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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.
Operational meaning of HTTP 426 for HTTP_STATUS_UPGRADE_REQUIRED
HTTP 426 (HTTP_STATUS_UPGRADE_REQUIRED) is a client-error response. AllStat records its meaning as “426 Upgrade Required”. For HTTP_STATUS_UPGRADE_REQUIRED, interpret it with the request method, selected representation, response headers, and the specification or product that defines the code; the number alone does not prove that application work completed.
Client and server handling for HTTP_STATUS_UPGRADE_REQUIRED
- Treat HTTP_STATUS_UPGRADE_REQUIRED as a condition visible at the request boundary; inspect authentication, syntax, preconditions, routing, and resource state before retrying.
- For HTTP_STATUS_UPGRADE_REQUIRED, preserve the response body and challenge or retry-related headers because they often carry the actionable reason.
- For HTTP_STATUS_UPGRADE_REQUIRED, automatic retry is safe only when the method and status contract allow it and the request has been corrected.
For telemetry, record HTTP 426, method, authority, route template rather than secrets, response headers relevant to caching or retry, latency, upstream identity, and whether the response originated at the application or an intermediary.
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