# Auth And Identity

> Use this skill for authentication, authorization, or identity in a Cratis Arc project — backend, frontend, or both. Covers identity providers (`IProvideIdentityDetails`), protecting commands/queries with authorization attributes, Microsoft Identity Platform, connecting backend identity to React, multi-tenant identity, and local-dev generated principals. Trigger on auth, login, roles, permissions, identity details, user context, or protecting endpoints.

- Skill: `cratis/auth-and-identity` (Agent Skill, multi-file: 6 files)
- Install (CLI): `npx skillmds@latest add cratis/auth-and-identity`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cratis/auth-and-identity/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Web & Frontend
- Author: Cratis (https://skillmd.com/u/cratis)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/cratis/auth-and-identity

---


# Auth & Identity in Cratis Arc

This skill covers the full auth and identity stack in a Cratis Arc application. Read the relevant reference files below for detailed API usage.

> **Read the relevant instruction files first.** This skill references concepts from the core copilot instructions in `.github/copilot-instructions.md`. If you need details on vertical slices, commands, queries, or proxy generation, consult those instructions.

## Architecture Overview

Identity and auth in Arc follow a cookie-first, convention-based pattern:

```
Frontend (React)                          Backend (ASP.NET Core)
─────────────────                         ──────────────────────
<IdentityProvider>                        app.MapIdentityProvider()
  └── useIdentity() hook                    └── GET /.cratis/me
        │                                        │
        ├─ 1. Read .cratis-identity cookie        │
        └─ 2. If no cookie → fetch /.cratis/me    │
                                                  ▼
                                    AuthenticationMiddleware
                                      └── IAuthenticationHandler[]
                                            │ sets HttpContext.User
                                            ▼
                                    IIdentityProvider.Get()
                                      └── IProvideIdentityDetails.Provide()
                                            │
                                            ▼
                                    IdentityProviderResult
                                      → JSON response
                                      → .cratis-identity cookie (base64)

Authorization:
  [Authorize] / [Roles("Admin")] / [AllowAnonymous]
    └── AuthorizationEvaluator checks per command/query
```

**Key design decisions:**
- The `.cratis-identity` cookie is `HttpOnly=false` so the frontend JavaScript can read it directly — no extra HTTP call needed on page load.
- Identity details are base64-encoded JSON in the cookie, automatically decoded by the frontend `IdentityProvider`.
- Only one `IProvideIdentityDetails` implementation is allowed per application (auto-discovered). If none exists, a default provider grants access to everyone.
- Cratis has its own `[Authorize]`, `[AllowAnonymous]`, and `[Roles]` attributes in `Cratis.Arc.Authorization` — these are distinct from ASP.NET Core's and are evaluated by `AuthorizationEvaluator` in the command and query pipeline.

---

## Decision Tree — Which Reference to Read

Use this decision tree to determine which reference file(s) to read based on the user's task:

| User wants to... | Read this reference |
|---|---|
| Add identity details to their app | [references/backend-identity.md](references/backend-identity.md) |
| Customize `IProvideIdentityDetails` (enrich from DB, block users, multi-tenant, preferences) | [references/backend-identity.md](references/backend-identity.md) |
| Modify identity at runtime (stateless selections, `ModifyDetails`) | [references/backend-identity.md](references/backend-identity.md) |
| Use Azure AD / Entra ID / Microsoft Identity | [references/authentication.md](references/authentication.md) |
| Write a custom authentication handler (API key, JWT, etc.) | [references/authentication.md](references/authentication.md) |
| Protect commands or queries with roles | [references/authorization.md](references/authorization.md) |
| Set up `[Authorize]`, `[AllowAnonymous]`, or `[Roles]` | [references/authorization.md](references/authorization.md) |
| Consume identity in React | [references/frontend.md](references/frontend.md) |
| Use identity in MVVM or vanilla TypeScript | [references/frontend.md](references/frontend.md) |
| Test identity locally without Azure | [references/local-development.md](references/local-development.md) |
| Full-stack setup (backend + frontend) | Read all references in order |

**For full-stack tasks, read in this order:**
1. `references/backend-identity.md` — identity provider and startup
2. `references/authentication.md` — how users are authenticated
3. `references/authorization.md` — protecting commands and queries
4. `references/frontend.md` — consuming identity in the UI
5. `references/local-development.md` — testing without real infrastructure

---

## Quick-Start: Full-Stack Identity Setup

This is the minimum checklist for an application with identity. Read the reference files for details on each step.

