Add aspnet-openapi skill (#dotnet-aspnet)

plugins/dotnet-aspnet/skills/aspnet-openapi/SKILL.md

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+---
+name: aspnet-openapi
+description: >
+  Add OpenAPI documentation to ASP.NET Core APIs (.NET 8+). Covers technology selection
+  (Microsoft.AspNetCore.OpenApi vs Swashbuckle vs NSwag), Scalar and Swagger UI setup,
+  JWT Bearer security scheme configuration, endpoint metadata for minimal APIs and MVC
+  controllers, and document/operation transformers.
+  USE FOR: adding an OpenAPI spec endpoint to a new or existing ASP.NET Core project,
+  setting up an interactive API explorer (Scalar, Swagger UI), configuring JWT Bearer
+  security in the OpenAPI document, annotating minimal API endpoints with response types
+  and tags, customizing the generated spec with transformers, or migrating from
+  Swashbuckle.AspNetCore to the built-in package on .NET 9+.
+  DO NOT USE FOR: generating typed API client code from an OpenAPI spec (use NSwag CLI
+  or the dotnet-openapi tooling instead), adding OpenAPI to non-ASP.NET projects, or
+  validating incoming requests against a schema at runtime.
+---
+
+# ASP.NET Core OpenAPI
+
+.NET 9 changed the landscape. `Microsoft.AspNetCore.OpenApi` is now the first-party
+solution, backed by the ASP.NET Core team, and ships in the default web project templates.
+Swashbuckle — the de-facto standard for the previous decade — still works but is no longer
+the default, and its major versions have lagged .NET releases by months. NSwag remains the
+right choice when client code generation is the primary goal.
+
+## Stop Signals
+
+- **Need to generate C# or TypeScript API clients?** — Use NSwag or `dotnet-openapi`. This skill covers spec *generation*, not *consumption*.
+- **Targeting .NET 8 or earlier?** — `Microsoft.AspNetCore.OpenApi` is .NET 9+ only. Use Swashbuckle 6.x on .NET 8.
+- **Already on Swashbuckle and no .NET 9 migration planned?** — Don't migrate just for its own sake. See [references/swashbuckle-migration.md](references/swashbuckle-migration.md) if migration is wanted later.
+- **Review request on existing OpenAPI config?** — Jump directly to the Validation checklist.
+
+## Inputs
+
+| Input | Required | Description |
+|-------|----------|-------------|
+| Target framework | Yes | Determines which packages are available |
+| API style | Yes | Minimal APIs or MVC controllers — affects annotation syntax |
+| Auth scheme | Recommended | JWT Bearer, OAuth2, or API key — drives security scheme config |
+| Existing packages | Recommended | Check whether Swashbuckle or NSwag is already referenced |
+
+## Workflow
+
+### Step 1: Choose the Package
+
+| Situation | Package | Reason |
+|-----------|---------|--------|
+| New project on .NET 9+ | `Microsoft.AspNetCore.OpenApi` | First-party, default in templates, actively maintained by the ASP.NET Core team |
+| New project on .NET 8 | `Swashbuckle.AspNetCore` 6.x | Built-in package requires .NET 9+ |
+| Need typed client code generation | `NSwag.AspNetCore` | Only mature option with C#/TypeScript codegen; pair with the spec output |
+| Complex polymorphism or discriminators | `NSwag.AspNetCore` | More schema control than either alternative |
+| Existing project already on Swashbuckle | Keep Swashbuckle | Migration is optional — see [references/swashbuckle-migration.md](references/swashbuckle-migration.md) |
+
+**Hard rule:** Do not mix `Microsoft.AspNetCore.OpenApi` and Swashbuckle in the same project.
+Both enumerate endpoints at startup and produce conflicting output. Pick one.
