| Previous | Next |
| HTTP_STATUS_TOO_EARLY | HTTP_STATUS_PRECONDITION_REQUIRED |
HTTP_STATUS_UPGRADE_REQUIRED
The server must include an Upgrade header naming acceptable protocols. The response does not itself change the protocol; the client must open or negotiate a suitable connection.
Check which protocol versions the endpoint supports and whether an intermediary strips upgrade headers. Retrying the same request over the same unsupported protocol will not help.
The response should indicate an acceptable protocol in the Upgrade header when the HTTP semantics apply. Clients need to establish the required protocol according to that protocol's negotiation rules; simply resending the same request on the unchanged connection will normally produce the same result.
Looking for a different code? Search another status or error code.