Register ping handler in McpClientImpl so server-initiated pings succeed

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Client/McpClientImpl.cs

@@ -85,6 +85,12 @@ private void RegisterHandlers(McpClientOptions options, NotificationHandlers not
             notificationHandlers.RegisterRange(notificationHandlersFromOptions);
         }
 
+        requestHandlers.Set(
+            RequestMethods.Ping,
+            (request, _, cancellationToken) => new ValueTask<PingResult>(new PingResult()),
+            McpJsonUtilities.JsonContext.Default.JsonNode,
+            McpJsonUtilities.JsonContext.Default.PingResult);
+
         if (taskStatusHandler is not null)
         {
             notificationHandlers.Register(

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);
+    }
 }