Simulate slow or flaky backends

[[rules]]
when.request.url_path = "/fast"
respond.text = "instant response"

[[rules]]
when.request.url_path = "/slow"
respond = { text = "eventually...", delay_response_milliseconds = 800 }

respond.delay_response_milliseconds sleeps before responding — useful for exercising a client’s timeout, retry, or loading-state handling against a predictable, artificial delay.

Set it per rule, on respond — or set a default for a whole rule-set file with [default] delay_response_milliseconds, which applies to every rule in that file that does not set its own. A per-rule value always wins, and respond.delay_response_milliseconds = 0 cancels the default for that one rule. See Rule-set schema.

Corrected 2026-10-06. This guide previously said the rule-set-wide default “has no effect on any response”. That was false — it has worked since RFC 045 (Defect 2), and this page was steering you away from a feature that does what its name says.

There’s no built-in mechanism for a genuinely flaky backend (randomly failing a fraction of requests) — only a fixed, deterministic delay. If you need actual failure injection, Rhai middleware can implement it directly.

A worked, verified example with three endpoints at increasing delays: crates/apimock/examples/simulate-slow-backend/.