To make an agent-facing API safe to retry, define a server-side deduplication contract for mutations that could create duplicate effects, and give clients a bounded retry policy for failures the API identifies as transient. The key does not guarantee exactly-once delivery: it lets the service recognize the same logical operation within a defined scope and retention period. Because a timeout can occur after a mutation commits, the client must preserve the original key and request when retrying—and report an ambiguous outcome when it cannot establish what happened.
What idempotency guarantees—and what it does not
RFC 9110 defines an idempotent method by its intended effect: sending multiple identical requests has the same intended server effect as sending one. That does not require identical responses, nor does it prevent the server from logging each request or keeping separate request history.
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HTTP method semantics are a useful starting point, not a substitute for an endpoint contract. RFC 9110 identifies safe methods, PUT, and DELETE as idempotent, but cautions clients against automatically retrying a non-idempotent request unless they know its semantics are idempotent or can determine that the original request was never applied. A POST that starts a workflow, sends a payment, or creates a resource can cause a second effect if an agent repeats it after losing the response. For such actions, explicitly define deduplication behavior.
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Specify the server contract before clients retry
A key is useful only when both sides know how the service interprets it. Document the accepted key format, its scope, request matching rules, replay behavior, concurrency behavior, and retention period. These are API design choices, not universal HTTP rules.
Scope keys to the caller and operation
Associate a key with an authenticated principal or another explicit caller scope, the operation identity, and a fingerprint of the request parameters that affect the operation. This prevents an unrelated caller or endpoint from accidentally colliding with the same string. Define which request details are included in the fingerprint: semantically meaningful inputs belong in it; transport-only details should not turn a valid retry into a mismatch.
The precise storage schema and transaction pattern are implementation-specific. The contract should nevertheless make the effective scope understandable to clients and sufficiently narrow to avoid treating a different action as a replay.
Define matching and replay behavior
For the same key and matching request, return the saved outcome or a documented in-progress response. For the same key with different meaningful parameters, reject the request rather than silently treating it as a new operation. Stripe documents parameter comparison and an error for a mismatch as one concrete implementation; it is an example, not a general HTTP requirement.
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State what “saved outcome” means. An API might replay the original status and response body, or return a stable operation reference that the client can use to retrieve current status. The contract should explain what happens if the original operation is still running and what information a caller may use to resolve an uncertain result.
Make concurrent duplicates explicit
Two retries may arrive while the first request is still executing. Specify whether the second request waits, receives an in-progress response, or gets another documented result. Stripe says a request that conflicts with one already executing is not saved as an idempotent result and can be retried. Other APIs may choose differently; clients cannot safely infer the behavior from the presence of a key alone.
Publish retention and key limits
Deduplication is only as durable as the server’s record. Stripe says it may prune keys automatically after they are at least 24 hours old; reusing a pruned key can create a new request. That is Stripe’s policy, not a universal retention period. Publish your own retention window and tell clients not to reuse a key for a new logical action. If a client may retry after the record expires, it needs another recovery path, such as checking the operation’s status, rather than assuming the old key still protects it.
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Keep the operation record and side effect consistent
A dangerous failure is one in which the business side effect commits but the deduplication record does not, or vice versa. A later retry can then duplicate an effect or incorrectly report that it already happened. Design persistence and recovery so the recorded outcome cannot diverge silently from the actual mutation. The exact database transaction, outbox, or recovery mechanism depends on the system; there is no single implementation recipe established by the cited HTTP and vendor guidance. Test partial failures at the boundary between recording the key and performing the operation.
Implement the request lifecycle
The following sequence is a practical contract pattern, not a protocol mandated by HTTP. It makes the server’s decision points explicit and gives agent runtimes behavior they can reason about.
- Identify a logical action. The client creates one unique key for the intended mutation and retains it with the exact request inputs. It must not generate a fresh key merely because a response was delayed.
- Authenticate and establish scope. The server resolves the caller and operation identity before looking up the key. It should not let an untrusted caller use another principal’s key namespace.
- Look up the key and compare the request. If there is no record, the server reserves the key for the request. If a matching completed record exists, it follows the documented replay behavior. If the key exists with different semantic parameters, it rejects the mismatch.
- Handle an in-progress match. If another request with the same key is executing, the server returns the documented in-progress or conflict behavior. It must not accidentally start a second copy merely because the first has not yet written a completed result.
- Commit the mutation and outcome coherently. The service records enough information to recover or replay the result consistently with the side effect. On completion, the client receives the result or a stable operation reference.
- Expire records according to the published policy. After retention ends, the service may no longer recognize a reused key. Clients should use status recovery or a new deliberate action, not treat expired deduplication as indefinite protection.
Choose a bounded client retry policy
Retry only when the failure is transient according to the API contract, and only when the operation is inherently idempotent or protected by a valid key. The absence of a response is not proof that a mutation did not happen: the connection may have failed after the server committed it. If the operation is non-idempotent and has no deduplication mechanism, an agent should not blindly repeat it after an ambiguous timeout.