+
+---
+
+### Step 2: Set Up Microsoft.AspNetCore.OpenApi (.NET 9+)
+
+```bash
+dotnet add package Microsoft.AspNetCore.OpenApi
+```
+
+```csharp
+// Program.cs
+var builder = WebApplication.CreateBuilder(args);
+
+builder.Services.AddOpenApi();              // Register document generation
+
+var app = builder.Build();
+
+if (app.Environment.IsDevelopment())
+{
+    app.MapOpenApi();                       // Serves spec JSON at /openapi/v1.json
+}
+
+app.Run();
+```
+
+> **CRITICAL:** `MapOpenApi()` serves raw JSON — it is **not** a UI. Add a UI package in Step 4.
+
+> **CRITICAL:** The default spec path is `/openapi/v1.json`. This is **not** the Swashbuckle
+> default of `/swagger/v1/swagger.json`. Any tooling, CI pipeline, or Postman collection
+> importing from the old path will silently get a 404.
+
+**Multiple document versions:**
+
+```csharp
+builder.Services.AddOpenApi("v1");
+builder.Services.AddOpenApi("v2");
+
+// Both served at their named paths: /openapi/v1.json and /openapi/v2.json
+app.MapOpenApi("/openapi/{documentName}.json");
+```
+
+---
+
+### Step 3: Set Up Swashbuckle (.NET 8 / existing projects)
+
+```bash
+# 6.x for .NET 8 — 7.x for .NET 9
+dotnet add package Swashbuckle.AspNetCore
+```
+
+```csharp
+// Program.cs
+var builder = WebApplication.CreateBuilder(args);
+
+// CRITICAL: Required for minimal API endpoints to appear in the spec.
+// Swashbuckle uses the ApiExplorer infrastructure, which minimal APIs do NOT
+// register automatically. Without this line every MapGet/MapPost endpoint is
+// invisible in the generated spec. MVC controllers are unaffected.
+builder.Services.AddEndpointsApiExplorer();
+builder.Services.AddSwaggerGen();
+
+var app = builder.Build();
+
+if (app.Environment.IsDevelopment())
+{
+    app.UseSwagger();       // Serves spec at /swagger/v1/swagger.json
+    app.UseSwaggerUI();     // Serves Swagger UI at /swagger
+}
+
+app.Run();
+```
+
+---
+
+### Step 4: Add an API Explorer UI
+
+#### Scalar (recommended with Microsoft.AspNetCore.OpenApi)
+
+```bash
+dotnet add package Scalar.AspNetCore
+```
+
+```csharp
+using Scalar.AspNetCore;
+
+if (app.Environment.IsDevelopment())
+{
+    app.MapOpenApi();
+    app.MapScalarApiReference();    // Default at /scalar/v1
+}
+```
+
+Scalar reads from `MapOpenApi()` automatically. For customization:
+
+```csharp
+app.MapScalarApiReference(options =>
+{
+    options.WithTitle("Contoso API");
+    options.WithDefaultHttpClient(ScalarTarget.CSharp, ScalarClient.HttpClient);
+    options.WithPreferredScheme("Bearer");
+});
+```
+
+#### Swagger UI alongside Microsoft.AspNetCore.OpenApi
+
+If Swagger UI is a hard requirement, use only the UI package — not the full Swashbuckle stack:
+
+```bash
+dotnet add package Swashbuckle.AspNetCore.SwaggerUi
+```
+
+```csharp
+if (app.Environment.IsDevelopment())
+{
+    app.MapOpenApi();
+    app.UseSwaggerUI(options =>
+    {
+        // CRITICAL: Point at the built-in package's path, not the Swashbuckle default.
+        options.SwaggerEndpoint("/openapi/v1.json", "v1");
+        options.RoutePrefix = "swagger";
+    });
+}
+```
+
+---
+
+### Step 5: Annotate Endpoints
+
+The spec is only as useful as the metadata you put in. Sparse annotations produce a sparse,
+useless spec — and waste the entire investment in Step 2–4.
+
+**Minimal APIs:**
+
+```csharp
+app.MapGet("/products/{id:int}", async (int id, IProductRepository repo) =>
+{
+    var product = await repo.GetByIdAsync(id);
+    return product is null ? Results.NotFound() : Results.Ok(product);
+})
+.WithName("GetProductById")
+.WithSummary("Get a product by ID")
+.WithDescription("Returns a single product including pricing and stock level.")
