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

tests/ModelContextProtocol.Tests/Protocol/RequestIdTests.cs

@@ -35,4 +35,32 @@ public class RequestIdTests
 
         Assert.Equal(id, JsonSerializer.Deserialize<RequestId>(JsonSerializer.Serialize(id, McpJsonUtilities.DefaultOptions), McpJsonUtilities.DefaultOptions));
     }
+
+    [Fact]
+    public void Null_DeserializesAsDefault()
+    {
+        // Per JSON-RPC 2.0 §5.1, error responses produced before the request id can be determined
+        // MUST carry id=null. Deserialization needs to tolerate that shape so callers can handle
+        // such error envelopes (instead of throwing on the bare RequestId conversion).
+        var id = JsonSerializer.Deserialize<RequestId>("null", McpJsonUtilities.DefaultOptions);
+        Assert.Equal(default(RequestId), id);
+        Assert.Null(id.Id);
+    }
+
+    [Fact]
+    public void Null_SerializesAsJsonNull()
+    {
+        // The default RequestId (Id == null) is the id-less-error-response shape. It MUST serialize as
+        // JSON null — not "" — so the wire form is spec-conformant and round-trips losslessly.
+        Assert.Equal("null", JsonSerializer.Serialize(default(RequestId), McpJsonUtilities.DefaultOptions));
+    }
+
+    [Fact]
+    public void Null_Roundtrips()
+    {
+        var json = JsonSerializer.Serialize(default(RequestId), McpJsonUtilities.DefaultOptions);
+        var id = JsonSerializer.Deserialize<RequestId>(json, McpJsonUtilities.DefaultOptions);
+        Assert.Equal(default(RequestId), id);
+        Assert.Null(id.Id);
+    }
 }