fix(mcp): notify only the session whose compatibility shim transitioned

src/Repl.Mcp/McpServerHandler.cs

@@ -311,7 +311,14 @@ private async ValueTask<ListToolsResult> ListToolsAsync(
 			&& !IsSessionlessRequest()
 			&& context.TryClaimCompatibilityIntro())
 		{
-			SignalToolListChanged();
+			// Sent to this request's own server, not through _toolListChanged: that collection's SDK
+			// fan-out reaches every attached session, and only this one's catalog is about to change.
+			if (request.Server is { } server)
+			{
+				await server.SendNotificationAsync(NotificationMethods.ToolListChangedNotification, cancellationToken)
+					.ConfigureAwait(false);
+			}
+
 			return McpCacheHints.MarkPrivateToThisClient(request, new ListToolsResult
 			{
 				Tools =
@@ -849,20 +856,18 @@ private void OnRoutingInvalidated(bool isVisibilityRetraction)
 		}
 	}
 
-	private void SignalDiscoveryChanged()
-	{
-		_toolListChanged.Clear();
-		_resourceListChanged.Clear();
-		_promptListChanged.Clear();
-	}
-
 	// Clearing an already-empty primitive collection raises its Changed event without mutating
 	// anything, which is what lets an empty collection act as a pure signal. That the event fires
 	// unconditionally is NOT documented on Clear(), so it is pinned by
 	// Given_McpSubscriptions.When_ClearingAnEmptyCollection_Then_ChangedStillFires: if a future SDK
 	// turns Clear() into a no-op, that test fails loudly instead of discovery notifications silently
 	// disappearing.
-	private void SignalToolListChanged() => _toolListChanged.Clear();
+	private void SignalDiscoveryChanged()
+	{
+		_toolListChanged.Clear();
+		_resourceListChanged.Clear();
+		_promptListChanged.Clear();
+	}
 
 	private void UnsubscribeFromRoutingChanges()
 	{

src/Repl.McpTests/Given_McpConcurrentSessions.cs

@@ -705,6 +705,54 @@ public async Task When_ShimEnabledAndTwoLegacySessionsList_Then_EachSessionGetsT
 		firstListB.Select(static tool => tool.Name).Should().BeEquivalentTo(["discover_tools", "call_tool"]);
 	}
 
+	[TestMethod]
+	[Description("The compatibility-shim transition is connection-local — each session claims its own intro — so its tools/list_changed must reach that session alone. It used to clear a handler-wide collection whose SDK fan-out notifies every attached session, sending a spurious list_changed to sessions whose catalog had not changed. A ping on the other session after the first session's own notification arrives is the ordering barrier: the SDK starts every session's send synchronously inside that one clear, so any notification bound for the other session is already queued ahead of the ping's response.")]
+	public async Task When_OneLegacySessionConsumesItsShimIntro_Then_NoOtherSessionIsNotified()
+	{
+		var app = ReplApp.Create();
+		app.UseMcpServer();
+		app.Map("alpha", () => "a");
+		var handler = CreateHandler(app, DynamicToolCompatibilityMode.DiscoverAndCallShim);
+		using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(30));
+
+		var legacy = new McpClientOptions { ProtocolVersion = McpProtocolRevisions.LastWithSessions };
+		var sessionA = await StartSessionAsync(handler, legacy, cts.Token).ConfigureAwait(false);
+		await using var scopeA = sessionA.ConfigureAwait(false);
+		var sessionB = await StartSessionAsync(handler, legacy, cts.Token).ConfigureAwait(false);
+		await using var scopeB = sessionB.ConfigureAwait(false);
+
+		var notifiedA = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+		await using var registrationA = sessionA.Client.RegisterNotificationHandler(
+			NotificationMethods.ToolListChangedNotification,
+			(_, _) =>
+			{
+				notifiedA.TrySetResult();
+				return ValueTask.CompletedTask;
+			}).ConfigureAwait(false);
+		var notificationsToB = 0;
+		var notifiedB = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+		await using var registrationB = sessionB.Client.RegisterNotificationHandler(
+			NotificationMethods.ToolListChangedNotification,
+			(_, _) =>
+			{
+				Interlocked.Increment(ref notificationsToB);
+				notifiedB.TrySetResult();
+				return ValueTask.CompletedTask;
+			}).ConfigureAwait(false);
+
+		(await sessionA.Client.ListToolsAsync(cancellationToken: cts.Token).ConfigureAwait(false))
+			.Select(static tool => tool.Name).Should().BeEquivalentTo(["discover_tools", "call_tool"]);
+		await notifiedA.Task.WaitAsync(TimeSpan.FromSeconds(10)).ConfigureAwait(false);
+		await sessionB.Client.PingAsync(cancellationToken: cts.Token).ConfigureAwait(false);
+
+		Volatile.Read(ref notificationsToB).Should().Be(0, "session A's transition changed nothing session B can see");
+
+		// Positive control: B's own transition does reach B, so the handler above can observe one.
+		await sessionB.Client.ListToolsAsync(cancellationToken: cts.Token).ConfigureAwait(false);
+		await notifiedB.Task.WaitAsync(TimeSpan.FromSeconds(10)).ConfigureAwait(false);
+		Volatile.Read(ref notificationsToB).Should().Be(1);
+	}
+
 	[TestMethod]
 	[Description("Regression guard: the compatibility bootstrap must not run on 2026-07-28. Serving discover_tools/call_tool on the first tools/list and the real catalog on the next is a connection-local change caused by another request on that connection — the second half of the MUST NOT, and observable with a single connection. A modern client gets the real catalog immediately, and twice in a row it gets the same one.")]
 	public async Task When_ShimEnabledAndAModernSessionLists_Then_TheCatalogIsTheSameEveryTime()