SSE client transport: tolerate return of RPC responses in POST requests. (#445)

* SSE client transport: handle return of RPC responses in POST requests.

* Remove accidental change.

* Address feedback

* Remove unused namespace.

Author: eiriktsarpalis

src/ModelContextProtocol/Client/SseClientSessionTransport.cs

@@ -92,26 +92,18 @@ public override async Task SendMessageAsync(
         StreamableHttpClientSessionTransport.CopyAdditionalHeaders(httpRequestMessage.Headers, _options.AdditionalHeaders);
         var response = await _httpClient.SendAsync(httpRequestMessage, cancellationToken).ConfigureAwait(false);
 
-        response.EnsureSuccessStatusCode();
-
-        var responseContent = await response.Content.ReadAsStringAsync(cancellationToken).ConfigureAwait(false);
-
-        if (string.IsNullOrEmpty(responseContent) || responseContent.Equals("accepted", StringComparison.OrdinalIgnoreCase))
-        {
-            LogAcceptedPost(Name, messageId);
-        }
-        else
+        if (!response.IsSuccessStatusCode)
         {
             if (_logger.IsEnabled(LogLevel.Trace))
             {
-                LogRejectedPostSensitive(Name, messageId, responseContent);
+                LogRejectedPostSensitive(Name, messageId, await response.Content.ReadAsStringAsync(cancellationToken).ConfigureAwait(false));
             }
             else
             {
                 LogRejectedPost(Name, messageId);
             }
 
-            throw new InvalidOperationException("Failed to send message");
+            response.EnsureSuccessStatusCode();
         }
     }
 

tests/ModelContextProtocol.AspNetCore.Tests/SseIntegrationTests.cs

@@ -58,6 +58,22 @@ public async Task ConnectAndReceiveMessage_InMemoryServer_WithFullEndpointEventU
         Assert.True(true);
     }
 
+    [Fact]
+    public async Task ConnectAndReceiveMessage_ServerReturningJsonInPostRequest()
+    {
+        await using var app = Builder.Build();
+        MapAbsoluteEndpointUriMcp(app, respondInJson: true);
+
+        await app.StartAsync(TestContext.Current.CancellationToken);
+
+        await using var mcpClient = await ConnectMcpClientAsync();
+
+        // Send a test message through POST endpoint
+        await mcpClient.SendNotificationAsync("test/message", new Envelope { Message = "Hello, SSE!" }, serializerOptions: JsonContext.Default.Options, cancellationToken: TestContext.Current.CancellationToken);
+
+        Assert.True(true);
+    }
+
     [Fact]
     public async Task ConnectAndReceiveNotification_InMemoryServer()
     {
@@ -220,7 +236,7 @@ public async Task EmptyAdditionalHeadersKey_Throws_InvalidOperationException()
         Assert.Equal("Failed to add header '' with value '' from AdditionalHeaders.", ex.Message);
     }
 
-    private static void MapAbsoluteEndpointUriMcp(IEndpointRouteBuilder endpoints)
+    private static void MapAbsoluteEndpointUriMcp(IEndpointRouteBuilder endpoints, bool respondInJson = false)
     {
         var loggerFactory = endpoints.ServiceProvider.GetRequiredService<ILoggerFactory>();
         var optionsSnapshot = endpoints.ServiceProvider.GetRequiredService<IOptions<McpServerOptions>>();
@@ -267,7 +283,7 @@ private static void MapAbsoluteEndpointUriMcp(IEndpointRouteBuilder endpoints)
                 await Results.BadRequest("Session not started.").ExecuteAsync(context);
                 return;
             }
-            var message = (JsonRpcMessage?)await context.Request.ReadFromJsonAsync(McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(JsonRpcMessage)), context.RequestAborted);
+            var message = await context.Request.ReadFromJsonAsync<JsonRpcMessage>(McpJsonUtilities.DefaultOptions, context.RequestAborted);
             if (message is null)
             {
                 await Results.BadRequest("No message in request body.").ExecuteAsync(context);
@@ -276,7 +292,15 @@ private static void MapAbsoluteEndpointUriMcp(IEndpointRouteBuilder endpoints)
 
             await session.OnMessageReceivedAsync(message, context.RequestAborted);
             context.Response.StatusCode = StatusCodes.Status202Accepted;
-            await context.Response.WriteAsync("Accepted");
+
+            if (respondInJson)
+            {
+                await context.Response.WriteAsJsonAsync(message, McpJsonUtilities.DefaultOptions, cancellationToken: context.RequestAborted);
+            }
+            else
+            {
+                await context.Response.WriteAsync("Accepted");
+            }
         });
     }