fix: ignore empty Streamable HTTP JSON responses (#1626)

src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

@@ -186,7 +186,10 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
         if (response.Content.Headers.ContentType?.MediaType == "application/json")
         {
             var responseContent = await response.Content.ReadAsStringAsync(cancellationToken).ConfigureAwait(false);
-            rpcResponseOrError = await ProcessMessageAsync(responseContent, rpcRequest, cancellationToken).ConfigureAwait(false);
+            if (responseContent.Length > 0)
+            {
+                rpcResponseOrError = await ProcessMessageAsync(responseContent, rpcRequest, cancellationToken).ConfigureAwait(false);
+            }
         }
         else if (response.Content.Headers.ContentType?.MediaType == "text/event-stream")
         {

tests/ModelContextProtocol.Tests/Transport/HttpClientTransportTests.cs

@@ -147,6 +147,40 @@ public class HttpClientTransportTests : LoggedTest
         Assert.True(true);
     }
 
+    [Fact]
+    public async Task StreamableHttp_NotificationWithEmptyAcceptedJsonResponse_DoesNotLogParseFailure()
+    {
+        var options = new HttpClientTransportOptions
+        {
+            Endpoint = new Uri("http://localhost:8080/mcp"),
+            TransportMode = HttpTransportMode.StreamableHttp,
+        };
+
+        using var mockHttpHandler = new MockHttpHandler();
+        using var httpClient = new HttpClient(mockHttpHandler);
+        await using var transport = new HttpClientTransport(options, httpClient, LoggerFactory);
+
+        mockHttpHandler.RequestHandler = request =>
+        {
+            Assert.Equal(HttpMethod.Post, request.Method);
+            Assert.Equal("http://localhost:8080/mcp", request.RequestUri?.AbsoluteUri);
+
+            return Task.FromResult(new HttpResponseMessage
+            {
+                StatusCode = HttpStatusCode.Accepted,
+                Content = new StringContent("", Encoding.UTF8, "application/json"),
+            });
+        };
+
+        await using var session = await transport.ConnectAsync(TestContext.Current.CancellationToken);
+        await session.SendMessageAsync(
+            new JsonRpcNotification { Method = "notifications/initialized" },
+            TestContext.Current.CancellationToken);
+
+        Assert.DoesNotContain(MockLoggerProvider.LogMessages, log =>
+            log.Message.Contains("transport message parsing failed", StringComparison.Ordinal));
+    }
+
     [Fact]
     public async Task SendMessageAsync_Throws_HttpRequestException_With_ResponseBody_On_ErrorStatusCode()
     {
@@ -377,4 +411,4 @@ public class HttpClientTransportTests : LoggedTest
         // Assert - Total GET requests = 1 initial connection + MaxReconnectionAttempts reconnections.
         Assert.Equal(1 + MaxReconnectionAttempts, getRequestCount);
     }
-}
+}