Which response should a client retry automatically, after a delay?

Options

• ✗ 400 Bad Request
• ✗ 403 Forbidden
• ✗ 404 Not Found
• ✓ 503 Service Unavailable Correct

Why

503 (like 502, 504 and 429 Too Many Requests) usually means a temporary condition, so retrying with backoff and jitter, honouring Retry-After, can succeed. Most other 4xx errors mean the request itself is wrong, and sending it again gets the same answer.

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