dotnet · guardrex · Jun 10, 2025 · Jun 6, 2025 · Jun 6, 2025 · Jun 6, 2025
@@ -1289,8 +1289,6 @@ Calling <xref:System.Net.Http.HttpContent.ReadAsStreamAsync%2A?displayProperty=n
 
 [!INCLUDE[](~/includes/aspnetcore-repo-ref-source-links.md)]
 
-<!-- UPDATE 10.0 - Tracking on https://github.com/dotnet/runtime/issues/97449
-
 To opt-out of response streaming globally, use either of the following approaches:
 
 * Add the `<WasmEnableStreamingResponse>` property to the project file with a value of `false`:
@@ -1301,12 +1299,6 @@ To opt-out of response streaming globally, use either of the following approache
 
 * Set the `DOTNET_WASM_ENABLE_STREAMING_RESPONSE` environment variable to `false` or `0`.
 
-............. AND REMOVE THE NEXT LINE .............
-
--->
-
-To opt-out of response streaming globally, set the `DOTNET_WASM_ENABLE_STREAMING_RESPONSE` environment variable to `false` or `0`.
-
 To opt-out of response streaming for an individual request, set <xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions.SetBrowserResponseStreamingEnabled%2A> to `false` on the <xref:System.Net.Http.HttpRequestMessage> (`request` in the following example):
 
 ```csharp

@@ -730,35 +730,33 @@ When a component is rendered statically (static SSR) and `NavigationManager.NotF
 }
 ```
 
-Two approaches for providing Not Found content for global interactive rendering:
+To provide Not Found content for global interactive rendering, use a Not Found page (Razor component).
 
-* Use a Not Found page (Razor component).
-* Specify Not Found content in the [`Router` component's](xref:blazor/fundamentals/routing#route-templates) <xref:Microsoft.AspNetCore.Components.Routing.Router.NotFound%2A> property (`<NotFound>...</NotFound>` markup or by setting the `NotFound` parameter to a render fragment in C# code).
-
-The following example uses a Not Found page (`NotFoundPage` component) to render Not Found content.
+> [!NOTE]
+> The Blazor project template includes a `NotFound.razor` page by default. This page automatically renders whenever `NavigationManager.NotFound` is called, making it easier to handle missing routes with a consistent user experience.
 
-`NotFoundPage.razor`:
+`NotFound.razor`:
 
 ```razor
 <h1>Not Found</h1>
 
 <p>Sorry! Nothing to show.</p>
 ```
 
-Specify the `NotFoundPage` component to the `Router` component in `Routes.razor`. You might need to specify the component's namespace with an [`@using`](xref:mvc/views/razor#using) directive either at the top of the `Routes.razor` file or in an [`_Imports.razor` file](xref:blazor/components/index#component-name-class-name-and-namespace).
+Assign the `NotFound` component to the router's `NotFoundPage` parameter. `NotFoundPage` supports routing that can be used across re-execution middleware, including non-Blazor middleware. If the `NotFound` render fragment is defined together with `NotFoundPage`, the page has higher priority.
+
+In the following example, the preceding `NotFound` component is present in the app's `Pages` folder and passed to the `NotFoundPage` parameter:
 
 ```razor
-<Router ...>
-    <Found ...>
-        ...
+<Router AppAssembly="@typeof(Program).Assembly" NotFoundPage="typeof(Pages.NotFound)">
+    <Found Context="routeData">
+        <RouteView RouteData="@routeData" />
+        <FocusOnNavigate RouteData="@routeData" Selector="h1" />
     </Found>
-    <NotFound>
-        <NotFoundPage />
-    </NotFound>
 </Router>
 ```
 
