Check and validate ETag response headers for proper caching behavior with weak vs strong ETag analysis.
An ETag response header identifies a selected representation with one quoted entity-tag. This checker examines the entered header value, determines whether it matches HTTP entity-tag syntax, and distinguishes a strong tag such as "abc" from a weak tag such as W/"abc". The distinction is semantic, not a judgment about hash length or cryptographic quality. A frequent mistake is treating W/ as an invalid prefix. It is valid syntax, but it signals that the validator is unsuitable wherever HTTP requires strong comparison.
HTTP defines an entity-tag as an optional, case-sensitive W/ weakness indicator followed by an opaque tag in double quotes. The checker evaluates that complete shape. An unquoted token, lowercase w/, missing slash, or text outside the quoted tag does not form the response header's entity-tag syntax, even if the value resembles a file hash or version identifier.
A tag without W/ is strong; one with W/ is weak. Strong comparison requires both tags to be strong and their opaque tag text to match. Weak comparison ignores weakness and compares the opaque tag text. This matters because conditional request fields select comparison rules: If-Match uses strong comparison, while If-None-Match uses weak comparison.
No. A weak ETag can still support cache revalidation: a client can send it in If-None-Match, whose comparison is weak. Weakness means the validator can identify representations the origin considers semantically equivalent; it does not set freshness or cacheability. Cache-Control and other response metadata govern those separate questions.
They contain the same opaque tag, but they do not match under every comparison rule. Weak comparison treats W/"v1" and "v1" as matching. Strong comparison does not, because both operands must be strong. The checker's weak or strong classification therefore matters even when the text inside the quotation marks is identical.
The behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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