Streamable HTTP transport doesn't correctly handle empty response

[DmitryLukyanov]

Describe the bug
I always see System.Text.Json.JsonException: The input does not contain any JSON tokens when I use MCP client (c#). Then it's restarted and result is returned, but it would be good to avoid initial failure.

To Reproduce
Steps to reproduce the behavior:

1. Create MCP client

     var transport = new SseClientTransport(
         new SseClientTransportOptions
         {
             Endpoint = new Uri(_mcpConfig.Endpoint),
             Name = name
         },
         loggerFactory: _loggerFactory,
         httpClient: _httpClient,
         ownsHttpClient: false);
     return await McpClientFactory.CreateAsync(transport, cancellationToken: cancellationToken);
   

where httpClient is created:

        services
            .AddHttpClient<McpHttpClient>()
            .AddPolicyHandler(s => McpHttpClient.GetRetryPolicy());

    public static IAsyncPolicy<HttpResponseMessage> GetRetryPolicy()
    {
        return HttpPolicyExtensions
            .HandleTransientHttpError() // Handles 5xx and 408 (Request Timeout)
            .OrResult(response => !response.IsSuccessStatusCode && (int)response.StatusCode >= 500)
            .WaitAndRetryAsync(
                retryCount: 2,
                sleepDurationProvider: attempt => TimeSpan.FromMilliseconds(500 * attempt));
    }

3. Initialize MCP tools in Semantic Kernel

        kernel.Plugins.AddFromFunctions(
            pluginName: PluginName,
            description: serverInstructions,
            functions: tools.Select(t => t.AsKernelFunction()));
   

4. Allow model to call MCP tool

Expected behavior
MCP tool is called without errors

Logs

	[2026-01-07 22:00:52.350] The maximum limit of 0 auto invocations per user request has been reached. Auto invocation is now disabled., Exception:<no exception>
	[2026-01-07 22:00:52.351] Initializing MCP client with endpoint: https://REMOTE_ADDRESS/mcp/, Exception:<no exception>
	[2026-01-07 22:00:52.351] MCP_NAME attempting to connect using Streamable HTTP transport., Exception:<no exception>
	[2026-01-07 22:00:52.351] Start processing HTTP request POST https://REMOTE_ADDRESS/mcp/, Exception:<no exception>
	[2026-01-07 22:00:52.351] Sending HTTP request POST https://REMOTE_ADDRESS/mcp/, Exception:<no exception>
	[2026-01-07 22:00:52.422] Received HTTP response headers after 71.2331ms - 200, Exception:<no exception>
	[2026-01-07 22:00:52.422] End processing HTTP request after 71.4301ms - 200, Exception:<no exception>
	[2026-01-07 22:00:52.422] MCP_NAME transport received message with ID '1'., Exception:<no exception>
	[2026-01-07 22:00:52.423] Start processing HTTP request GET https://REMOTE_ADDRESS/mcp/, Exception:<no exception>
	[2026-01-07 22:00:52.423] Sending HTTP request GET https://REMOTE_ADDRESS/mcp/, Exception:<no exception>
	[2026-01-07 22:00:52.423] MCP_NAME using Streamable HTTP transport., Exception:<no exception>
	[2026-01-07 22:00:52.423] Start processing HTTP request POST https://REMOTE_ADDRESS/mcp/, Exception:<no exception>
	[2026-01-07 22:00:52.424] Sending HTTP request POST https://REMOTE_ADDRESS/mcp/, Exception:<no exception>
	[2026-01-07 22:00:52.426] Received HTTP response headers after 3.023ms - 200, Exception:<no exception>
	[2026-01-07 22:00:52.426] End processing HTTP request after 3.3766ms - 200, Exception:<no exception>
	[2026-01-07 22:00:52.486] Received HTTP response headers after 62.7538ms - 202, Exception:<no exception>
	[2026-01-07 22:00:52.486] End processing HTTP request after 62.931ms - 202, Exception:<no exception>
	[2026-01-07 22:00:52.487] MCP_NAME transport message parsing failed., Exception:System.Text.Json.JsonException: The input does not contain any JSON tokens. Expected the input to start with a valid JSON token, when isFinalBlock is true. Path: $ | LineNumber: 0 | BytePositionInLine: 0.
	 ---> System.Text.Json.JsonReaderException: The input does not contain any JSON tokens. Expected the input to start with a valid JSON token, when isFinalBlock is true. LineNumber: 0 | BytePositionInLine: 0.
	   at System.Text.Json.ThrowHelper.ThrowJsonReaderException(Utf8JsonReader& json, ExceptionResource resource, Byte nextByte, ReadOnlySpan`1 bytes)
	   at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state)
	   --- End of inner exception stack trace ---
	   at System.Text.Json.ThrowHelper.ReThrowWithPath(ReadStack& state, JsonReaderException ex)
	   at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state)
	   at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 json, JsonTypeInfo`1 jsonTypeInfo)
	   at ModelContextProtocol.Client.StreamableHttpClientSessionTransport.ProcessMessageAsync(String data, JsonRpcRequest relatedRpcRequest, CancellationToken cancellationToken)
	[2026-01-07 22:00:52.487] Start processing HTTP request POST https://REMOTE_ADDRESS/mcp/, Exception:<no exception>
	[2026-01-07 22:00:52.487] Sending HTTP request POST https://REMOTE_ADDRESS/mcp/, Exception:<no exception>
	[2026-01-07 22:00:52.491] Received HTTP response headers after 3.454ms - 200, Exception:<no exception>
	[2026-01-07 22:00:52.491] End processing HTTP request after 3.5049ms - 200, Exception:<no exception>
	[2026-01-07 22:00:52.492] MCP_NAME transport received message with ID '2'., Exception:<no exception>

Additional context

nuget: ModelContextProtocol, v0.3.0-preview.4

UPDATE: just checked, the same behavior happens with versions:

• 0.4.1-preview.1
• 0.5.0-preview.1