The Birrell/Nelson call identifier find is great. One thing that still surprises me is how inconsistent idempotency key semantics are across modern APIs, forty years later. Stripe stores the first response and replays it for 24h, even if the first attempt was a 500. Some other payment APIs only dedupe successful requests, so a retry after a failure creates a brand new attempt. Same header name, very different contracts. The scary part is these details usually live in one paragraph of prose in the docs, not in the OpenAPI spec, so client generators and retry middleware have no way to know about them. I have been bitten by a provider quietly changing their key retention window in a changelog nobody read.