Idempotency keys, properly

Placeholder article. This body stands in for the real post; replace it and delete the placeholder flag in the frontmatter.

The point this article will make: storing the response is the easy half; the hard half is deciding what counts as the same request when a client retries with a changed payload.

A fingerprint of the request body belongs in the key record, checked on every replay:

Idempotency-Key: 8f3c1a9e

Two more paragraphs of substance will go here.