Scalar

Core Principles

1. Scalar replaces Swagger UI — Scalar is the recommended API documentation UI for .NET 10. Faster rendering, built-in dark mode, code generation for dozens of languages, and full OpenAPI 3.1 support.
2. Development only by default — Wrap MapScalarApiReference() in an IsDevelopment() check. API documentation exposes internal structure. If needed in production, add authorization.
3. Disable the proxy for sensitive APIs — Scalar's "Try It" feature routes through proxy.scalar.com by default. Disable it with .WithProxy(null) to keep auth headers local.
4. Security schemes come from OpenAPI — Scalar reads security schemes from the OpenAPI document. Configure them via document transformers, not in Scalar directly.

Patterns

Basic Setup

using Scalar.AspNetCore;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddOpenApi();

var app = builder.Build();

if (app.Environment.IsDevelopment())
{
    app.MapOpenApi();
    app.MapScalarApiReference();  // UI at /scalar/v1
}

app.Run();

Customized Configuration

app.MapScalarApiReference(options =>
{
    options
        .WithTitle("Checkout API")
        .WithTheme(ScalarTheme.Mars)
        .WithDefaultHttpClient(ScalarTarget.CSharp, ScalarClient.HttpClient)
        .WithPreferredScheme("Bearer")
        .WithProxy(null)  // Disable external proxy
        .WithSidebar(true);
});

Authentication Prefill (Development Only)

Pre-fill credentials so developers don't have to paste tokens manually. The OpenAPI document must already include the security scheme via a document transformer.

if (app.Environment.IsDevelopment())
{
    app.MapScalarApiReference(options =>
    {
        options
            .WithPreferredScheme("Bearer")
            .AddHttpAuthentication("Bearer", auth =>
            {
                auth.Token = "dev-only-test-token";
            });
    });
}

Other auth types:

// API Key
options.WithApiKeyAuthentication(apiKey =>
{
    apiKey.Token = "dev-api-key";
});

// OAuth2
options.WithOAuth2Authentication(oauth =>
{
    oauth.ClientId = "your-client-id";
    oauth.Scopes = ["openid", "profile"];
});

Available Themes

// ScalarTheme options: Default, Moon, Purple, BluePlanet, Saturn, Mars, DeepSpace, Kepler, Solarized, Laserwave
options.WithTheme(ScalarTheme.Mars);

Multiple API Documents

// Register multiple OpenAPI documents
builder.Services.AddOpenApi("v1");
builder.Services.AddOpenApi("v2-beta");

// Scalar picks them up automatically
app.MapOpenApi();
app.MapScalarApiReference();
// Available at /scalar/v1 and /scalar/v2-beta

Or configure documents explicitly:

app.MapScalarApiReference(options =>
{
    options
        .AddDocument("v1", "Production API")
        .AddDocument("v2-beta", "Beta API", isDefault: true);
});

Custom Route Prefix

// Default is /scalar/{documentName}
app.MapScalarApiReference("/api-docs");
// Now at /api-docs/v1

Production with Authorization

// When partners need access to docs in production
app.MapOpenApi().RequireAuthorization("ApiDocs");
app.MapScalarApiReference().RequireAuthorization("ApiDocs");

Force Dark Mode

options.ForceDarkMode();

Classic Layout (Swagger-like)

options.WithClassicLayout();

Anti-patterns

Don't Expose Scalar in Production Without Auth

// BAD — anyone can see your API structure
app.MapOpenApi();
app.MapScalarApiReference();

// GOOD — development only
if (app.Environment.IsDevelopment())
{
    app.MapOpenApi();
    app.MapScalarApiReference();
}

// GOOD — production with auth
app.MapOpenApi().RequireAuthorization("ApiDocs");
app.MapScalarApiReference().RequireAuthorization("ApiDocs");

Don't Pre-fill Real Credentials

// BAD — real tokens visible in browser
options.AddHttpAuthentication("Bearer", auth =>
{
    auth.Token = "eyJhbG...real-production-token";
});

// GOOD — dev-only test tokens
if (app.Environment.IsDevelopment())
{
    options.AddHttpAuthentication("Bearer", auth =>
    {
        auth.Token = "dev-only-test-token";
    });
}

Don't Forget the Security Scheme Transformer

// BAD — no auth UI in Scalar because OpenAPI doc has no security schemes
builder.Services.AddOpenApi();
app.MapScalarApiReference(options =>
{
    options.WithPreferredScheme("Bearer"); // Does nothing!
});

// GOOD — register the document transformer first
builder.Services.AddOpenApi(options =>
{
    options.AddDocumentTransformer<BearerSecuritySchemeTransformer>();
});
app.MapScalarApiReference(options =>
{
    options.WithPreferredScheme("Bearer");
});

Don't Leave the Proxy Enabled for Sensitive APIs

// BAD — auth headers flow through proxy.scalar.com
app.MapScalarApiReference();

// GOOD — disable proxy for APIs with sensitive data
app.MapScalarApiReference(options =>
{
    options.WithProxy(null);
});

Don't Use Swagger UI for New .NET 10 Projects

// BAD — Swashbuckle removed from templates, maintenance concerns
builder.Services.AddSwaggerGen();
app.UseSwaggerUI();

// GOOD — built-in OpenAPI + Scalar
builder.Services.AddOpenApi();
app.MapOpenApi();
app.MapScalarApiReference();

Decision Guide