Prerendering is on by default for all interactive render modes. The server renders the component as static HTML and ships it to the browser immediately. Then the interactive runtime (Server/WebAssembly) loads and re-renders the component with full interactivity.
\nThis means:
\nOnInitializedAsync runs twice — once during prerender (static), once when the interactive runtime attaches.OnAfterRenderAsync is NOT called during prerender — only after the interactive render.Check the project's AGENTS.md for the Interactivity Mode and Interactivity Scope:
| Mode | \nPrerendering applies? | \n
|---|---|
| None (Static SSR) | \nNo — there's no interactive handoff | \n
| Server | \nYes | \n
| WebAssembly | \nYes | \n
| Auto | \nYes | \n
If the mode is None, this skill doesn't apply.
The most common prerendering problem: data loaded in OnInitializedAsync during prerender is thrown away and re-fetched when the interactive runtime attaches. This causes flicker and duplicate API/DB calls.
[PersistentState] attributeAnnotate properties to automatically serialize during prerender and restore on interactive activation:
\n@page \"/forecasts\"\n@rendermode InteractiveServer\n\n<h1>Weather</h1>\n\n@if (Forecasts is null)\n{\n <p>Loading...</p>\n}\nelse\n{\n @foreach (var f in Forecasts)\n {\n <p>@f.Date: @f.TemperatureC°C</p>\n }\n}\n\n@code {\n [PersistentState]\n public WeatherForecast[]? Forecasts { get; set; }\n\n protected override async Task OnInitializedAsync()\n {\n Forecasts ??= await ForecastService.GetForecastsAsync();\n }\n}\nThe ??= pattern is critical — it means \"only fetch if the property wasn't already restored from prerender state.\"
When the same component type appears multiple times, use @key to disambiguate state:
@foreach (var item in items)\n{\n <ItemCard @key=\"item.Id\" />\n}\nPersistentComponentState serviceFor complex scenarios (dynamic keys, custom serialization), use the imperative API:
\n@inject PersistentComponentState ApplicationState\n\n@code {\n private List<Order>? orders;\n\n protected override async Task OnInitializedAsync()\n {\n ApplicationState.RegisterOnPersisting(PersistOrders);\n\n if (!ApplicationState.TryTakeFromJson<List<Order>>(\"orders\", out var restored))\n {\n orders = await OrderService.GetOrdersAsync();\n }\n else\n {\n orders = restored;\n }\n }\n\n private Task PersistOrders()\n {\n ApplicationState.PersistAsJson(\"orders\", orders);\n return Task.CompletedTask;\n }\n}\nDisable prerendering when a component depends on browser APIs immediately or when the prerender+interactive double render causes problems you can't solve with [PersistentState].
@rendermode @(new InteractiveServerRenderMode(prerender: false))\nReplace InteractiveServerRenderMode with InteractiveWebAssemblyRenderMode or InteractiveAutoRenderMode as needed.
<MyChart @rendermode=\"new InteractiveServerRenderMode(prerender: false)\" />\nIn App.razor:
<HeadOutlet @rendermode=\"new InteractiveServerRenderMode(prerender: false)\" />\n<Routes @rendermode=\"new InteractiveServerRenderMode(prerender: false)\" />\nNote: A parent's prerendering setting overrides children. If <Routes> disables prerendering, individual pages cannot re-enable it.
In a globally interactive app, some pages may need HttpContext (cookies, request headers, response status codes). These pages must render via static SSR, not inside the interactive runtime.
Use [ExcludeFromInteractiveRouting]:
@page \"/privacy\"\n@attribute [ExcludeFromInteractiveRouting]\n\n<h1>Privacy Policy</h1>\nThis forces a full page reload when navigating to this page, exiting interactive routing. The page renders as static SSR with full HttpContext access.
In App.razor, conditionally apply the render mode:
<!DOCTYPE html>\n<html>\n<head>\n <HeadOutlet @rendermode=\"RenderModeForPage\" />\n</head>\n<body>\n <Routes @rendermode=\"RenderModeForPage\" />\n <script src=\"_framework/blazor.web.js\"></script>\n</body>\n</html>\n\n@code {\n [CascadingParameter]\n public HttpContext HttpContext { get; set; } = default!;\n\n private IComponentRenderMode? RenderModeForPage =>\n HttpContext.AcceptsInteractiveRouting() ? InteractiveServer : null;\n}\nReplace InteractiveServer with the app's configured render mode.
Use RendererInfo to guard code that should only run interactively:
protected override async Task OnInitializedAsync()\n{\n if (RendererInfo.IsInteractive)\n {\n // Only runs during the interactive render, not during prerender\n await StartSignalRConnection();\n }\n}\nRendererInfo properties:
IsInteractive — false during prerender, true after interactive runtime attachesName — \"Static\" during prerender, \"Server\" or \"WebAssembly\" when interactiveComponents in the .Client project prerender on the server. Services registered only in the client Program.cs (e.g., IWebAssemblyHostEnvironment) won't be available during prerender.
Fix by one of:
\nProgram.cs files provide the servicepublic MyComponent(IMyService? svc = null).Client, implementations in both projectsOnInitializedAsync — JS isn't available during prerender. Use OnAfterRenderAsync(firstRender).OnInitializedAsync runs once — it runs twice with prerendering. Always use [PersistentState] or ??= guards.HttpContext in interactive components — it's only available during the static prerender, not during the interactive lifetime. Use [ExcludeFromInteractiveRouting] for pages that need it.[PersistentState] to preserve state instead.