ApiSimul logo ApiSimul — the switchboard

HTTP 426 — Upgrade Required

Client error responses (4xx) · Updated 2026-10-18

HTTP 426 is the compatibility cliff: the server will only talk to clients that switch protocols — the Upgrade header names what is required, whether TLS or a newer application protocol.

When 426 shows up

Mocking a 426 response

A response with this status can be faked end to end with a local mock rule — no backend changes needed. The configuration below mirrors the rule fields the ApiSimul editor uses (method, path, status, delay in milliseconds, response body):

{
  "method": "GET",
  "path": "/api/legacy",
  "status": 426,
  "delay": 170,
  "enabled": true,
  "body": {
    "error": "upgrade_required",
    "protocol": "TLS 1.3"
  }
}

Dynamic placeholders such as {{random.name}}, {{string.uuid}} or {{number.int 1 100}} can appear inside the body to vary each response, which keeps client-side list rendering and retry logic honest during development.

Frequently asked questions

What does HTTP 426 mean?
The client must switch protocols as directed by the Upgrade header before the server will process anything further.

How can I test a 426 response without a real backend?
Define a mock rule with the method and path your client requests and set the status to 426. The example configuration above returns this response after the listed delay, so the client exercises its 426 handling exactly as it would against a live server.

Related status codes

400 Bad Request 401 Unauthorized 402 Payment Required 403 Forbidden 404 Not Found 405 Method Not Allowed 406 Not Acceptable 407 Proxy Authentication Required 408 Request Timeout 409 Conflict 410 Gone 411 Length Required 412 Precondition Failed 413 Content Too Large 414 URI Too Long 415 Unsupported Media Type 416 Range Not Satisfiable 417 Expectation Failed 418 I’m a teapot 421 Misdirected Request 422 Unprocessable Content 423 Locked 424 Failed Dependency 425 Too Early 428 Precondition Required 429 Too Many Requests 431 Request Header Fields Too Large 451 Unavailable For Legal Reasons

Need the response in your own project? Spin up rules for any status with the ApiSimul mock server, or browse every status code page.