---
title: Errors
description: Error values and HTTP status code mapping.
---

Nexus defines sentinel errors in the root package. These are used throughout the codebase and mapped to HTTP status codes in the API layer.

## Error Values

```go
var (
    ErrProviderNotFound    = errors.New("nexus: provider not found")
    ErrModelNotFound       = errors.New("nexus: model not found")
    ErrNoProviderAvailable = errors.New("nexus: no provider available")
    ErrAuthRequired        = errors.New("nexus: authentication required")
    ErrForbidden           = errors.New("nexus: forbidden")
    ErrTenantRequired      = errors.New("nexus: tenant context required")
    ErrTenantNotFound      = errors.New("nexus: tenant not found")
    ErrTenantDisabled      = errors.New("nexus: tenant disabled")
    ErrGuardrailBlocked    = errors.New("nexus: blocked by guardrail")
    ErrCacheMiss           = errors.New("nexus: cache miss")
    ErrRateLimited         = errors.New("nexus: rate limit exceeded")
    ErrBudgetExceeded      = errors.New("nexus: budget exceeded")
    ErrTokenOverflow       = errors.New("nexus: token limit exceeded")
    ErrAliasNotFound       = errors.New("nexus: alias not found")
)
```

## HTTP Mapping

The API layer maps these errors to standard HTTP responses:

| Error | HTTP Status |
|-------|-------------|
| `ErrAuthRequired` | 401 Unauthorized |
| `ErrForbidden` | 403 Forbidden |
| `ErrProviderNotFound` | 404 Not Found |
| `ErrTenantNotFound` | 404 Not Found |
| `ErrGuardrailBlocked` | 400 Bad Request |
| `ErrRateLimited` | 429 Too Many Requests |
| `ErrBudgetExceeded` | 402 Payment Required |
| `ErrTokenOverflow` | 413 Payload Too Large |