+.WithTags("Products")
+.Produces<Product>(StatusCodes.Status200OK)
+.ProducesProblem(StatusCodes.Status404NotFound);
+
+app.MapPost("/products", async ([FromBody] CreateProductRequest req, IProductRepository repo) =>
+{
+    var id = await repo.CreateAsync(req);
+    return Results.CreatedAtRoute("GetProductById", new { id });
+})
+.WithName("CreateProduct")
+.WithTags("Products")
+.Accepts<CreateProductRequest>("application/json")
+.Produces<Product>(StatusCodes.Status201Created)
+.ProducesValidationProblem()
+.ProducesProblem(StatusCodes.Status409Conflict);
+
+// Exclude internal/infra endpoints entirely
+app.MapGet("/internal/health", () => "ok")
+   .ExcludeFromDescription();
+```
+
+**MVC Controllers:** Use `[ProducesResponseType]`, `[Produces]`, `[Consumes]`, and XML
+`<summary>` / `<remarks>` doc comments. Both packages consume these through the ApiExplorer
+infrastructure. Enable XML doc generation:
+
+```xml
+<!-- In your .csproj -->
+<PropertyGroup>
+    <GenerateDocumentationFile>true</GenerateDocumentationFile>
+</PropertyGroup>
+```
+
+For Swashbuckle, wire up the XML file in `AddSwaggerGen`:
+
+```csharp
+builder.Services.AddSwaggerGen(options =>
+{
+    var xmlFile = $"{Assembly.GetExecutingAssembly().GetName().Name}.xml";
+    options.IncludeXmlComments(Path.Combine(AppContext.BaseDirectory, xmlFile));
+});
+```
+
+---
+
+### Step 6: Configure JWT Bearer Security Scheme
+
+The built-in package does not auto-detect `AddAuthentication()`. You must declare the
+security scheme via a document transformer.
+
+```csharp
+// Program.cs
+builder.Services.AddOpenApi(options =>
+{
+    options.AddDocumentTransformer<BearerSecuritySchemeTransformer>();
+});
+```
+
+```csharp
+// BearerSecuritySchemeTransformer.cs
+using Microsoft.AspNetCore.OpenApi;
+using Microsoft.OpenApi.Models;
+
+internal sealed class BearerSecuritySchemeTransformer : IOpenApiDocumentTransformer
+{
+    public Task TransformAsync(
+        OpenApiDocument document,
+        OpenApiDocumentTransformerContext context,
+        CancellationToken cancellationToken)
+    {
+        document.Components ??= new OpenApiComponents();
+        document.Components.SecuritySchemes["Bearer"] = new OpenApiSecurityScheme
+        {
+            Type = SecuritySchemeType.Http,
+            Scheme = "bearer",
+            In = ParameterLocation.Header,
+            BearerFormat = "JWT",
+            Description = "Enter a valid JWT. Example: eyJhbGci..."
+        };
+
+        // Apply to every operation. For per-endpoint selective auth,
+        // use an operation transformer — see references/transformers.md.
+        var securityRequirement = new OpenApiSecurityRequirement
+        {
+            [new OpenApiSecurityScheme
+            {
+                Reference = new OpenApiReference
+                {
+                    Type = ReferenceType.SecurityScheme,
+                    Id = "Bearer"
+                }
+            }] = []
+        };
+
+        foreach (var pathItem in document.Paths.Values)
+        {
+            foreach (var operation in pathItem.Operations.Values)
+            {
+                operation.Security ??= [];
+                operation.Security.Add(securityRequirement);
+            }
+        }
+
+        return Task.CompletedTask;
+    }
+}
+```
+
+> For **per-endpoint** security (some routes public, some require auth), or for **OAuth2**
+> and **API key** schemes, see [references/transformers.md](references/transformers.md).
+
+For **Swashbuckle**, the equivalent uses `AddSecurityDefinition` and `AddSecurityRequirement`
+inside `AddSwaggerGen` — full examples in [references/transformers.md](references/transformers.md).
+
+---
+
+### Step 7: Secure the Spec Endpoint
+
+The OpenAPI document lists every endpoint, parameter, and auth scheme. Do not expose it
+publicly in production.
