diff --git a/AGENTS.md b/AGENTS.md
index d030b130a9f..25d7bb60d18 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -1,6 +1,7 @@
 # Repository workflow
 
 - Open pull requests against `dotnet/orleans`.
+- Unless otherwise specified, create new branches from `main` in the upstream `dotnet/orleans` repository (`upstream/main`).
 - Do not push feature branches directly to `dotnet/orleans` or the `upstream` remote.
 - Push feature branches to your personal fork, which is configured as `origin`.
 - Create PRs using `dotnet/orleans` as the base repository and `origin`/personal fork branches as the head.
@@ -8,6 +9,8 @@
 Example:
 
 ```powershell
+git fetch upstream main
+git switch -c <branch> upstream/main
 git push --set-upstream origin <branch>
 gh pr create --repo dotnet/orleans --base main --head ReubenBond:<branch>
 ```
diff --git a/src/Orleans.Core/Messaging/Message.cs b/src/Orleans.Core/Messaging/Message.cs
index 807555c397f..f327a284ff3 100644
--- a/src/Orleans.Core/Messaging/Message.cs
+++ b/src/Orleans.Core/Messaging/Message.cs
@@ -11,6 +11,7 @@ internal sealed class Message : ISpanFormattable, IMessageReceiverCache
     {
         public const int LENGTH_HEADER_SIZE = 8;
         public const int LENGTH_META_HEADER = 4;
+        internal const int MaxCacheInvalidationHeaderEntries = 16;
 
         [NonSerialized]
         private short _retryCount;
@@ -282,15 +283,16 @@ public bool IsExpirableMessage()
         internal void AddToCacheInvalidationHeader(GrainAddress invalidAddress, GrainAddress? validAddress)
         {
             var grainAddressCacheUpdate = new GrainAddressCacheUpdate(invalidAddress, validAddress);
-            if (_cacheInvalidationHeader is null)
+            var cacheInvalidationHeader = _cacheInvalidationHeader;
+            if (cacheInvalidationHeader is null)
             {
                 var newList = new List<GrainAddressCacheUpdate> { grainAddressCacheUpdate };
-                if (Interlocked.CompareExchange(ref _cacheInvalidationHeader, newList, null) is not null)
+                if (Interlocked.CompareExchange(ref _cacheInvalidationHeader, newList, null) is { } existingCacheInvalidationHeader)
                 {
                     // Another thread initialized it, add to the existing list
-                    lock (_cacheInvalidationHeader)
+                    lock (existingCacheInvalidationHeader)
                     {
-                        _cacheInvalidationHeader.Add(grainAddressCacheUpdate);
+                        AddCacheInvalidationHeaderEntry(existingCacheInvalidationHeader, grainAddressCacheUpdate);
                     }
                 }
                 else
@@ -300,13 +302,36 @@ internal void AddToCacheInvalidationHeader(GrainAddress invalidAddress, GrainAdd
             }
             else
             {
-                lock (_cacheInvalidationHeader)
+                lock (cacheInvalidationHeader)
                 {
-                    _cacheInvalidationHeader.Add(grainAddressCacheUpdate);
+                    AddCacheInvalidationHeaderEntry(cacheInvalidationHeader, grainAddressCacheUpdate);
                 }
             }
         }
 
+        private static void AddCacheInvalidationHeaderEntry(List<GrainAddressCacheUpdate> cacheInvalidationHeader, GrainAddressCacheUpdate grainAddressCacheUpdate)
+        {
+            if (cacheInvalidationHeader.Count >= MaxCacheInvalidationHeaderEntries || ContainsCacheInvalidationHeaderEntry(cacheInvalidationHeader, grainAddressCacheUpdate.GrainId))
+            {
+                return;
+            }
+
+            cacheInvalidationHeader.Add(grainAddressCacheUpdate);
+        }
+
+        private static bool ContainsCacheInvalidationHeaderEntry(List<GrainAddressCacheUpdate> cacheInvalidationHeader, GrainId grainId)
+        {
+            foreach (var entry in cacheInvalidationHeader)
+            {
+                if (entry.GrainId.Equals(grainId))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
         public override string ToString() => $"{this}";
 
         string IFormattable.ToString(string? format, IFormatProvider? formatProvider) => ToString();
diff --git a/test/Orleans.Core.Tests/Messaging/MessageTests.cs b/test/Orleans.Core.Tests/Messaging/MessageTests.cs
new file mode 100644
index 00000000000..731c1db5654
--- /dev/null
+++ b/test/Orleans.Core.Tests/Messaging/MessageTests.cs
@@ -0,0 +1,52 @@
+using System.Net;
+using Orleans.Runtime;
+using TestExtensions;
+using Xunit;
+
+namespace UnitTests.Messaging;
+
+public class MessageTests
+{
+    [Fact, TestCategory("BVT")]
+    public void AddToCacheInvalidationHeader_LimitsHeaderLength()
+    {
+        var message = new Message();
+
+        for (var i = 0; i < Message.MaxCacheInvalidationHeaderEntries + 1; i++)
+        {
+            var grainId = GrainId.Create("test", i.ToString());
+            message.AddToCacheInvalidationHeader(CreateAddress(grainId, i), CreateAddress(grainId, i + 100));
+        }
+
+        var header = message.CacheInvalidationHeader;
+        Assert.NotNull(header);
+        Assert.Equal(Message.MaxCacheInvalidationHeaderEntries, header.Count);
+        Assert.DoesNotContain(header, update => update.GrainId.Equals(GrainId.Create("test", Message.MaxCacheInvalidationHeaderEntries.ToString())));
+    }
+
+    [Fact, TestCategory("BVT")]
+    public void AddToCacheInvalidationHeader_DeduplicatesByGrainId()
+    {
+        var message = new Message();
+        var grainId = GrainId.Create("test", "duplicate");
+        var invalidAddress = CreateAddress(grainId, 1);
+        var validAddress = CreateAddress(grainId, 2);
+
+        message.AddToCacheInvalidationHeader(invalidAddress, validAddress);
+        message.AddToCacheInvalidationHeader(CreateAddress(grainId, 3), CreateAddress(grainId, 4));
+
+        var header = message.CacheInvalidationHeader;
+        Assert.NotNull(header);
+        var update = Assert.Single(header);
+        Assert.Equal(grainId, update.GrainId);
+        Assert.Equal(invalidAddress, update.InvalidGrainAddress);
+        Assert.Equal(validAddress, update.ValidGrainAddress);
+    }
+
+    private static GrainAddress CreateAddress(GrainId grainId, int offset) => new()
+    {
+        GrainId = grainId,
+        ActivationId = ActivationId.NewId(),
+        SiloAddress = SiloAddress.New(IPAddress.Loopback, 10_000 + offset, offset + 1)
+    };
+}