### Backend

1. **Details record**: Define a C# record for application-specific user information
2. **Identity provider**: Implement `IProvideIdentityDetails<TDetails>` (auto-discovered, one per app)
3. **Startup**: Call `app.MapIdentityProvider()` to register `GET /.cratis/me`
4. **Authentication**: Add `AddMicrosoftIdentityPlatformIdentityAuthentication()` or implement `IAuthenticationHandler`
5. **Authorization**: Add `[Authorize]` / `[Roles]` / `[AllowAnonymous]` attributes from `Cratis.Arc.Authorization` to commands and queries

### Frontend

6. **Provider**: Wrap app root with `<IdentityProvider>` from `@cratis/arc.react/identity`
7. **Hook**: Use `useIdentity<TDetails>()` to access identity anywhere in the component tree
8. **Roles**: Use `identity.isInRole('Admin')` for UI-level role gating

### Proxy Generation

9. Using `IProvideIdentityDetails<TDetails>` (generic) enables automatic TypeScript type generation at `dotnet build` time — the generated types can be imported in the frontend for end-to-end type safety.

---

## Critical Rules

These rules are frequently violated — always enforce them:

1. **One identity provider per app**: Only one `IProvideIdentityDetails` implementation is allowed. Multiple throws `MultipleIdentityDetailsProvidersFound`.
2. **Use Cratis attributes, not ASP.NET Core's**: `[Authorize]`, `[Roles]`, `[AllowAnonymous]` must come from `Cratis.Arc.Authorization`, not `Microsoft.AspNetCore.Authorization`.
3. **Never combine `[Authorize]` and `[AllowAnonymous]` on the same target**: This throws `AmbiguousAuthorizationLevel`.
4. **Prefer the generic interface**: Use `IProvideIdentityDetails<TDetails>` over `IProvideIdentityDetails` to enable proxy generation.
5. **Auto-discovery**: Both `IProvideIdentityDetails` and `IAuthenticationHandler` implementations are auto-discovered — no DI registration needed.
6. **Frontend role checks are UX, not security**: `isInRole()` on the frontend hides UI elements. The backend `[Roles]` attribute is the actual security boundary.
7. **Build before frontend**: TypeScript proxy types for identity details are generated by `dotnet build`. The backend must compile before the frontend can import them.
8. **Authorize at the boundary, secure by default**: express access with `[Authorize]`/`[Roles]` attributes on the command/query — **never** gate behavior with an `if (identity.IsInRole(...))` inside `Handle()`. For app-wide protection, configure a default-deny fallback policy so a target is protected unless it explicitly opts out with `[AllowAnonymous]`. Cross-cutting auth that spans many commands belongs in an `ICommandFilter`, not duplicated per handler.
9. **Command-specific scope is validation, not an attribute**: a rule like "may only act on resources in your own organization" belongs in the `CommandValidator<T>` (inject the identity and reject with a validation error) — not in `Handle()`, and not expressible by a role attribute alone.
10. **Read the current user from the authenticated principal**: in backend code resolve the current user from `IHttpContextAccessor.User` (the ASP.NET principal the authentication handler populates). Do not invent a bespoke `IIdentityAccessor` abstraction — that is a product-specific wrapper, not part of generic Cratis.

---

## Common Code Patterns

### Protecting a command with roles

```csharp
[Command]
[Roles("Admin")]
public record PromoteUser(UserId Id)
{
    public void Handle(IUserService users) => users.Promote(Id);
}
```

### Conditionally rendering UI based on roles

```tsx
const identity = useIdentity();

return identity.isInRole('Admin')
    ? <AdminPanel />
    : <AccessDenied />;
```

### Modifying identity at runtime (stateless selections)

```csharp
public class SetDepartment(IIdentityProvider identityProvider)
{
    public async Task Handle(string department) =>
        await identityProvider.ModifyDetails<UserDetails>(
            details => details with { SelectedDepartment = department });
}
```

---

## Reference Documentation

### Skill references (detailed implementation guidance)

- [Backend Identity Provider](references/backend-identity.md) — `IProvideIdentityDetails`, `IdentityProviderContext`, cookie mechanics, proxy generation, `ModifyDetails`
- [Authentication](references/authentication.md) — `IAuthenticationHandler`, `AuthenticationResult`, Microsoft Identity Platform, combining handlers
- [Authorization](references/authorization.md) — `[Authorize]`, `[Roles]`, `[AllowAnonymous]`, inheritance rules, fallback policies
- [Frontend Identity](references/frontend.md) — React `IdentityProvider`, `useIdentity()`, MVVM, core identity, role checking
- [Local Development](references/local-development.md) — Generating principals, ModHeader, cookie fallback, dev testing