Default draft protocol support: sessionless + handshake-less (SEP-2575 + SEP-2567) (#1610)

tests/ModelContextProtocol.ConformanceClient/Program.cs

@@ -36,6 +36,16 @@ McpClientOptions options = new()
     },
 };
 
+// The default client now prefers the draft revision (probing with server/discover and falling back
+// to a legacy initialize handshake). The "initialize" and "sse-retry" scenarios specifically exercise
+// the legacy initialize handshake and SSE resumability (removed in draft) and strictly expect
+// initialize as the first message, so pin them to the latest stable version. Other scenarios run on
+// the draft default and exercise the server/discover probe plus the transparent legacy fallback.
+if (scenario is "initialize" or "sse-retry")
+{
+    options.ProtocolVersion = "2025-11-25";
+}
+
 var consoleLoggerFactory = LoggerFactory.Create(builder =>
 {
     builder.AddConsole();