Retry-After tells an HTTP client how long the server recommends waiting before a follow-up request. After a 429 Too Many Requests response, treat a valid value as the server’s wait guidance—but decide separately whether the original operation is safe to send again. The header controls timing; it does not guarantee that retrying is safe.
What does Retry-After mean?
RFC 9110 defines Retry-After as a server-provided indication of how long a user agent ought to wait before making a follow-up request. In rate-limit handling, it commonly appears with 429 Too Many Requests, the status code for a client that has sent too many requests in a given period. RFC 6585 says a 429 response may include the header; it is optional, not guaranteed.
The header answers “when should I try again?” It does not answer “should I repeat this operation?” Those are separate decisions: the server supplies timing guidance, while the client must consider whether another attempt is safe.
How to read the two Retry-After formats
RFC 9110 permits either an HTTP date or a non-negative integer delay in seconds.
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| Format | Example | Meaning |
|---|---|---|
| Delay in seconds | Retry-After: 120 |
Wait 120 seconds—two minutes—from receipt of the response before the follow-up request. |
| HTTP date | Retry-After: Wed, 21 Oct 2026 07:28:00 GMT |
Wait until the specified date and time before the follow-up request. |
The numeric form is a relative delay measured from when the response is received. The date form names an absolute time, so interpreting it requires parsing the HTTP date and comparing it with the client’s current time. A client implementing the standard should recognize both formats.
How should an API client handle Retry-After on a 429?
- Check the response status. Confirm that the response is
429 Too Many Requests; the status indicates that the server is applying a request limit. - Read and parse the header. Accept a valid HTTP date or a decimal delay in seconds, as defined by RFC 9110.
- Wait for the stated interval or time. Use the value as the server’s timing guidance before making a new request.
- Check whether the request is safe to repeat. Consider whether the operation is idempotent and whether the first attempt may already have taken effect.
- Apply your client’s retry controls. Bound attempts and avoid retry loops. The cited RFC provisions do not prescribe a universal attempt count or fallback for a missing or invalid header.
If a 429 has no usable Retry-After value, the standards cited here do not specify one universal waiting algorithm. The client therefore needs its own documented policy; do not present that policy as an HTTP requirement.
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Does Retry-After mean you can safely retry?
No. A wait instruction is not authorization to duplicate a consequential request. RFC 9110 says clients should not automatically retry non-idempotent requests unless they know the operation’s semantics are idempotent or can determine that the original request was not applied.
For example, if a client cannot tell whether a request that changes state succeeded before the 429 response was returned, repeating it could apply the change twice. Assess retry eligibility independently of the header’s timing value.
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How the header differs for 503 and redirects
Retry-After is not exclusively a rate-limit signal. Its meaning depends on the response context.
| Response | Meaning of Retry-After |
|---|---|
429 Too Many Requests |
How long to wait before making a new request after too many requests in a period. The header may be absent. |
503 Service Unavailable |
How long the service is expected to be unavailable. |
3xx redirection |
The minimum time to wait before issuing the redirected request. |
Use the response status to interpret the guidance rather than treating every occurrence as a rate-limit timer.
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What a 429 does—and does not—tell you about the limit
A 429 tells you that the server considers the request rate too high, but it does not establish a universal quota or identify how requests are counted. RFC 6585 leaves the counting and user-identification approach to the server: limits may apply per resource, across a server or multiple servers, or according to credentials or cookies.
RFC 6585 also specifies that responses with status 429 must not be stored by a cache. That cache rule does not reveal the limit’s scope or tell a client how to choose a fallback delay when the header is absent.
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What the HTTP standards specify
RFC 9110, HTTP Semantics, published by the IETF in June 2022, defines the header’s formats and its use with 503 and redirects, and addresses automatic retry safety. RFC 6585, Additional HTTP Status Codes, published by the IETF in April 2012, defines 429 and its optional use of Retry-After. Read the standards at RFC 9110 and RFC 6585.
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