feat: expose clientName in SDK session metadata

dotnet/src/Types.cs

@@ -2736,6 +2736,10 @@ public sealed class SessionMetadata
     /// </summary>
     public string? Summary { get; set; }
     /// <summary>
+    /// Identifier of the client driving the session.
+    /// </summary>
+    public string? ClientName { get; set; }
+    /// <summary>
     /// Whether the session is running on a remote server.
     /// </summary>
     public bool IsRemote { get; set; }

dotnet/test/Unit/SerializationTests.cs

@@ -171,6 +171,30 @@ public void ResumeSessionRequest_CanSerializeInstructionDirectories_WithSdkOptio
         Assert.Equal("C:\\resume-instructions", root.GetProperty("instructionDirectories")[0].GetString());
     }
 
+    [Fact]
+    public void SessionMetadata_CanRoundTripClientName_WithSdkOptions()
+    {
+        var options = GetSerializerOptions();
+        var original = new SessionMetadata
+        {
+            SessionId = "session-1",
+            StartTime = DateTimeOffset.Parse("2025-01-01T00:00:00Z"),
+            ModifiedTime = DateTimeOffset.Parse("2025-01-01T01:00:00Z"),
+            Summary = "loaded session",
+            ClientName = "my-app",
+            IsRemote = false
+        };
+
+        var json = JsonSerializer.Serialize(original, options);
+        using var document = JsonDocument.Parse(json);
+        var root = document.RootElement;
+        Assert.Equal("my-app", root.GetProperty("clientName").GetString());
+
+        var deserialized = JsonSerializer.Deserialize<SessionMetadata>(json, options);
+        Assert.NotNull(deserialized);
+        Assert.Equal("my-app", deserialized.ClientName);
+    }
+
     [Fact]
     public void CreateSessionRequest_CanSerializeEnableSessionTelemetry_WithSdkOptions()
     {

go/client_test.go

@@ -402,6 +402,33 @@ func TestResumeSessionRequest_ClientName(t *testing.T) {
 	})
 }
 
+func TestSessionMetadata_ClientName(t *testing.T) {
+	t.Run("round-trips clientName in JSON", func(t *testing.T) {
+		var metadata SessionMetadata
+		if err := json.Unmarshal([]byte(`{
+			"sessionId":"s1",
+			"startTime":"2025-01-01T00:00:00Z",
+			"modifiedTime":"2025-01-01T01:00:00Z",
+			"summary":"loaded session",
+			"clientName":"my-app",
+			"isRemote":false
+		}`), &metadata); err != nil {
+			t.Fatalf("Failed to unmarshal: %v", err)
+		}
+		if metadata.ClientName == nil || *metadata.ClientName != "my-app" {
+			t.Fatalf("Expected clientName to be my-app, got %v", metadata.ClientName)
+		}
+
+		data, err := json.Marshal(metadata)
+		if err != nil {
+			t.Fatalf("Failed to marshal: %v", err)
+		}
+		if !strings.Contains(string(data), `"clientName":"my-app"`) {
+			t.Fatalf("Expected marshaled JSON to include clientName, got %s", string(data))
+		}
+	})
+}
+
 func TestCreateSessionRequest_Agent(t *testing.T) {
 	t.Run("includes agent in JSON when set", func(t *testing.T) {
 		req := createSessionRequest{Agent: "test-agent"}