+
+**Option A — Development only** (most common for internal or backend APIs):
+
+```csharp
+// Already shown — wrap MapOpenApi() and the UI in IsDevelopment().
+if (app.Environment.IsDevelopment())
+{
+    app.MapOpenApi();
+    app.MapScalarApiReference();
+}
+```
+
+**Option B — Authenticated users only** (staging, internal portals):
+
+```csharp
+app.MapOpenApi()
+   .RequireAuthorization();
+```
+
+**Option C — Specific policy or role**:
+
+```csharp
+app.MapOpenApi()
+   .RequireAuthorization("InternalToolsPolicy");
+```
+
+> **CRITICAL:** Do not combine Option B/C with the JWT transformer from Step 6 without
+> verifying the flow end-to-end. If the spec endpoint requires a token, and the UI reads the
+> spec before the user authenticates, the UI will load blank. Either serve the UI and spec
+> without auth (Options A), or ensure both are behind the same authenticated session.
+
+---
+
+## Validation
+
+- [ ] `dotnet build -warnaserror` completes cleanly
+- [ ] Navigate to the spec path in a running instance — verify well-formed JSON is returned
+- [ ] Every endpoint with a non-trivial return type has at least one `Produces<T>()` or `[ProducesResponseType]` annotation
+- [ ] Error paths return `ProblemDetails` — not anonymous `{ error: "..." }` objects
+- [ ] If auth is configured, the `securitySchemes` section appears in the spec JSON and the scheme name matches the `$ref` value exactly (case-sensitive)
+- [ ] The spec endpoint is either restricted to Development or requires authorization — never `AllowAnonymous` in production
+
+## Common Pitfalls
+
+| Pitfall | Solution |
+|---------|----------|
+| Forgot `AddEndpointsApiExplorer()` with Swashbuckle | Minimal API endpoints won't appear — MVC controllers are unaffected. This is the single most common Swashbuckle setup bug |
+| Spec path mismatch after switching packages | Built-in defaults to `/openapi/v1.json`; Swashbuckle defaults to `/swagger/v1/swagger.json`. Update all downstream tooling explicitly |
+| Mixed `Microsoft.AspNetCore.OpenApi` + Swashbuckle | Both packages enumerate endpoints at startup. Behavior is undefined. Remove one |
+| Security scheme `$ref` name mismatch | The `Id` in `OpenApiReference` must exactly match the key in `SecuritySchemes` — `"Bearer"` ≠ `"bearer"` |
+| Spec endpoint returns 401 unexpectedly | Either wrapped in `IsDevelopment()` (run in Development env) or secured with `RequireAuthorization()` (authenticate first, or remove the auth requirement from the spec endpoint) |
+| Swashbuckle 6.x on .NET 9 | Swashbuckle 6.x does not support .NET 9+ reflection APIs. Upgrade to 7.x or migrate to the built-in package |
+| `MapOpenApi()` produces empty `paths` | All endpoints were registered after `app.Build()` was called, or the project targets .NET 8 where the built-in package is unavailable |
+| Scalar UI loads but shows no auth button | `WithPreferredScheme("Bearer")` not set, or the security scheme was not added to `document.Components.SecuritySchemes` |
+
+## Reference Files
+
+- **[references/transformers.md](references/transformers.md)** — Document, operation, and schema transformers for `Microsoft.AspNetCore.OpenApi`; equivalent `IDocumentFilter`, `IOperationFilter`, and `ISchemaFilter` patterns for Swashbuckle; OAuth2 and API key scheme setup. **Load when** customizing the generated spec, configuring non-JWT security schemes, adding XML documentation to the spec, or applying per-endpoint auth annotations.
+
+- **[references/swashbuckle-migration.md](references/swashbuckle-migration.md)** — Step-by-step migration from `Swashbuckle.AspNetCore` to `Microsoft.AspNetCore.OpenApi` on .NET 9+, including package changes, middleware rewrites, filter-to-transformer mapping, and common post-migration failures. **Load when** the project currently uses Swashbuckle and the developer wants to move to the built-in package.