Register ping handler in McpSessionHandler to support server-initiated pings (#1391)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>

Author: Copilot

src/ModelContextProtocol.Core/McpSessionHandler.cs

@@ -132,6 +132,14 @@ public McpSessionHandler(
         _incomingMessageFilter = incomingMessageFilter ?? (next => next);
         _outgoingMessageFilter = outgoingMessageFilter ?? (next => next);
         _logger = logger;
+
+        // Per the MCP spec, ping may be initiated by either party and must always be handled.
+        _requestHandlers.Set(
+            RequestMethods.Ping,
+            (request, _, cancellationToken) => new ValueTask<PingResult>(new PingResult()),
+            McpJsonUtilities.JsonContext.Default.JsonNode,
+            McpJsonUtilities.JsonContext.Default.PingResult);
+
         LogSessionCreated(EndpointName, _sessionId, _transportKind);
     }
 

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -91,7 +91,6 @@ public McpServerImpl(ITransport transport, McpServerOptions options, ILoggerFact
         ConfigureLogging(options);
         ConfigureCompletion(options);
         ConfigureExperimental(options);
-        ConfigurePing();
 
         // Register any notification handlers that were provided.
         if (options.Handlers.NotificationHandlers is { } notificationHandlers)
@@ -204,14 +203,6 @@ public override async ValueTask DisposeAsync()
         await _sessionHandler.DisposeAsync().ConfigureAwait(false);
     }
 
-    private void ConfigurePing()
-    {
-        SetHandler(RequestMethods.Ping,
-            async (request, _) => new PingResult(),
-            McpJsonUtilities.JsonContext.Default.JsonNode,
-            McpJsonUtilities.JsonContext.Default.PingResult);
-    }
-
     private void ConfigureInitialize(McpServerOptions options)
     {
         _requestHandlers.Set(RequestMethods.Initialize,

tests/ModelContextProtocol.Tests/Client/McpClientTests.cs

@@ -782,4 +782,16 @@ public async Task SetLoggingLevelAsync_WithRequestParams_NullThrows()
         await Assert.ThrowsAsync<ArgumentNullException>("requestParams",
             () => client.SetLoggingLevelAsync((SetLevelRequestParams)null!, TestContext.Current.CancellationToken));
     }
+
+    [Fact]
+    public async Task ServerCanPingClient()
+    {
+        await using McpClient client = await CreateMcpClientForServer();
+
+        var pingRequest = new JsonRpcRequest { Method = RequestMethods.Ping };
+        var response = await Server.SendRequestAsync(pingRequest, TestContext.Current.CancellationToken);
+
+        Assert.NotNull(response);
+        Assert.NotNull(response.Result);
+    }
 }