Earlier posts in this series argued about where idempotency keys should come from. This one is hands-on: get a free sandbox key (no account), issue a permit, call a tool, retry with the same key and watch the original receipt come back instead of a new dispatch, then verify the receipt offline. Code: https://github.com/agentmiddleware/amw-demo . If you try it, tell me in the comments which real tool you'd put behind it.