diff --git a/CHANGELOG.md b/CHANGELOG.md
index e7772486..8730a50c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## Unreleased
 - Change the default large payload externalization threshold (`LargePayloadStorageOptions.ThresholdBytes`) from 900,000 bytes to 256 KiB (262,144 bytes)
+- **BREAKING**: `EntityInstanceId` now throws if its string form exceeds 100 characters (backend limit). Migration: shorten entity name/key; if entities already exist with overlong IDs, use the truncated 100-character value when addressing them.
 
 
 ## v1.25.0-preview.2
diff --git a/src/Abstractions/Entities/EntityInstanceId.cs b/src/Abstractions/Entities/EntityInstanceId.cs
index fceaaaee..0b7836d4 100644
--- a/src/Abstractions/Entities/EntityInstanceId.cs
+++ b/src/Abstractions/Entities/EntityInstanceId.cs
@@ -12,6 +12,8 @@ namespace Microsoft.DurableTask.Entities;
 [JsonConverter(typeof(EntityInstanceId.JsonConverter))]
 public readonly record struct EntityInstanceId
 {
+    const int MaxInstanceIdLength = 100;
+
     /// <summary>
     /// Initializes a new instance of the <see cref="EntityInstanceId"/> struct.
     /// </summary>
@@ -28,6 +30,12 @@ public EntityInstanceId(string name, string key)
         Check.NotNull(key);
         this.Name = name.ToLowerInvariant();
         this.Key = key;
+
+        int length = this.ToString().Length;
+        if (length > MaxInstanceIdLength)
+        {
+            throw new ArgumentException($"entity instance ids may not exceed 100 characters in length, actual length {length}.");
+        }
     }
 
     /// <summary>
diff --git a/test/Abstractions.Tests/Entities/EntityInstanceIdTests.cs b/test/Abstractions.Tests/Entities/EntityInstanceIdTests.cs
new file mode 100644
index 00000000..b5f4c24b
--- /dev/null
+++ b/test/Abstractions.Tests/Entities/EntityInstanceIdTests.cs
@@ -0,0 +1,64 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using Microsoft.DurableTask.Entities;
+
+namespace Microsoft.DurableTask.Abstractions.Tests.Entities;
+
+public class EntityInstanceIdTests
+{
+    [Fact]
+    public void TestValidEntityInstanceId()
+    {
+        var entityId = new EntityInstanceId("entity", "key1");
+        Assert.Equal("@entity@key1", entityId.ToString());
+    }
+
+    [Fact]
+    public void TestEntityNameLowercased()
+    {
+        var entityId = new EntityInstanceId("Entity", "key1");
+        Assert.Equal("entity", entityId.Name);
+        Assert.Equal("@entity@key1", entityId.ToString());
+    }
+
+    [Fact]
+    public void TestEntityNameWithAtSymbolThrows()
+    {
+        Assert.Throws<ArgumentException>(() => new EntityInstanceId("entity@name", "key1"));
+    }
+
+    [Fact]
+    public void TestEntityInstanceIdExceedsMaxLengthThrows()
+    {
+        string longName = new string('a', 50);
+        string longKey = new string('b', 51);
+        Assert.Throws<ArgumentException>(() => new EntityInstanceId(longName, longKey));
+    }
+
+    [Fact]
+    public void TestEntityInstanceIdEqualsMaxLength()
+    {
+        string longName = new string('a', 49);
+        string longKey = new string('b', 49);
+        var entityId = new EntityInstanceId(longName, longKey);
+        Assert.Equal($"@{longName}@{longKey}", entityId.ToString());
+    }
+
+    [Fact]
+    public void TestFromStringValid()
+    {
+        var entityId = EntityInstanceId.FromString("@entity@key1");
+        Assert.Equal("entity", entityId.Name);
+        Assert.Equal("key1", entityId.Key);
+    }
+
+    [Fact]
+    public void TestFromStringInvalidThrows()
+    {
+        Assert.Throws<ArgumentException>(() => EntityInstanceId.FromString("invalid"));
+        Assert.Throws<ArgumentException>(() => EntityInstanceId.FromString("@entitykey1"));
+        Assert.Throws<ArgumentException>(() => EntityInstanceId.FromString("@@key1"));
+        Assert.Throws<ArgumentException>(() => EntityInstanceId.FromString($"@entity@{new string('a', 100)}"));
+    }
+}