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A useful policy has four inputs: retryable failure classification, delay and jitter, an attempt cap, and an overall deadline. Set them together. A high attempt limit is not safe if the operation can outlive the agent’s task deadline, and a generous deadline is not a reason to retry a permanent validation error.
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Classify failures using the API’s contract
Document which failures are retryable for each endpoint. Transient service unavailability or throttling may be candidates; malformed requests, authorization failures, and semantic key mismatches generally require a change or intervention rather than repetition. There is no single universally correct retryable status-code list established by the sources here, so do not copy a generic list without checking the service’s behavior.
Distinguish a known rejection from an ambiguous result. A received response may say the server rejected the action; a timeout or dropped connection after sending the request may leave its outcome unknown. For an ambiguous result, retry with the same key or query the operation status if the API supports it. Do not create a new key simply to make the uncertainty disappear.
Use exponential backoff with jitter
Exponential backoff increases the wait after successive attempts; jitter randomizes that wait so many agents do not retry in synchrony. AWS SDK standard mode documents this formula: random(0, 1) × min(20,000 ms, base_delay × 2^retry). In that AWS-specific mode, the maximum backoff is 20 seconds, the transient-error base delay is 50 ms, and the throttling base delay is 1,000 ms. These are AWS SDK parameters, not universal recommendations; SDK, mode, and configuration matter.
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Set attempt limits and an overall deadline
Stop when either the attempt budget or the caller’s deadline is exhausted. Google Cloud IAM guidance explicitly describes stopping after a deadline. AWS SDK standard mode also uses a retry quota that can be exhausted, at which point it fails fast. These are vendor-specific mechanisms; your API client should document its own limits and terminal behavior.
For agent runtimes, the deadline should fit inside the agent’s remaining task budget. Reserve time to return a useful result or perform a status check instead of spending the entire budget sleeping between attempts. The exact attempt count and deadline are service- and task-specific; the cited guidance does not establish one value suitable for all clients.
Honor Retry-After without outliving the deadline
RFC 9110 defines Retry-After as either an HTTP date or a non-negative number of seconds. On a 503 response, it indicates how long the service is expected to remain unavailable. Parse the header in either form, treat the specified delay as a minimum wait, and do not retry earlier than requested. If that wait exceeds the remaining deadline, stop and report the outcome instead of retrying forever. Where jitter is used, add it without reducing the server-requested minimum.
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When retries stop, the runtime needs more than “request failed.” It should preserve whether the operation is known to have succeeded, known to have failed, or remains ambiguous; state whether retrying is allowed; and include the operation or key reference needed to check status where available. This response shape is a design recommendation, not a standard-defined agent schema.
For example, a runtime can expose a structured result with fields such as outcome, retryable, operation_id, and retry_after. A timeout after sending a keyed mutation might be represented as an unknown outcome that is safe to resolve using the same operation identity. Avoid instructing an agent simply to “try again”: without the original key and an explicit retryability decision, that instruction can turn uncertainty into a duplicate side effect.
How the cited policies differ
The standards and vendor documents answer different questions. RFC 9110 establishes HTTP semantics and the meaning of Retry-After; it does not define a universal idempotency-key protocol. Stripe, AWS, and Google Cloud document implementation policies within their own products.
Quick Recap
| Source | What it establishes | Important boundary |
|---|---|---|
| RFC 9110 | Idempotency concerns intended effect; clients should not automatically retry non-idempotent methods without a safety basis. Retry-After can be an HTTP date or delay in seconds. | Does not prescribe an idempotency-key header, storage model, retention window, or universal status-code retry list. |
| Stripe API documentation | One key-based policy: request parameter comparison, defined behavior for a concurrent conflict, automatic pruning once keys are at least 24 hours old, and a documented 255-character key maximum. | These are Stripe behaviors, not general HTTP guarantees. |
| AWS SDK standard mode | Exponential backoff with jitter, a 20-second maximum backoff, cited transient and throttling base delays, and a retry quota. | Values and quota behavior are AWS SDK mode/configuration specific. |
| Google Cloud IAM guidance | Truncated exponential backoff with jitter, a maximum-backoff cap, and stopping after a deadline. | Does not establish one delay or attempt count for every API client. |
Review the contract before shipping
- Can the API say whether a repeated request is the same logical operation, and is the caller and operation scope clear?
- Will a matching retry replay a saved outcome or return an explicitly documented in-progress result?
- Does the server reject a reused key with changed semantic parameters?
- Are concurrent duplicates and key expiry behavior documented, including the recovery path after retention?
- Can the service keep the business mutation and its deduplication record consistent through partial failure?
- Does the client retry only contractually transient failures, preserve the same key, honor Retry-After, and stop at an attempt cap or deadline?
- Will the agent receive a terminal result that distinguishes known failure from an ambiguous outcome?
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