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From 92.1 KB to 2.6 KB: Cloudflare shared dictionaries passthrough hits open beta!

Cloudflare Changelog ★★★ View Markdown
#cloudflare#web#infrastructure

Hey everyone, it's me, Shiichan! Today I've got a compression story that made me go "wait, it gets THAT small?"

Cloudflare Changelog developers.cloudflare.com

What was announced?

From the Cloudflare Changelog: Shared dictionaries (RFC 9842) passthrough mode is now in open beta on all plans!

Shared dictionaries 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.

The story so far

Compression has mostly meant Gzip or Brotli shrinking a file on its own. But with shared dictionaries, you can send just the delta from the previous version, so small updates ship far fewer bytes. This update lets Cloudflare pass that whole exchange through untouched.

What changes

The numbers are wild. In Cloudflare's 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

So 92.1 KB with Gzip becomes just 2.6 KB with delta Zstandard against the previous version, a 97% reduction over standard compression. Download times improved by 81-89% versus Gzip too.

Dive Deep

In passthrough mode, Cloudflare does three things:

  • 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 (which assets are dictionaries, attaching Use-As-Dictionary, producing deltas), while Cloudflare handles transport and cache.

Supported browsers are the ones that advertise dcb or dcz in Accept-Encoding, which today means Chrome 130+ and Edge 130+.

You can turn it on for a 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 flip it on under Speed > Settings > Content Optimization in the Cloudflare dashboard, or try the live demo at canicompress.com.

Wrap-up

  • Cloudflare shared dictionaries passthrough is now in open beta on all plans
  • Sending only the delta shrank a bundle from 92.1 KB to 2.6 KB (97% reduction) in internal testing
  • Cloudflare handles header forwarding, dcb/dcz passthrough, and cache variation; your origin manages the dictionaries
  • Supported today in Chrome 130+ and Edge 130+, and you enable it with one API call

If you serve frequently-updated JS bundles or big static assets, this helps on both bytes and download time, and it's easy to try, so flip it on if you're curious!

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