
+
+    [Fact]
+    public void RemapStreamAssignment_selects_new_peers()
+    {
+        // Go reference: jetstream_cluster.go:7077 — retain existing minus removed, add candidate
+        var meta = new JetStreamMetaGroup(3);
+
+        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2", "peer-3"] };
+        var sa = new StreamAssignment { StreamName = "EVENTS", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        var available = new List<string> { "peer-1", "peer-2", "peer-3", "peer-4" };
+        var result = meta.RemapStreamAssignment(sa, available, removePeer: "peer-3");
+
+        result.ShouldBeTrue();
+        var updated = meta.GetStreamAssignment("EVENTS")!;
+        updated.Group.Peers.ShouldNotContain("peer-3");
+        updated.Group.Peers.Count.ShouldBe(3);
+        updated.Group.Peers.ShouldContain("peer-4");
+    }
+
+    [Fact]
+    public void RemapStreamAssignment_retains_existing_peers()
+    {
+        // Retained peers (not removed) remain in the new assignment.
+        var meta = new JetStreamMetaGroup(3);
+
+        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2", "peer-3"] };
+        var sa = new StreamAssignment { StreamName = "S", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        var available = new List<string> { "peer-1", "peer-2", "peer-3", "peer-4" };
+        meta.RemapStreamAssignment(sa, available, removePeer: "peer-1");
+
+        var updated = meta.GetStreamAssignment("S")!;
+        updated.Group.Peers.ShouldContain("peer-2");
+        updated.Group.Peers.ShouldContain("peer-3");
+    }
+
+    [Fact]
+    public void RemapStreamAssignment_returns_false_when_no_replacement()
+    {
+        // Go reference: jetstream_cluster.go:7098-7110 — no placement, R1 returns false
+        var meta = new JetStreamMetaGroup(3);
+
+        var group = new RaftGroup { Name = "rg", Peers = ["peer-1"] };
+        var sa = new StreamAssignment { StreamName = "R1", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        var available = new List<string> { "peer-1" };
+        var result = meta.RemapStreamAssignment(sa, available, removePeer: "peer-1");
+
+        // Only peer-1 available and it is the removed one → nothing to add
+        result.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void RemapStreamAssignment_empty_available_shrinks_group()
+    {
+        // When the available-peer list is empty, the group simply loses the removed peer.
+        var meta = new JetStreamMetaGroup(3);
+
+        var group = new RaftGroup { Name = "rg", Peers = ["peer-1", "peer-2"] };
+        var sa = new StreamAssignment { StreamName = "S", Group = group };
+        meta.AddStreamAssignment(sa);
+
+        var result = meta.RemapStreamAssignment(sa, [], removePeer: "peer-1");
+
+        result.ShouldBeFalse();
+        meta.GetStreamAssignment("S")!.Group.Peers.ShouldNotContain("peer-1");
+    }
+
+    // ---------------------------------------------------------------
+    // AddKnownPeer / RemoveKnownPeer
+    // ---------------------------------------------------------------
+
+    [Fact]
+    public void AddKnownPeer_and_RemoveKnownPeer_are_consistent()
+    {
+        var meta = new JetStreamMetaGroup(3);
+
+        meta.AddKnownPeer("p1");
+        meta.AddKnownPeer("p2");
+        meta.RemoveKnownPeer("p1");
+
+        var known = meta.GetKnownPeers();
+        known.ShouldNotContain("p1");
+        known.ShouldContain("p2");
+    }
+
+    [Fact]
+    public void RemoveKnownPeer_unknown_peer_is_noop()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        meta.AddKnownPeer("p1");
+
+        // Should not throw
+        meta.RemoveKnownPeer("p99");
+
+        meta.GetKnownPeers().Count.ShouldBe(1);
+    }
+}