IdempotencyKey

A key with this exact value, such as one read back from an external system's log.

The key of the call toolCallId issued by a model call that ran on threadId at checkpoint checkpointId, in superstep superstep, as task taskId (its RunContext.position): 64 lowercase hexadecimal characters, the SHA-256 of the five values, so it is deterministic, of fixed length and reveals none of them.

Together they name one model call on the thread, whatever the provider's call ids and run ids (design §5.3). The superstep counts on across a thread's runs, so a later turn's model call has a later one even when its run reuses a RunId - and so the checkpoint ids of an earlier run; the task tells apart model calls running side by side in one superstep; and the checkpoint tells apart a model request made again after its response was lost, which runs at a new checkpoint.

Encodes as a plain JSON string. Built from upickle's string codecs: inside this scope an IdempotencyKey is a String, so summoning ReadWriter[String] would find this given.