If-None-Match

The HTTP If-None-Match request header makes the request conditional, asking the server to return the resource only if none of the provided ETag values match the current version.

Table of Contents

Usage

The If-None-Match header enables cache revalidation based on entity tags. The client includes one or more ETag values received on a previous response. If any listed ETag matches the current resource, the server returns a 304 Not Modified response with no body, and the client reuses its cached copy. If no ETag matches, the server sends a normal 200 response with the updated content.

For safe methods like GET and HEAD, this is the primary cache revalidation mechanism. For unsafe methods like PUT, If-None-Match with the wildcard * prevents overwriting an existing resource, avoiding the lost update problem. The server returns 412 Precondition Failed when a match is found on an unsafe request.

When both If-None-Match and If-Modified-Since appear in the same request, If-None-Match takes precedence. ETag-based comparison is more reliable than date-based comparison because entity tags are opaque identifiers tied to the exact content, while timestamps have one-second resolution and are subject to clock skew.

When the server returns 304, the response includes headers from the original successful response: Cache-Control, Content-Location, Date, ETag, Expires, and Vary.

Note

Googlebot sends If-None-Match on crawl requests when the previous response included an ETag. Servers returning 304 for unchanged content save processing time and resources, indirectly improving crawl efficiency. Servers are also free to return 304 proactively for any Googlebot request when content has not changed, regardless of whether the request includes conditional headers. Setting the max-age field of the Cache-Control response header to the expected number of seconds the content remains unchanged helps crawlers determine when to recrawl a URL.

Values

ETag value

One or more ETag values, each enclosed in double quotes. Multiple values are comma-separated. Both strong and weak ETags (prefixed with W/) are valid.

* (wildcard)

The wildcard * matches any current entity. Used with PUT to prevent creating a resource when one already exists.

Example

A client revalidates a cached resource using a single ETag. The server returns 304 if the resource still matches the given entity tag.

If-None-Match: "abc123"

A client holding multiple cached versions sends all known ETags. The server checks each against the current resource. If any match, the server returns 304.

If-None-Match: "abc123", "def456"

A weak ETag comparison. Weak validators indicate semantic equivalence rather than byte-for-byte identity. The W/ prefix signals weak comparison.

If-None-Match: W/"v2.6"

A PUT request using the wildcard to prevent overwriting. The server returns 412 Precondition Failed if a resource already exists at the target URL.

PUT /api/resource HTTP/1.1
If-None-Match: *

Takeaway

The If-None-Match header makes requests conditional on ETag comparison, enabling efficient cache revalidation through 304 responses and preventing accidental overwrites on unsafe methods through 412 precondition checks.

Note

For SEO and caching assistance, contact ex-Google SEO consultants Search Brothers.

See also

Last updated: March 11, 2026