This code system https://fhir.nhs.uk/CodeSystem/http-error-codes defines the following codes:
| Code | Display | Definition |
| SEND_BAD_REQUEST | 400: The API was unable to process the request. | The API has given a 400 response to the Senders request. |
| REC_BAD_REQUEST | 400: The Receiver was unable to process the request. | The Receiver has given a 400 response to the Senders request. |
| PROXY_BAD_REQUEST | 400: The Proxy was unable to process the request. | The Proxy has given a 400 response internally to the Senders request. |
| BAD_REQUEST | 400: The Server was unable to process the request. | The Server has given a 400 response to the Senders request. |
| SEND_UNAUTHORIZED | 401: The API deemed you unauthorized to make this request. | The API has given a 401 response to the Senders request. |
| REC_UNAUTHORIZED | 401: The Receiver deemed you unauthorized to make this request. | The Receiver has given a 401 response to the Senders request. |
| PROXY_UNAUTHORIZED | 401: The Proxy deemed you unauthorized to make this request. | The Proxy has given a 401 response internally to the Senders request. |
| UNAUTHORIZED | 401: The Server deemed you unauthorized to make this request. | The Server has given a 401 response to the Senders request. |
| SEND_FORBIDDEN | 403: Failed to authenticate with the API. | The API has given a 403 response to the Senders request. |
| REC_FORBIDDEN | 403: Failed to authenticate with the Receiver. | The Receiver has given a 403 response to the Senders request. |
| PROXY_FORBIDDEN | 403: Failed to authenticate with the Receiver. | The Proxy has given a 403 response to the Senders request. |
| FORBIDDEN | 403: Failed to Authenticate with the Server. | The Server has given a 403 response to the Senders request. |
| PROXY_NOT_FOUND | 404: The Proxy was unable to find the specified resource. | The Proxy has given a 404 response to the Senders request. |
| REC_NOT_FOUND | 404: The Receiver was unable to find the specified resource. | The Receiver has given a 404 response to the Senders request. |
| NOT_FOUND | 404: The Server was unable to find the specified resource. | The Server has given a 404 response to the Senders request. |
| SEND_METHOD_NOT_ALLOWED | 405: This API doesnt allow this method. | The API has given a 405 response to the Senders request. |
| REC_METHOD_NOT_ALLOWED | 405: The Receiver doesnt allow this method. | The Receiver has given a 405 response to the Senders request. |
| PROXY_METHOD_NOT_ALLOWED | 405: The Proxy doesnt allow this method. | The Proxy has given a 405 response to the Senders request. |
| METHOD_NOT_ALLOWED | 405: This method is not allowed. | The Server has given a 405 response to the Senders request. |
| SEND_NOT_ACCEPTABLE | 406: Senders message had an incorrect content type defined for a response. | The API has given a 406 response to the Senders request. |
| NOT_ACCEPTABLE | 406: Senders message had an incorrect content type defined for a response. | The Server has given a 406 response to the Senders request. |
| REC_TIMEOUT | 408: The request timed out to the receiver. | The Receiver has given a 408 resonse to the Senders request |
| PROXY_TIMEOUT | 408: The request timed out internally. | The Proxy has given a 408 response internally to the Senders request. |
| TIMEOUT | 408: The request timed out. | The Server has given a 408 response to the Senders request. |
| SEND_CONFLICT | 409: The API identified a conflict. | The API has given a 409 response to the Senders request. |
| REC_CONFLICT | 409: The Receiver identified a conflict. | The Receiver has given a 409 response to the Senders request. |
| PROXY_CONFLICT | 409: The Proxy identified a conflict. | The Proxy has given a 409 response to the Senders request. |
| CONFLICT | 409: The Server identified a conflict. | The Server has given a 409 response to the Senders request. |
| SEND_UNPROCESSABLE_ENTITY | 422: Message was not malformed but deemed unprocessable by the API. | The API has given a 422 response to the Senders request. |
| REC_UNPROCESSABLE_ENTITY | 422: Message was not malformed but deemed unprocessable by the Receiver. | The Receiver has given a 422 response to the Senders request. |
| PROXY_UNPROCESSABLE_ENTITY | 422: Message was not malformed but deemed unprocessable by the Proxy. | The Proxy has given a 422 response to the Senders request. |
| UNPROCESSABLE_ENTITY | 422: Message was not malformed but deemed unprocessable by the server. | The Server has given a 422 response to the Senders request. |
| PROXY_TOO_MANY_REQUESTS | 429: Too many requests have been made by this source to the Proxy in a given amount of time. | The Proxy has given a 429 response internally to the Senders request. |
| SEND_TOO_MANY_REQUESTS | 429: Too many requests have been made by this source to the API in a given amount of time. | The API has given a 429 response to the Senders request. |
| TOO_MANY_REQUESTS | 429: Too many requests have been made by this source in a given amount of time. | The Server has given a 429 response to the Senders request. |
| REC_SERVER_ERROR | 500: The Receiver has encountered an error processing the request. | The Receiver has given a 500 response to the Senders request. |
| PROXY_SERVER_ERROR | 500: The Proxy has encountered an error processing the request. | The Proxy has given a 500 response to the Senders request. |
| SERVER_ERROR | 500: The Server has encountered an error processing the request. | The Server has given a 500 response to the Senders request. |
| SEND_NOT_IMPLEMENTED | 501:The Request was not recognized by the API. | The API has given a 501 response to the Senders request. |
| REC_NOT_IMPLEMENTED | 501: The Request was not recognized by the Receiver. | The Receiver has given a 501 response to the Senders request. |
| PROXY_NOT_IMPLEMENTED | 501: The Request was not recognized by the Proxy. | The Proxy has given a 501 response to the Senders request. |
| NOT_IMPLEMENTED | 501: The Request was not recognized by the server. | The Server has given a 501 response to the Senders request. |
| PROXY_UNAVAILABLE | 503: An internal component is unavailable | The Proxy has given a 503 response internally to the Senders request. |
| REC_UNAVAILABLE | 503: The Receiver is currently unavailable. | The Receiver has given a 503 response to the Senders request. |
| UNAVAILABLE | 503: The Server is currently unavailable. | The Receiver has given a 503 response to the Senders request. |