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For Stripe, there's two headers that can help clients with retrying or not: "Idempotent-Replayed: true"[1] and "Stripe-Should-Retry"[2].

Whether they're helpful in all server-side error situations I'm not sure.

1: https://stripe.com/docs/idempotency#sending-idempotency-keys

2: https://stripe.com/docs/error-handling#the-stripe-should-ret...

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"Should-Retry" is useful, but "Idempotent-Replayed" while might be somewhat useful for debugging, makes the idempotent semantics worse: i.e. the side-effect idempotency stays the same, while result isn't idempotent.

I guess in HTTP you may say that only the body of the HTTP response is the result, not the headers. But I implemented a similar system for GraphQL, and there are these boolean flags (isRetry and isReplayed, etc.) are part of the mutation inputs and/or payloads. The same with the "Idempotency-Key" itself, which is called clientMutationId in Relay mutation spec. The added value of not using HTTP headers is that it also works over websockets.

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