4xx · Client Error
412Precondition Failed
请求头中的前置条件评估为 false。
What is HTTP 412?
412 Precondition Failed 表示服务器在请求头中评估前置条件时结果为 false。常与 If-Match、If-Unmodified-Since 等头配合使用，用于并发控制。
Example
PUT with If-Match: "old-etag" but server has "new-etag" → 412Check a URL in real-time
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