Simplify endpoint filter: tag before next() for child span inheritance

Move Activity.AddTag before next() so child spans created during
request processing inherit the transport session ID. Accept that the
first initialize request won't have the tag (no request header yet).
Update test to match the simplified pattern.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: halter73

docs/concepts/sessions/sessions.md

@@ -659,34 +659,23 @@ app.MapMcp().AddEndpointFilter(async (context, next) =>
 {
     var httpContext = context.HttpContext;
 
-    // Read from request headers first. This is available on all non-initialize
-    // requests in stateful mode, because the client echoes back the session ID
-    // it received from the server's initialize response.
-    var sessionId = httpContext.Request.Headers["Mcp-Session-Id"].FirstOrDefault();
-
-    var result = await next(context);
-
-    // After the handler runs, check response headers. In stateful mode, the server
-    // sets Mcp-Session-Id on every POST and GET response — not just initialize —
-    // so the session ID is always available here even for the first request.
-    // DELETE responses do not include the header, but the request header has it.
-    sessionId ??= httpContext.Response.Headers["Mcp-Session-Id"].FirstOrDefault();
-
-    // Tag the HTTP request Activity with the transport session ID so it appears
-    // alongside child MCP spans (which carry mcp.session.id) in your traces.
-    // sessionId is null only in stateless mode, where sessions don't exist.
-    if (sessionId is not null)
+    // The session ID is available in the request header on all non-initialize requests
+    // in stateful mode (the client echoes back the ID it received from the server's
+    // initialize response). It is null for the first initialize request and always null
+    // in stateless mode. Tag before next() so child spans inherit the value.
+    string? sessionId = httpContext.Request.Headers["Mcp-Session-Id"];
+    if (sessionId != null)
     {
         Activity.Current?.AddTag("mcp.transport.session.id", sessionId);
     }
 
-    return result;
+    return await next(context);
 });
 ```
 
 <!-- mlc-disable-next-line -->
 > [!NOTE]
-> In stateful mode, the `Mcp-Session-Id` response header is set on **every POST and GET response**, not just the `initialize` response. This means the session ID is always available in the filter after `await next(context)`. The only case where `sessionId` is `null` is in stateless mode, where the server doesn't use sessions at all.
+> The tag is added **before** calling `next()` so that any child activities created during request processing inherit it. The trade-off is that the very first `initialize` request won't have the tag, because the client doesn't have a session ID yet — the server assigns it in the response. All subsequent requests will have it.
 
 <!-- mlc-disable-next-line -->
 > [!NOTE]

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs

@@ -661,28 +661,28 @@ public async Task EndpointFilter_CanReadSessionId_BeforeAndAfterHandler()
         await using var app = Builder.Build();
 
         // This is the pattern documented in sessions.md — verify it actually works.
+        // Tag before next() so child spans inherit the value.
         app.MapMcp().AddEndpointFilter(async (context, next) =>
         {
             var httpContext = context.HttpContext;
 
             // Read from request headers — available on all non-initialize requests in stateful mode.
-            var beforeSessionId = httpContext.Request.Headers["Mcp-Session-Id"].FirstOrDefault();
+            string? beforeSessionId = httpContext.Request.Headers["Mcp-Session-Id"];
 
-            var result = await next(context);
-
-            // After the handler, check response headers.
-            var afterSessionId = httpContext.Response.Headers["Mcp-Session-Id"].FirstOrDefault();
-            var sessionId = beforeSessionId ?? afterSessionId;
-
-            capturedSessionIds.Add((beforeSessionId, afterSessionId, httpContext.Request.Method));
-
-            // Verify Activity.Current is available and AddTag works (the documented pattern).
+            // Tag before next() so child activities created during the handler inherit it.
             var activity = System.Diagnostics.Activity.Current;
-            if (sessionId is not null)
+            if (beforeSessionId != null)
             {
-                activity?.AddTag("mcp.transport.session.id", sessionId);
+                activity?.AddTag("mcp.transport.session.id", beforeSessionId);
             }
             var tagValue = activity?.GetTagItem("mcp.transport.session.id")?.ToString();
+
+            var result = await next(context);
+
+            // After the handler, check response headers too (for test validation only).
+            string? afterSessionId = httpContext.Response.Headers["Mcp-Session-Id"];
+
+            capturedSessionIds.Add((beforeSessionId, afterSessionId, httpContext.Request.Method));
             capturedActivityTags.Add((tagValue, activity is not null, httpContext.Request.Method));
 
             return result;
@@ -725,9 +725,11 @@ public async Task EndpointFilter_CanReadSessionId_BeforeAndAfterHandler()
             // (the initialized notification or list_tools — but not the initial initialize request).
             Assert.Contains(postCaptures, c => c.BeforeNext == client.SessionId);
 
-            // Verify Activity.Current was available and the AddTag pattern works.
-            var postActivityTags = capturedActivityTags.Where(c => c.Method is "POST").ToList();
-            Assert.All(postActivityTags, c =>
+            // Verify Activity.Current was available and the AddTag pattern works before next().
+            // The tag is only set on non-initialize requests (where the request header has the session ID).
+            var taggedPosts = capturedActivityTags.Where(c => c.Method is "POST" && c.TagValue is not null).ToList();
+            Assert.NotEmpty(taggedPosts);
+            Assert.All(taggedPosts, c =>
             {
                 Assert.True(c.HadActivity, "Activity.Current should be non-null in the endpoint filter");
                 Assert.Equal(client.SessionId, c.TagValue);