Idempotency means that repeating the same API request has the same intended effect on server state as doing it once. It matters whenever a client or AI agent might retry an action after a timeout: the response may have been lost even though the server completed the write. A stable idempotency key, where the endpoint supports one, lets the server recognize retries of the same logical operation and avoid applying its effect twice.
What idempotency means in an API
An operation is idempotent when repeating it leaves server state with the same intended result as performing it once. The response need not be identical: a first DELETE might report success, while a later one reports that the resource is already absent. Idempotency also does not rule out incidental effects such as logging or metrics. MDN explains the HTTP meaning of idempotency.
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HTTP method semantics are a useful guide, not a guarantee that every endpoint is implemented correctly:
- GET, HEAD, OPTIONS, TRACE, PUT, and DELETE are defined as idempotent.
- POST and PATCH are not guaranteed to be idempotent. A repeated POST, for example, may create another resource.
PUT commonly replaces the representation at a known target, so repeating the same replacement has the same intended result. The endpoint’s documented behavior still matters.
Why a timeout makes retries risky
A timeout or lost connection leaves the outcome unknown. The server may have completed a payment, order, or other write before its response disappeared. Retrying an unprotected operation can then create a duplicate; assuming it failed can leave a client out of sync.
Retrying is safer when the operation is idempotent by design or the exact endpoint supports idempotency keys. A missing response is not evidence that the write did not happen. The client should retry the same logical operation with the same identity, rather than treating the retry as a new action.
How idempotency keys work
An idempotency key identifies one logical write. The client creates a unique key for a new operation and reuses it unchanged for that operation’s retries. A genuinely new operation gets a new key. The server records the key and associates it with the operation so it can recognize a duplicate request and avoid applying the effect again.
The Tool Desk
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Stripe’s documented behavior
Stripe documents that it saves the first request’s status code and body for a given key, then returns that result for later requests using the same key—including when the saved result is a 500 response. This is Stripe-specific behavior, not a rule to assume for other APIs.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What AI agents need to do differently
An agent may call a tool that changes external state, then be interrupted or encounter an ambiguous result. The model or orchestration layer may try the call again because it cannot tell whether the write completed. A prompt telling the agent not to repeat an action is not a substitute for deduplication in the software that executes it.
OpenAI Agents SDK guidance says: “If a tool has side effects, make it idempotent by call ID so an interrupted continuation cannot repeat the effect.” The SDK documentation points to a practical design: preserve the tool-call identifier, or create a stable operation ID before starting the write; persist the operation state; and return the stored result when that same operation is encountered again. When the downstream API accepts idempotency keys, pass the stable identity through according to its rules.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchIf a downstream service does not support idempotency, the integration should check or reconcile the service’s state before repeating an uncertain action. OpenAI’s agent recovery guidance similarly recommends checking completed actions after a failed turn, since a turn may already have changed files or called external tools.
Workspace Agents trigger example
OpenAI’s Workspace Agents trigger API documents an Idempotency-Key for retrying the same trigger event. Reusing the key returns the original accepted outcome instead of adding a second trigger event to the queue. Use it only for the same event. See the Workspace Agents trigger API documentation.
Quick Recap
A practical retry checklist
- Identify the effect. Decide whether the call changes external state, such as creating a record, sending a payment, or starting a run.
- Check the endpoint’s semantics. HTTP method semantics help, but confirm how this endpoint is implemented and whether it documents key support.
- Assign an operation identity before the write. Use a stable tool-call ID or operation ID; generate a fresh one for a genuinely new operation.
- Persist progress and outcome. Store enough state to recognize a retry after a timeout or interrupted run and return the prior result for the same operation.
- Retry with the same key and request. Observe the endpoint’s rules for payload matching, concurrent requests, and key expiry.
- Reconcile if deduplication is unavailable. Query the downstream state or otherwise verify what happened before replaying an uncertain write.
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