> ## Documentation Index > Fetch the complete documentation index at: https://docs.isaacus.com/llms.txt > Use this file to discover all available pages before exploring further. # Errors > How the Isaacus API communicates errors The Isaacus API uses conventional HTTP response codes to indicate whether an API request has succeeded or failed. In general, codes in the `2xx` range indicate success, codes in the `4xx` range indicate a problem with the request that was sent, and codes in the `5xx` range indicate an error with our servers. Errors returned by our API servers (as opposed to errors generated by gateway servers) will conform to the [RFC 7807](https://tools.ietf.org/html/rfc7807) problem detail standard: ```json theme={null} { "type": "A URL that explains the error or 'about:blank' if no such URL is available.", "title": "A short, human-readable summary of the problem.", "status": "The integer HTTP status code (normally 400 or 500) generated by the origin server for this occurrence of the problem.", "detail": "An optional human-readable explanation specific to this occurrence of the problem.", "instance": "An optional URL that identifies this specific occurrence of the problem." } ``` ## Error types This is a non-exhaustive list of the types of errors you may encounter when using the Isaacus API. ### 400 validation error A validation error indicates that the API request was **invalid**, perhaps because a constraint was violated or a required field was missing, or perhaps because the request was incapable of being processed due to some quality of the request itself. A validation error will return the `400 Bad Request` status code. A validation error should always include a non-generic `detail` field that explains, in some way, what was wrong with the request. If you ever encounter a validation error that does not include a `detail` field (or that lacks sufficient information to help you understand what went wrong with your request), please [let us know](https://isaacus.com/support). ### 401 unauthenticated error An unauthenticated error indicates that the request was **not properly authenticated**. This is typically because the request used an API key that does not exist, is expired or is revoked. An unauthenticated error will return the `401 Unauthorized` status code. If you encounter an unauthenticated error, please double-check that the API key you are using to make the request is correct and active. If you are certain that the API key is correct and active, please [let us know](https://isaacus.com/support). ### 402 payment required error A payment required error indicates that the request was not processed because the account associated with the API key used to make the request has an **outstanding balance**. You can pay overdue invoices [here](https://platform.isaacus.com/billing). A payment required error will return the `402 Payment Required` status code. If you encounter a payment required error but you have no outstanding balance, please [let us know](https://isaacus.com/support). ### 403 access forbidden error An access forbidden error indicates that the request was not processed because the account associated with the API key used to make the request **does not have permission** to access the requested resource. This could be because you cancelled your subscription to our zero flat fee, usage-based API plan, which you can resubscribe to [here](https://platform.isaacus.com/billing). An access forbidden error will return the `403 Forbidden` status code. If you encounter an access forbidden error but you have not unsubscribed from our API plan, please [let us know](https://isaacus.com/support). ### 404 not found error A not found error indicates that the requested resource **could not be found**, either because it never existed or because it was deleted. A not found error will return the `404 Not Found` status code. If you encounter a not found error, please double-check the URL you are using to make the request. If you are certain that the URL is correct, please [let us know](https://isaacus.com/support). ### 405 method not allowed error A method not allowed error indicates that the request was made using a **HTTP method that is not supported** by the requested resource, for example, by sending a `GET` request to an endpoint that only supports `POST` requests. A method not allowed error will return the `405 Method Not Allowed` status code. ### 413 request too large error A request too large error indicates that the **request was too large** for the server to process, typically because the request body exceeded the maximum size allowed by the server. A request too large error will return the `413 Payload Too Large` status code. If you want to be able to send larger requests, please [let us know](https://isaacus.com/support). ### 500 internal server error An internal server error indicates that the server encountered an **unexpected problem** that prevented it from fulfilling the request. An internal server error will return the `500 Internal Server Error` status code. You are advised to retry your request if you encounter an internal server error. If the problem persists, please [let us know](https://isaacus.com/support), or else try waiting a bit later, as we are pinged whenever internal server errors occur and we may be in the process of fixing the problem.