Flash data

Flash data is carried across the redirect (and any further redirects) and consumed by the next response that is not a redirect. It appears as page.flash, only when there is some.

app.MapPost("/users", (CreateUserInput input, HttpContext context) =>
{
    context.Inertia().Flash("toast", new { type = "success", message = "Saved" });
    return Results.Redirect("/users");
});

// or on Back():
return Back().WithFlash("toast", "Saved");

{
  "component": "Users/Index",
  "props": { "errors": {} },
  "flash": { "toast": { "type": "success", "message": "Saved" } }
}

Read it with usePage().flash or the onFlash visit callback:

const { flash } = usePage()

{flash.toast && <Toast {...flash.toast} />}

Where state is kept

Flash data, errors, clearHistory and preserveFragment set during a request that ends in a redirect (or a Location() / fragment 409) are saved in an IInertiaStateStore and loaded at the start of the next request. If that request redirects again, the state is carried on, so Back() to a URL that itself redirects still shows the flash. Otherwise its response consumes it. The state is also kept when the response is the version-mismatch 409.

Encrypted cookie (default)

The default store is an encrypted cookie named .Inertia.State. It is protected with ASP.NET Core Data Protection, HttpOnly, SameSite=Lax, and works without sessions and under Native AOT. Tampered or undecryptable cookies are ignored and deleted. It logs a warning above about 4 KB because browsers drop larger cookies.

Session store

builder.Services.AddSession();
builder.Services.AddInertia(o => o.State.UseSession());

app.UseSession();       // before UseInertia
app.UseInertia();

Use the session store when flash data can be large or must not reach the browser, and you already run sessions.

MVC TempData store

builder.Services.AddInertiaMvc(o => o.UseTempDataStateStore());

This keeps the state in MVC TempData, so it follows your TempData provider: the default cookie provider, or AddSessionStateTempDataProvider() with AddSession() and app.UseSession() before app.UseInertia(). It works for Minimal API endpoints too, because the store saves TempData itself. Not Native AOT compatible.

Your own store

Implement IInertiaStateStore and register it before AddInertia:

public interface IInertiaStateStore
{
    ValueTask<byte[]?> LoadAsync(HttpContext httpContext);
    ValueTask SaveAsync(HttpContext httpContext, byte[] state);
    ValueTask ClearAsync(HttpContext httpContext);
}

The methods move opaque bytes; the format is Inertia.Net's.

Choosing a store

History flags

encryptHistory is written only when set and is not carried across redirects. clearHistory and preserveFragment go through the state store and apply to the next page rendered after the redirects, then are consumed.

app.MapPost("/logout", (HttpContext context) =>
{
    context.Inertia().ClearHistory();
    return Results.Redirect("/login");
});

See Redirects and history.