Changelog

Shared dictionaries (RFC 9842 ↗) let an origin compress a response against a previous version of the same resource that the browser already has cached, so only the difference between versions travels over the wire. Shared dictionaries passthrough is now in open beta on all plans.

In passthrough mode, Cloudflare:

• Forwards the Use-As-Dictionary and Available-Dictionary headers between client and origin without modification.
• Treats dcb (Dictionary-Compressed Brotli) and dcz (Dictionary-Compressed Zstandard) as valid Content-Encoding values end to end, without recompressing them.
• Extends the cache key to vary on Available-Dictionary and Accept-Encoding so each delta-compressed variant is cached correctly.

Your origin manages the dictionary lifecycle: deciding which assets are dictionaries, attaching Use-As-Dictionary headers, and producing deltas in response to Available-Dictionary requests. Cloudflare handles the transport and the cache.

In internal testing on a 272 KB JavaScript bundle, the asset shrinks from 92.1 KB with Gzip to 2.6 KB with delta Zstandard against the previous version — a 97% reduction over standard compression — with download times improving by 81–89% versus Gzip.

Shared dictionaries work with browsers that advertise dcb or dcz in Accept-Encoding. Today, this includes Chrome 130 or later and Edge 130 or later.

Turn on passthrough for your zone with a single API call:

curl "https://api.cloudflare.com/client/v4/zones/$ZONE_ID/settings/shared_dictionary_mode" \
  --request PATCH \
  --header "Authorization: Bearer $CLOUDFLARE_API_TOKEN" \
  --json '{
    "value": "passthrough"
  }'

You can also turn it on under Speed > Settings > Content Optimization in the Cloudflare dashboard ↗. For full origin setup instructions and a working test recipe, refer to Shared dictionaries, or try the live demo at canicompress.com ↗.