-When a component is rendered with a global interactive render mode, calling `NotFound` signals the Blazor router to render Not Found content, which is the `NotFoundPage` component:
+When a component is rendered with a global interactive render mode, calling `NotFound` signals the Blazor router to render the `NotFound` component:
 
 ```razor
 @page "/render-not-found-interactive"

@@ -1,11 +1,11 @@
 ---
 title: ASP.NET Core Blazor performance best practices
 author: guardrex
-description: Tips for increasing the performance of ASP.NET Core Blazor apps and avoiding common performance problems.
+description: Guidance on ASP.NET Core Blazor metrics and tracing, improving app performance, and avoiding common performance problems.
 monikerRange: '>= aspnetcore-3.1'
 ms.author: wpickett
 ms.custom: mvc
-ms.date: 04/16/2025
+ms.date: 06/08/2025
 uid: blazor/performance/index
 ---
 # ASP.NET Core Blazor performance best practices
@@ -24,3 +24,91 @@ Blazor is optimized for high performance in most realistic application UI scenar
 Ahead-of-time (AOT) compilation compiles a Blazor app's .NET code directly into native WebAssembly for direct execution by the browser. AOT-compiled apps result in larger apps that take longer to download, but AOT-compiled apps usually provide better runtime performance, especially for apps that execute CPU-intensive tasks. For more information, see <xref:blazor/tooling/webassembly#ahead-of-time-aot-compilation>.
 
 :::moniker-end
+
+:::moniker range=">= aspnetcore-10.0"
+
+## Metrics and tracing
+
+Metrics and tracing capabilities help you monitor and diagnose app performance, track user interactions, and understand component behavior in production environments.
+
+### Configuration
+
+To enable Blazor metrics and tracing in your app, configure [OpenTelemetry](https://github.com/open-telemetry/opentelemetry-dotnet) with the following meters and activity sources in the app's `Program` file where services are registered:
+
+```csharp
+builder.Services.ConfigureOpenTelemetryMeterProvider(meterProvider =>
+{
+    meterProvider.AddMeter("Microsoft.AspNetCore.Components");
+    meterProvider.AddMeter("Microsoft.AspNetCore.Components.Lifecycle");
+    meterProvider.AddMeter("Microsoft.AspNetCore.Components.Server.Circuits");
+});
+
+builder.Services.ConfigureOpenTelemetryTracerProvider(tracerProvider =>
+{
+    tracerProvider.AddSource("Microsoft.AspNetCore.Components");
+});
+```
+
+### Performance meters
+
+For more information on the following performance meters, see <xref:log-mon/metrics/built-in>.
+
+`Microsoft.AspNetCore.Components` meter:
+
+* `aspnetcore.components.navigation`: Tracks the total number of route changes in the app.
+* `aspnetcore.components.event_handler`: Measures the duration of processing browser events, including business logic.
+
+`Microsoft.AspNetCore.Components.Lifecycle` meter:
+
+* `aspnetcore.components.update_parameters`: Measures the duration of processing component parameters, including business logic.
+* `aspnetcore.components.render_diff`: Tracks the duration of rendering batches.
+
+`Microsoft.AspNetCore.Components.Server.Circuits` meter:
+
+In server-side Blazor apps, additional circuit-specific metrics include:
+
+* `aspnetcore.components.circuit.active`: Shows the number of active circuits currently in memory.
+* `aspnetcore.components.circuit.connected`: Tracks the number of circuits connected to clients.
+* `aspnetcore.components.circuit.duration`: Measures circuit lifetime duration and provides total circuit count.
+
+### Blazor tracing
+
+For more information on the following tracing activities, see <xref:log-mon/metrics/built-in>.
+
+The new activity tracing capabilities use the `Microsoft.AspNetCore.Components` activity source and provide three main types of tracing activities: circuit lifecycle, navigation, and event handling.
+
+Circuit lifecycle tracing:
+
+`Microsoft.AspNetCore.Components.CircuitStart`: Traces circuit initialization with the format `Circuit {circuitId}`.
+
+* Tag: `aspnetcore.components.circuit.id`
+* Link: HTTP activity
+
+Navigation tracing:
+
+`Microsoft.AspNetCore.Components.RouteChange`: Tracks route changes with the format `Route {route} -> {componentType}`.
+
+* Tags
+  * `aspnetcore.components.circuit.id`
+  * `aspnetcore.components.route`
+  * `aspnetcore.components.type`
+* Links
+  * HTTP trace
+  * Circuit trace
+
+Event handling tracing:
+
+`Microsoft.AspNetCore.Components.HandleEvent`: Traces event handling with the format `Event {attributeName} -> {componentType}.{methodName}`.
+
+* Tags
+  * `aspnetcore.components.attribute.name`
