What is the difference between a strong and weak ETag?

A strong ETag (like "abc") guarantees byte-for-byte identity, so it can be used for range requests and caching. A weak ETag (prefixed W/, like W/"abc") only guarantees semantic equivalence — the content means the same thing but may differ byte-for-byte, e.g. after recompression.

When does a server return 304 Not Modified?

When a conditional GET carries If-None-Match (or If-Modified-Since) and the resource has not changed, the server returns 304 with no body. The client then reuses its cached copy, saving bandwidth while revalidating freshness.

How does If-Match enable optimistic concurrency?

A client sends If-Match with the ETag it last read on a PUT or PATCH. If the resource's current ETag differs, someone else changed it, and the server returns 412 Precondition Failed instead of overwriting — preventing the lost-update problem.

Which comparison do range requests use?

Range requests and cache updates require strong comparison: both ETags must be present, neither weak, and identical. Weak ETags fail strong comparison, so a weak validator cannot be used to assemble partial responses.

Does this tool send any request?

No. The comparator runs entirely in your browser. It performs no network calls and stores nothing.

ETag & Conditional Request Reference

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Revalidate without re-downloading

Conditional requests let a client ask “has this changed?” and get back a tiny 304 Not Modified instead of the whole body. They also power optimistic concurrency, so two clients don’t silently overwrite each other. This reference covers the validator headers, the strong-versus-weak comparison rules, and the 304 flow — plus a comparator for two ETags.

How it works

A response advertises validators:

ETag: "33a64df5"
Last-Modified: Tue, 10 Jun 2026 12:00:00 GMT

The client echoes them on the next request:

If-None-Match: "33a64df5"
If-Modified-Since: Tue, 10 Jun 2026 12:00:00 GMT

If unchanged, the server replies 304 Not Modified with no body. Strong comparison (used by range requests) requires both ETags present, neither weak, and identical. Weak comparison (used by If-None-Match for cache revalidation) treats W/"x" and "x" as matching. For writes, If-Match with the last-known ETag yields 412 Precondition Failed if the resource moved on, preventing lost updates.

Tips and notes

Prefer strong ETags unless your representation legitimately changes byte-for-byte.
ETag beats Last-Modified for precision (sub-second changes, content hashing).
Use If-Match/If-None-Match: * to make creates and replaces idempotent.
A 304 should still carry headers like ETag, Cache-Control and Vary.