Make a retry of one logical operation reuse the same idempotency key and the same request parameters. The server must implement that key’s deduplication contract; a timeout alone cannot tell the client whether the original request took effect. Use a new key for a genuinely new operation, and retry only when the API’s guidance says the failure is retryable.
Why a timeout can cause duplicate work
A client can send a request, the server can apply it, and the response can be lost before it reaches the client. From the client’s perspective, a timeout leaves the outcome unknown: it does not prove the request was never processed. Sending the operation again without a deduplication mechanism may create a second charge, task, or resource.
HTTP distinguishes idempotent methods from non-idempotent ones. RFC 9110 defines an idempotent method by its intended effect: repeating the same request has the same intended effect as making it once. Incidental activity, such as logging each request, may still happen more than once. The standard lists PUT, DELETE, and all safe methods as idempotent; GET, HEAD, OPTIONS, and TRACE are safe. Safety and idempotency are distinct properties.
POST is not generally idempotent under its standard method semantics. RFC 9110, section 9.2.2, says: “A client SHOULD NOT automatically retry a request with a non-idempotent method unless it has some means to know that the request semantics are actually idempotent, regardless of the method, or some means to detect that the original request was never applied.” The IETF published RFC 9110 in June 2022; its authors are Roy T. Fielding, Mark Nottingham, and Julian Reschke. Read RFC 9110, section 9.2.2.
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The standard also says clients SHOULD NOT automatically retry a failed automatic retry. That is standards guidance, not a substitute for a service’s specific retry policy or an SDK’s documented behavior.
What an idempotency key does
An idempotency key is an API-specific identifier for one logical mutation. The client sends it using the API’s documented header or parameter. If the client must retry because the response is missing or the connection fails, it sends the same operation with the same key and semantically identical parameters. A server that supports the contract recognizes the key and applies its duplicate-request policy instead of treating the retry as a new operation.
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The key does not make a request idempotent by itself. If an API does not implement key-based deduplication, sending an arbitrary key has no protective effect. Nor does the phrase “supports idempotency keys” fully describe the behavior: the API must specify scope, parameter matching, concurrent requests, saved outcomes, duplicate responses, and retention.
How provider contracts differ
These examples illustrate service-specific behavior, not a shared contract. Confirm the current documentation for the exact endpoint or action you use.
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| API | Key and matching behavior | Duplicate, error, and scope behavior |
|---|---|---|
| Stripe | Uses an idempotency key; keys may be up to 255 characters. Incoming parameters are compared with the original request, and a mismatch returns an error. | Stripe says it saves and replays the first request’s status code and body, including a 500, once endpoint execution begins. Validation failures and conflicts with an already executing request are not saved as idempotent results. Keys may be pruned once at least 24 hours old; after pruning, reusing one starts a new request. Stripe: Idempotent requests |
| Amazon ECS | Client-token idempotency is available only for documented actions. Tokens are case-sensitive; reuse the token only with matching parameters and do not use it for another request. | For a successfully completed request, a repeat with the same token and parameters returns the original result without further action. For RunTask, changed parameters can produce a ConflictException. AWS ECS: Ensuring idempotency |
| Amazon EC2 | Selected operations support client tokens, with regional or zonal scope depending on the operation. | The same token can identify separate operations in different regions; zonal scope also depends on Availability Zone. Relevant parameter changes can produce IdempotentParameterMismatch. Do not assume a token is globally unique across services or resources. AWS EC2: Ensuring idempotency |
These contracts differ in more than naming. Before relying on a key, check how the API supplies it, what counts as the same request, how concurrent duplicates are handled, which outcomes are stored, what a duplicate receives, where the key applies, and how long the record remains valid. For example, AWS Cloud Control API documentation describes a 36-hour token validity period; that service-specific duration is not a general AWS or industry rule.
How to make client retries safe
- Create the key with the operation. Generate it once when the application creates the logical mutation, before the first network attempt. Stripe recommends UUID v4 or another sufficiently random string. Follow the provider’s character, length, casing, and transport requirements.
- Keep the key with the operation state. If the client may restart before the outcome is resolved, persist or otherwise retain the key and the request data needed to retry. A retry after a restart still belongs to the original operation.
- Reuse the key and parameters for that operation. Do not mint a new key merely because a request timed out. Keep the payload semantically identical; changing parameters while retaining the old key can trigger a mismatch error.
- Use a new key for a new operation. A separate user action is a separate logical operation, even if its payload happens to match a previous request.
- Apply the API’s retry policy separately. Deduplication limits repeated effects; it does not make every response worth retrying. Follow documented status handling, rate limits, and pacing guidance. Stripe recommends exponential backoff for 429 Too Many Requests, but that advice is not a universal strategy for every status or provider. Stripe: Errors
- Handle mismatches as identity errors. Investigate why a key is paired with different parameters. Do not silently change the payload and keep the old key; decide whether this is still the same operation or a new one.
What API designers need to define
A reliable contract describes observable behavior, not just a header name. Document:
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- How the client supplies the key, including syntax and limits.
- Its scope: for example, per operation, account, endpoint, region, or availability zone.
- How the server determines that two requests are equivalent and what happens if parameters differ.
- How simultaneous requests with the same key behave, including what the client sees while the first is in flight.
- Which outcomes are recorded: successes, validation errors, conflicts, server errors, or other results.
- What response a duplicate receives, whether the original response is replayed, and how long the record is retained.
- Which failures clients should retry, and any rate-limit or backoff requirements.
The deduplication record and the protected operation need consistency appropriate to the system. In particular, avoid a design where the operation completes but its key and result are not recorded, or where a second request executes while the first is still in flight. The storage and external side-effect design determine how to manage those cases; HTTP itself does not supply the transaction.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What an idempotency key does not guarantee
Do not infer “exactly once” for an entire distributed workflow from the presence of a key. State the actual contract: perhaps duplicate effects are suppressed within a defined scope and retention window, or a prior response is replayed. Retries beyond that window, operations outside the key’s scope, and other systems in a workflow depend on their own behavior.
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For clients, the practical rule is to preserve operation identity across uncertain outcomes. For API designers, the corresponding responsibility is to define how that identity is recognized and what happens at every boundary of the contract.
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