+  * `aspnetcore.components.circuit.id`
+  * `aspnetcore.components.method`
+  * `aspnetcore.components.type`
+  * `error.type`
+* Links
+  * HTTP trace
+  * Circuit trace
+  * Router trace
+
+:::moniker-end
diff --git a/aspnetcore/log-mon/metrics/built-in/includes/built-in10.md b/aspnetcore/log-mon/metrics/built-in/includes/built-in10.md
@@ -3,7 +3,93 @@
 This article describes the metrics built-in for ASP.NET Core produced using the
 <xref:System.Diagnostics.Metrics?displayProperty=nameWithType> API. For a listing of metrics based on the older [EventCounters](/dotnet/core/diagnostics/event-counters) API, see [Available counters](/dotnet/core/diagnostics/available-counters).
 
-See [Using ASP.NET Core metrics](xref:log-mon/metrics/metrics) for information about how to collect, report, enrich, and test ASP.NET Core metrics
+For information about how to collect, report, enrich, and test ASP.NET Core metrics, see <xref:log-mon/metrics/metrics>.
+
+## `Microsoft.AspNetCore.Components`
+
+The `Microsoft.AspNetCore.Components` metrics report information on Razor component route changes and browser events:
+
+* [`aspnetcore.components.navigation`](#metric-aspnetcorecomponentsnavigation)
+* [`aspnetcore.components.event_handler`](#metric-aspnetcorecomponentsevent_handler)
+
+#### Metric: `aspnetcore.components.navigation`
+
+Name | Instrument Type | Unit (UCUM) | Description
+--- | --- | --- | ---
+`aspnetcore.components.navigation`<!--](https://opentelemetry.io/docs/specs/semconv/dotnet/dotnet-http-metrics/#metric-aspnetcorecomponentsnavigation)--> | Counter | `{route}` | Tracks the total number of route changes in the app.
+
+Attribute | Type | Description | Examples | Presence
+--- | --- | --- | --- | ---
+`aspnetcore.components.type` | string | Component navigated to. | `TestComponent` | Always
+`aspnetcore.components.route` | string | The component's route. | `/test-route` | Always
+
+#### Metric: `aspnetcore.components.event_handler`
+
+Name | Instrument Type | Unit (UCUM) | Description
+--- | --- | --- | ---
+`aspnetcore.components.event_handler`<!--](https://opentelemetry.io/docs/specs/semconv/dotnet/dotnet-http-metrics/#metric-aspnetcorecomponentsevent_handler)--> | Histogram | `s` | Measures the duration of processing browser events, including business logic.
+
+Attribute | Type | Description | Examples | Presence
+--- | --- | --- | --- | ---
+`aspnetcore.components.type` | string | Component type handling the event. | `TestComponent` | Always
+`aspnetcore.components.method` | string | C# method handling the event. | `OnClick` | Always
+`aspnetcore.components.attribute.name` | string | Component attribute name handling the event. | `onclick` | Always
+`error.type` | string | The full name of exception type. | `System.InvalidOperationException`; `Contoso.MyException` | If an exception is thrown.
+
+## `Microsoft.AspNetCore.Components.Lifecycle`
+
+The `Microsoft.AspNetCore.Components.Lifecycle` metrics report information on Razor component lifecycle events:
+
+* [`aspnetcore.components.update_parameters`](#metric-aspnetcorecomponentsupdate_parameters)
+* [`aspnetcore.components.render_diff`](#metric-aspnetcorecomponentsrender_diff)
+
+#### Metric: `aspnetcore.components.update_parameters`
+
+Name | Instrument Type | Unit (UCUM) | Description
+--- | --- | --- | ---
+`aspnetcore.components.update_parameters`<!--](https://opentelemetry.io/docs/specs/semconv/dotnet/dotnet-http-metrics/#metric-aspnetcorecomponentsupdate_parameters)--> | Histogram | `s` | Measures the duration of processing component parameters, including business logic.
+
+Attribute | Type | Description | Examples | Presence
+--- | --- | --- | --- | ---
+`aspnetcore.components.type` | string | Component type handling the event. | `TestComponent` | Always
+`error.type` | string | The full name of exception type. | `System.InvalidOperationException`; `Contoso.MyException` | If an exception is thrown.
+
+#### Metric: `aspnetcore.components.render_diff`
+
+Name | Instrument Type | Unit (UCUM) | Description
+--- | --- | --- | ---
+`aspnetcore.components.render_diff`<!--](https://opentelemetry.io/docs/specs/semconv/dotnet/dotnet-http-metrics/#metric-aspnetcorecomponentsrender_diff)--> | Histogram | `s` | Tracks the duration of rendering batches.
+
+Attribute | Type | Description | Examples | Presence
+--- | --- | --- | --- | ---
+`aspnetcore.components.diff.length` | int | The length of the render diff. | 50 | Always
+`error.type` | string | The full name of exception type. | `System.InvalidOperationException`; `Contoso.MyException` | If an exception is thrown.
