ETag & Conditional Request Builder

Generate ETag values and matching If-None-Match / If-Modified-Since request headers.

Inputs

ETag from

Weak validator (W/)

Last-Modified

Content to hash

Response headers (server sends)

Last-Modified: Thu, 04 Jun 2026 19:59:00 GMT

strong ETagfirst response: 200 OK

Conditional request (client sends next)

If-Modified-Since: Thu, 04 Jun 2026 19:59:00 GMT

matched: 304 Not Modifiedchanged: 200 OK + new body

How 304 works

The server returns the ETag (and optionally Last-Modified) with the first 200 response. A cache stores both the body and these validators.

On the next request, the client echoes the ETag in If-None-Match and the date in If-Modified-Since. If the resource is unchanged, the server replies 304 Not Modified with no body, and the client reuses its cached copy. If it changed, the server replies 200 OK with the new body and a fresh ETag.

A weak validator (W/) signals the entity is semantically equivalent but may differ byte-for-byte; it cannot be used for byte-range (partial) requests, while a strong validator can.

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ETag & Conditional Request Builder

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