Fix protocol version tag for stateless server spans (#1810)

Co-authored-by: Luis Rodriguez <25299418+luisangelrod@users.noreply.github.com>
Co-authored-by: Tarek Mahmoud Sayed <tarekms@ntdev.microsoft.com>

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStatelessTests.cs

@@ -2,6 +2,8 @@
 using Microsoft.Extensions.DependencyInjection;
 using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Server;
+using OpenTelemetry.Trace;
+using System.Diagnostics;
 
 namespace ModelContextProtocol.AspNetCore.Tests;
 
@@ -10,6 +12,57 @@ public class MapMcpStatelessTests(ITestOutputHelper outputHelper) : MapMcpStream
     protected override bool UseStreamableHttp => true;
     protected override bool Stateless => true;
 
+    // In stateless mode each HTTP request is served by a fresh, un-negotiated McpServer, so the
+    // session-level NegotiatedProtocolVersion is null when the server span is tagged. The tag must
+    // therefore come from the per-request MCP-Protocol-Version header / _meta value. Exercising more
+    // than one version proves the tag tracks the per-request value rather than coincidentally matching
+    // a single hard-coded version (and would regress to an absent tag if the per-request fallback were
+    // removed, since the negotiated value is null at tagging time).
+    [Theory]
+    [InlineData("2025-11-25")]
+    [InlineData("2026-07-28")]
+    public async Task ServerActivity_TagsPerRequestProtocolVersion_InStatelessMode(string protocolVersion)
+    {
+        var activities = new List<Activity>();
+        string? capturedNegotiatedProtocolVersion = null;
+
+        var protocolVersionTool = McpServerTool.Create(
+            (RequestContext<CallToolRequestParams> context) =>
+            {
+                capturedNegotiatedProtocolVersion = context.Server.NegotiatedProtocolVersion;
+                return "ok";
+            },
+            new() { Name = "stateless-capture-version" });
+
+        using (var tracerProvider = OpenTelemetry.Sdk.CreateTracerProviderBuilder()
+            .AddSource("Experimental.ModelContextProtocol")
+            .AddInMemoryExporter(activities)
+            .Build())
+        {
+            Builder.Services.AddMcpServer()
+                .WithHttpTransport(ConfigureStateless)
+                .WithTools([protocolVersionTool]);
+
+            await using var app = Builder.Build();
+            app.MapMcp();
+            await app.StartAsync(TestContext.Current.CancellationToken);
+
+            await using var client = await ConnectAsync(configureClient: options =>
+                options.ProtocolVersion = protocolVersion);
+
+            await client.CallToolAsync("stateless-capture-version", cancellationToken: TestContext.Current.CancellationToken);
+        }
+
+        Assert.Contains(activities, activity =>
+            activity.DisplayName == "tools/call stateless-capture-version" &&
+            activity.Kind == ActivityKind.Server &&
+            activity.GetTagItem("mcp.protocol.version") as string == protocolVersion);
+
+        // Once the per-request version is applied, the request-scoped server settles on that same version,
+        // so the value tagged on the span and the version the session ends up negotiating agree.
+        Assert.Equal(protocolVersion, capturedNegotiatedProtocolVersion);
+    }
+
     [Fact]
     public async Task EnablePollingAsync_ThrowsInvalidOperationException_InStatelessMode()
     {