Which HTTP status code indicates that a client application is experiencing intentional rate limiting by the server?

The HTTP 429 Too Many Requests response status code is the standard indicator that a user has sent too many requests in a given amount of time, a practice known as rate limiting. This is an intentional server-side policy to protect the service from abuse or excessive use by a single client. The server is functioning correctly but is temporarily refusing to service the request from this specific client. The response may also include a Retry-After header indicating how long the client should wait before making a new request.

A. 202 (Accepted) indicates a request has been accepted for future processing but is not yet complete; it is not an error.
B. 401 (Unauthorized) indicates the request requires user authentication; it is related to identity, not request frequency.
D. 503 (Service Unavailable) indicates a general server-side failure (e.g., overload or maintenance), not a specific client-side rate-limiting policy.

1. IETF RFC 6585

"Additional HTTP Status Codes"

Section 4: "The 429 status code indicates that the user has sent too many requests in a given amount of time ('rate limiting'). The response representations SHOULD include details explaining the condition

and MAY include a Retry-After header indicating how long to wait before making a new request."

2. Cisco DevNet

Webex API Documentation

"API Usage and Rate Limiting": "If you exceed the rate limit for an API

your request will be rejected and you'll receive an HTTP 429 Too Many Requests response." This is a common implementation across Cisco's API platforms.

3. Mozilla Developer Network (MDN) Web Docs

"429 Too Many Requests": "The HTTP 429 Too Many Requests response status code indicates the user has sent too many requests in a given amount of time ('rate limiting'). A Retry-After header might be included to this response indicating how long to wait before making a new request." This resource is widely used in academic and professional settings for web standards.