+
+## `Microsoft.AspNetCore.Components.Server.Circuits`
+
+The `Microsoft.AspNetCore.Components.Server.Circuits` metrics report information on server-side Blazor circuits in Blazor Server and Blazor Web Apps:
+
+* [`aspnetcore.components.circuit.active`](#metric-aspnetcorecomponentscircuitactive)
+* [`aspnetcore.components.circuit.connected`](#metric-aspnetcorecomponentscircuitconnected)
+* [`aspnetcore.components.circuit.duration`](#metric-aspnetcorecomponentscircuitduration)
+
+#### Metric: `aspnetcore.components.circuit.active`
+
+Name | Instrument Type | Unit (UCUM) | Description
+--- | --- | --- | ---
+`aspnetcore.components.circuit.active`<!--](https://opentelemetry.io/docs/specs/semconv/dotnet/dotnet-http-metrics/#metric-aspnetcorecomponentscircuitactive)--> | UpDownCounter | `{circuit}` | Shows the number of active circuits currently in memory.
+
+#### Metric: `aspnetcore.components.circuit.connected`
+
+Name | Instrument Type | Unit (UCUM) | Description
+--- | --- | --- | ---
+`aspnetcore.components.circuit.connected`<!--](https://opentelemetry.io/docs/specs/semconv/dotnet/dotnet-http-metrics/#metric-aspnetcorecomponentscircuitconnected)--> | UpDownCounter | `{circuit}` | Tracks the number of circuits connected to clients.
+
+#### Metric: `aspnetcore.components.circuit.duration`
+
+Name | Instrument Type | Unit (UCUM) | Description
+--- | --- | --- | ---
+`aspnetcore.components.circuit.duration`<!--](https://opentelemetry.io/docs/specs/semconv/dotnet/dotnet-http-metrics/#metric-aspnetcorecomponentscircuitduration)--> | Histogram | `s` | Measures circuit lifetime duration and provides total circuit count.
 
 ## `Microsoft.AspNetCore.Hosting`
 

diff --git a/aspnetcore/release-notes/aspnetcore-10.0.md b/aspnetcore/release-notes/aspnetcore-10.0.md
@@ -4,7 +4,7 @@ author: wadepickett
 description: Learn about the new features in ASP.NET Core in .NET 10.
 ms.author: wpickett
 ms.custom: mvc
-ms.date: 06/10/2025
+ms.date: 6/9/2025
 uid: aspnetcore-10
 ---
 # What's new in ASP.NET Core in .NET 10
@@ -13,7 +13,6 @@ This article highlights the most significant changes in ASP.NET Core in .NET 10
 
 This article will be updated as new preview releases are made available. For breaking changes, see [Breaking changes in .NET](/dotnet/core/compatibility/breaking-changes).
 
-
 ## Blazor
 
 This section describes new features for Blazor.

diff --git a/aspnetcore/release-notes/aspnetcore-10/includes/blazor.md b/aspnetcore/release-notes/aspnetcore-10/includes/blazor.md
@@ -1,6 +1,6 @@
 ### New and updated Blazor Web App security samples
 
-We've added and updated Blazor Web App security samples cross-linked in the following articles:
+We've added and updated the Blazor Web App security samples linked in the following articles:
 
 * <xref:blazor/security/blazor-web-app-oidc>
 * <xref:blazor/security/blazor-web-app-entra>
@@ -110,10 +110,6 @@ In prior Blazor releases, response streaming for <xref:System.Net.Http.HttpClien
 
 This is a breaking change because calling <xref:System.Net.Http.HttpContent.ReadAsStreamAsync%2A?displayProperty=nameWithType> for an <xref:System.Net.Http.HttpResponseMessage.Content%2A?displayProperty=nameWithType> (`response.Content.ReadAsStreamAsync()`) returns a `BrowserHttpReadStream` and no longer a <xref:System.IO.MemoryStream>. `BrowserHttpReadStream` doesn't support synchronous operations, such as `Stream.Read(Span<Byte>)`. If your code uses synchronous operations, you can opt-out of response streaming or copy the <xref:System.IO.Stream> into a <xref:System.IO.MemoryStream> yourself.
 
-<!-- UNCOMMENT FOR PREVIEW 5 ...
-     Waiting on https://github.com/dotnet/runtime/issues/97449
-     ... and update the Call web API article Line 983
-
 To opt-out of response streaming globally, use either of the following approaches:
 
 * Add the `<WasmEnableStreamingResponse>` property to the project file with a value of `false`:
@@ -124,12 +120,6 @@ To opt-out of response streaming globally, use either of the following approache
 
 * Set the `DOTNET_WASM_ENABLE_STREAMING_RESPONSE` environment variable to `false` or `0`.
 
-............. AND REMOVE THE NEXT LINE .............
-
--->
-
-To opt-out of response streaming globally, set the `DOTNET_WASM_ENABLE_STREAMING_RESPONSE` environment variable to `false` or `0`.
-
 To opt-out of response streaming for an individual request, set <xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions.SetBrowserResponseStreamingEnabled%2A> to `false` on the <xref:System.Net.Http.HttpRequestMessage> (`requestMessage` in the following example):
 
