diff --git a/dotnet/src/Generated/SessionEvents.cs b/dotnet/src/Generated/SessionEvents.cs
index 996f3f010..dfdaf2178 100644
--- a/dotnet/src/Generated/SessionEvents.cs
+++ b/dotnet/src/Generated/SessionEvents.cs
@@ -119,6 +119,11 @@ public partial class SessionEvent
     [JsonPropertyName("ephemeral")]
     public bool? Ephemeral { get; set; }
 
+    /// <summary>Sub-agent instance identifier. Absent for events from the root/main agent and session-level events.</summary>
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    [JsonPropertyName("agentId")]
+    public string? AgentId { get; set; }
+
     /// <summary>
     /// The event type discriminator.
     /// </summary>
diff --git a/dotnet/test/ForwardCompatibilityTests.cs b/dotnet/test/ForwardCompatibilityTests.cs
index d3f5b7785..8b61868b5 100644
--- a/dotnet/test/ForwardCompatibilityTests.cs
+++ b/dotnet/test/ForwardCompatibilityTests.cs
@@ -97,4 +97,41 @@ public void SessionEvent_Type_DefaultsToUnknown()
 
         Assert.Equal("unknown", evt.Type);
     }
+
+        [Fact]
+    public void FromJson_WithAgentId_DeserializesAgentId()
+    {
+        var json = """
+        {
+            "id": "12345678-1234-1234-1234-123456789abc",
+            "timestamp": "2026-06-15T10:30:00Z",
+            "parentId": null,
+            "agentId": "sub-agent-42",
+            "type": "future.feature_from_server",
+            "data": {}
+        }
+        """;
+
+        var result = SessionEvent.FromJson(json);
+
+        Assert.Equal("sub-agent-42", result.AgentId);
+    }
+
+    [Fact]
+    public void FromJson_WithoutAgentId_AgentIdIsNull()
+    {
+        var json = """
+        {
+            "id": "12345678-1234-1234-1234-123456789abc",
+            "timestamp": "2026-06-15T10:30:00Z",
+            "parentId": null,
+            "type": "future.feature_from_server",
+            "data": {}
+        }
+        """;
+
+        var result = SessionEvent.FromJson(json);
+
+        Assert.Null(result.AgentId);
+    }
 }
diff --git a/scripts/codegen/csharp.ts b/scripts/codegen/csharp.ts
index 9c8332c09..01bd29cca 100644
--- a/scripts/codegen/csharp.ts
+++ b/scripts/codegen/csharp.ts
@@ -739,6 +739,8 @@ namespace GitHub.Copilot.SDK;
     lines.push(`    [JsonPropertyName("parentId")]`, `    public Guid? ParentId { get; set; }`, "");
     lines.push(...xmlDocComment(baseDesc("ephemeral"), "    "));
     lines.push(`    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]`, `    [JsonPropertyName("ephemeral")]`, `    public bool? Ephemeral { get; set; }`, "");
+    lines.push(...xmlDocComment(baseDesc("agentId"), "    "));
+    lines.push(`    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]`, `    [JsonPropertyName("agentId")]`, `    public string? AgentId { get; set; }`, "");
     lines.push(`    /// <summary>`, `    /// The event type discriminator.`, `    /// </summary>`);
     lines.push(`    [JsonIgnore]`, `    public virtual string Type => "unknown";`, "");
     lines.push(`    /// <summary>Deserializes a JSON string into a <see cref="SessionEvent"/>.</summary>`);