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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →Use an idempotency key to identify one logical state-changing operation, then send that same key with every retry. The server must coordinate requests that share a key, bind it to the original request, and define what outcome retries receive. This matters most after a timeout: the server may have completed the operation even though the client never received the response.
What an idempotency key does
An idempotency key is a client-supplied identifier that lets an API recognize repeated submissions as attempts at the same logical operation. If a request times out, retrying with the same key gives the server a way to return or honor the original outcome instead of performing the operation twice. Stripe describes its implementation as enabling safe retries without accidentally creating the same operation twice: Stripe’s idempotent requests documentation.
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A key is not a guarantee that every API request is deduplicated. The API must support the mechanism for the endpoint and method you are calling, and its contract determines how it handles conflicts, failures, and expired keys.
Implement idempotency step by step
- Choose the operation boundary. Create one key for one intended action, such as creating a particular payment or order. Reuse it only for transport retries of that action. Generate a new key when the user or system initiates a genuinely new action.
- Generate a unique, unpredictable value. Stripe recommends UUID v4 or another random string with enough entropy to avoid collisions, and advises against putting sensitive information such as an email address or personal identifier in the key. Stripe allows keys up to 255 characters; that is Stripe’s limit, not a universal limit. See Stripe’s key guidance.
- Use the API’s documented field. Send the key exactly where the provider expects it. Stripe uses the
Idempotency-Keyheader for supported POST requests. Checkout.com documentsCko-Idempotency-Keyfor its/paymentsendpoint in its support article published June 5, 2026. These examples are provider- and endpoint-specific; do not assume the same header or coverage elsewhere. - Bind the key to the request. Store enough context to detect a key accidentally reused for a different endpoint or payload. Stripe compares incoming parameters with the original request and errors if they differ. A general API should define which properties participate in the comparison and how it handles serialization or semantically equivalent values; those details are design choices, not rules established by the provider examples here.
- Make claiming a key and beginning the side effect coordinated. A separate “check whether this key exists” followed by “perform the operation” leaves a gap in which two concurrent requests can both pass the check. Use an atomic claim, transaction, lock, or an equivalent strategy so only one request starts the side effect. Define what a simultaneous duplicate receives. Stripe documents a concurrent conflict that is not stored as an idempotent result and can be retried, illustrating one possible contract.
- Persist the result and specify replay behavior. Decide when execution is considered to have begun, what response is retained, and how a caller learns that an operation is still running. Stripe stores the first resulting status code and body after endpoint execution begins; later requests with the same key receive that result, including a 500 response. That is Stripe’s behavior, not a general requirement to cache every error. See Stripe’s execution and replay rules.
- Set and document retention. Keep key records long enough to cover the operation’s realistic retry horizon and the consequences of accidentally repeating it. Stripe says it may remove keys once they are at least 24 hours old; reuse after pruning begins a new request. This is a Stripe policy, not a standard duration. Tell callers what happens when a key is reused after your retention period.
- Define which failures are safe to retry. Stripe does not save an idempotent result for validation failures and certain conflicts that occur before endpoint execution begins; its documentation says those requests can be retried. For other errors, follow the specific API’s guidance rather than assuming any failure is safe to repeat.
What the server needs to store
The exact storage design depends on the application, but the contract needs to preserve enough information to distinguish a retry from key misuse and to replay or report the operation’s state. A typical record associates the key with its scope, a request fingerprint or comparable request details, an execution state, and the outcome needed for replay. Define scope explicitly—for example, whether a key is unique across an account, endpoint, or another boundary—rather than leaving it implicit.
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- Unclaimed: no operation has claimed the key.
- In progress: one request has claimed it and the side effect is being processed; concurrent requests need a defined response.
- Completed: the operation’s outcome is recorded and subsequent matching requests follow the documented replay behavior.
This state model is an implementation approach, not a provider-wide standard. Keep the key claim and the side effect sufficiently coordinated for the system’s failure modes. If an operation spans services or a database commit cannot be atomic with an external side effect, design recovery and reconciliation deliberately; an idempotency key alone does not make those separate actions transactional.
How client retries should work
On an uncertain timeout, retry the same logical request with the same key, provided the API documents support for that endpoint. Do not mint a fresh key merely because the first response was lost: a new key can make the server treat the retry as a new operation. Conversely, do not reuse a key for a changed request or a new user action.
Before retrying other failure responses, check the provider’s rules. A server may replay a stored error, allow a pre-execution validation failure to be corrected and retried, or report an in-progress conflict. Those cases are not interchangeable. In Stripe’s documented behavior, execution-stage outcomes—including a 500—are retained, while certain failures before execution begins are not.
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Provider behavior is not interchangeable
Stripe and Checkout.com illustrate why the endpoint’s own documentation matters. The available Checkout.com support article confirms its key header and payments endpoint, but does not establish its behavior for payload mismatches, concurrent requests, replayed failures, key scope, or retention. Do not infer that those rules match Stripe.
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| Behavior | Stripe | Checkout.com |
|---|---|---|
| Documented key field and scope in the cited material | Idempotency-Key header for supported POST requests; see Stripe’s API documentation. |
Cko-Idempotency-Key for the /payments endpoint; see Checkout.com’s support article. |
| Key length | Maximum 255 characters, per Stripe’s API documentation. | Not stated in the cited Checkout.com article. |
| Payload mismatch | Stripe errors if parameters differ from the original request. | Not stated in the cited Checkout.com article. |
| Concurrent submissions | A concurrent conflict is not stored as an idempotent result and can be retried. | Not stated in the cited Checkout.com article. |
| Stored outcomes and replay | After execution begins, Stripe stores the first resulting status code and body, including a 500, and returns it for later requests with the same key. | Not stated in the cited Checkout.com article. |
| Retention | Stripe may prune keys once they are at least 24 hours old; reuse after pruning starts a new request. | Not stated in the cited Checkout.com article. |
Checklist before shipping
- Confirm the endpoint and HTTP method support idempotency.
- Generate a sufficiently random key per logical operation, keep it free of personal data, and reuse it only for that operation’s retries.
- Reject or clearly handle reuse of a key with a different request or scope.
- Ensure concurrent requests cannot both begin the same side effect.
- Document in-progress responses, stored outcomes, retryable failures, and expiry behavior.
- Test lost-response retries, simultaneous submissions, mismatched payloads, process failures, and retries after the retention window against the contract you implement.
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