 ```csharp
@@ -387,8 +377,6 @@ Calling <xref:Microsoft.AspNetCore.Components.NavigationManager.NavigateTo%2A?di
 
 Code that relied on <xref:Microsoft.AspNetCore.Components.NavigationException> being thrown should be updated. For example, in the default Blazor Identity UI, the `IdentityRedirectManager` previously threw an <xref:System.InvalidOperationException> after calling `RedirectTo` to ensure it wasn't invoked during interactive rendering. This exception and the [`[DoesNotReturn]` attributes](xref:System.Diagnostics.CodeAnalysis.DoesNotReturnAttribute) should now be removed.
 
-<!-- HOLD FOR PREVIEW 5
-
 To revert to the previous behavior of throwing a <xref:Microsoft.AspNetCore.Components.NavigationException>, set the following <xref:System.AppContext> switch:
 
 ```csharp
@@ -397,8 +385,6 @@ AppContext.SetSwitch(
     isEnabled: true);
 ```
 
--->
-
 ### Not Found responses using `NavigationManager` for static SSR and global interactive rendering
 
 The <xref:Microsoft.AspNetCore.Components.NavigationManager> now includes a `NotFound` method to handle scenarios where a requested resource isn't found during static server-side rendering (static SSR) or global interactive rendering:
@@ -412,3 +398,27 @@ Per-page/component rendering support is planned for Preview 5 in June, 2025.
 You can use the `NavigationManager.OnNotFound` event for notifications when `NotFound` is invoked.
 
 For more information and examples, see <xref:blazor/fundamentals/routing?view=aspnetcore-10.0#not-found-responses>.
+
+### Blazor router has a `NotFoundPage` parameter
+
+Blazor now provides an improved way to display a "Not Found" page when navigating to a non-existent page. You can specify a page to render when `NavigationManager.NotFound` by passing a page type to the `Router` component using the `NotFoundPage` parameter. This approach is recommended over the previous `NotFound` fragment, as it supports routing, works across code re-execution middleware, and is compatible even with non-Blazor scenarios. If both a `NotFound` fragment and `NotFoundPage` are defined, the page specified by `NotFoundPage` takes priority.
+
+```razor
+<Router AppAssembly="@typeof(Program).Assembly" NotFoundPage="typeof(Pages.NotFound)">
+    <Found Context="routeData">
+        <RouteView RouteData="@routeData" />
+        <FocusOnNavigate RouteData="@routeData" Selector="h1" />
+    </Found>
+    <NotFound>This content is ignored because NotFoundPage is defined.</NotFound>
+</Router>
+```
+
+The Blazor project template now includes a `NotFound.razor` page by default. This page automatically renders whenever `NavigationManager.NotFound` is called in your app, making it easier to handle missing routes with a consistent user experience.
+
+For more information, see <xref:blazor/fundamentals/routing#not-found-responses>.
+
+### Metrics and tracing
+
+This release introduces comprehensive metrics and tracing capabilities for Blazor apps, providing detailed observability of the component lifecycle, navigation, event handling, and circuit management.
+
+For more information, see <xref:blazor/performance/index#metrics-and-tracing>.