> ## Documentation Index
> Fetch the complete documentation index at: https://developers.myhero.so/llms.txt
> Use this file to discover all available pages before exploring further.

# Errors

> HERO REST API error response shape and status codes — a JSON envelope with error, message, and details fields, listed by HTTP status.

All errors return JSON in this shape:

```json theme={null}
{
  "error": "ValidationError",
  "message": "Request failed schema validation",
  "details": [...]
}
```

## Status codes

### 400

* 400 response
* Mcp-Session-Id header missing.
* Missing or invalid request.
* Validation error
* sessionId query param missing.

### 401

* 401 response
* OAuth token missing or invalid.

### 403

* 403 response
* Session belongs to a different user.

### 404

* 404 response
* No active session for the given session ID.
* Unknown client identifier.

### 409

* 409 response

### 413

* 413 response

### 429

* 429 response

### 500

* 500 response

### 503

* Extension not yet built on this server.