The reframe from "how do we resume?" to "is it safe to start again?" is the whole post, and it's correct. People reach for checkpoint-and-resume because it feels like the rigorous answer, but you can't cleanly resume a non-deterministic trajectory — the model may not take the same branch the second time, so "step 3" isn't a stable coordinate. Idempotency at the effect boundary is the only thing that actually holds.
The part I'd dig into more is how you classify a tool call as safe-to-replay versus world-touching. send_email is obviously an effect; a GET that hits a rate-limited or stateful API is sneakier, and a "pure" lookup that writes an audit row isn't pure at all. Do you tag that at the tool-registration level, or infer it? We landed on making every effectful tool declare an idempotency key it computes from its args, so a replay dedupes at the tool instead of relying on the runtime to reason about safety — curious whether you considered pushing the decision down to the tools rather than the orchestrator.
Thanks, you put it better than I did: “step 3 isn’t a stable coordinate” is exactly it.
Today we don’t classify at all: any recorded tool call blocks the automatic replay, reads included. Deliberately blunt, for exactly the cases you list. The cost is refusing some safe replays.
Agreed that the decision belongs in the tools, with one catch: a key hashed from the args breaks when the LLM rewrites them on replay. The key has to come from stable intent, never from generated content. That’s what we do in production where we’ve built it: Outlook webhooks are deduped on the provider’s message identity, dataset imports on a SHA-256 of the content, and outbound campaigns need human approval.
What’s missing is that rule generalized to every tool, and you’ve clearly already shipped that design. I’m opening an RFC issue on tool-level idempotency keys, and I’d love your help shaping it: a comment, a counter-proposal or a PR, whatever suits you.
The project is young, so early contributors really set the direction. Every contribution is credited by name, I’m happy to pair on a first PR, and I share contributors’ work with our network. Your comment already inspired my next post, and I’d love to credit you there too.
The reframe from "how do we resume?" to "is it safe to start again?" is the whole post, and it's correct. People reach for checkpoint-and-resume because it feels like the rigorous answer, but you can't cleanly resume a non-deterministic trajectory — the model may not take the same branch the second time, so "step 3" isn't a stable coordinate. Idempotency at the effect boundary is the only thing that actually holds.
The part I'd dig into more is how you classify a tool call as safe-to-replay versus world-touching.
send_emailis obviously an effect; aGETthat hits a rate-limited or stateful API is sneakier, and a "pure" lookup that writes an audit row isn't pure at all. Do you tag that at the tool-registration level, or infer it? We landed on making every effectful tool declare an idempotency key it computes from its args, so a replay dedupes at the tool instead of relying on the runtime to reason about safety — curious whether you considered pushing the decision down to the tools rather than the orchestrator.Thanks, you put it better than I did: “step 3 isn’t a stable coordinate” is exactly it.
Today we don’t classify at all: any recorded tool call blocks the automatic replay, reads included. Deliberately blunt, for exactly the cases you list. The cost is refusing some safe replays.
Agreed that the decision belongs in the tools, with one catch: a key hashed from the args breaks when the LLM rewrites them on replay. The key has to come from stable intent, never from generated content. That’s what we do in production where we’ve built it: Outlook webhooks are deduped on the provider’s message identity, dataset imports on a SHA-256 of the content, and outbound campaigns need human approval.
What’s missing is that rule generalized to every tool, and you’ve clearly already shipped that design. I’m opening an RFC issue on tool-level idempotency keys, and I’d love your help shaping it: a comment, a counter-proposal or a PR, whatever suits you.
The project is young, so early contributors really set the direction. Every contribution is credited by name, I’m happy to pair on a first PR, and I share contributors’ work with our network. Your comment already inspired my next post, and I’d love to credit you there too.
Repo: github.com/apowerb/apowerb
Thanks for the exchange!