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Shared dictionaries


Last updated Apr 30, 2026|Copy as Markdown|View as Markdown|Agent setup

OverviewAvailabilityRequirementsHow shared dictionaries workEnable shared dictionaries 1. Enable passthrough in Cloudflare 2. Mark assets as dictionaries at your origin 3. Compress new versions against the advertised dictionary 4. Fall back when no dictionary is available Implementation optionsTest shared dictionariesLimitations




Shared dictionaries (RFC 9842 ↗) let your origin compress a response against a copy of the same — or a different — resource that the visitor's browser already has cached. Only the difference between the two resources travels over the wire.

This is most effective for versioned assets that change incrementally between deploys, such as JavaScript bundles, CSS files, and framework chunks. After a deploy, returning visitors can receive the new asset as a small delta against the version they already have, instead of redownloading the full file.

Cloudflare supports shared dictionaries in passthrough mode: your origin manages dictionaries and produces delta-compressed responses. Cloudflare forwards the dictionary headers and dcb/dcz content encodings without modifying or recompressing them, and varies the cache so each delta-compressed variant is stored separately.

For background on the other compression algorithms Cloudflare supports, refer to Content compression.

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Availability


FreeProBusinessEnterprise

Availability

Yes (beta)

Yes (beta)

Yes (beta)

Yes (beta)


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Requirements


Shared dictionaries work when all of the following are true:

The visitor's browser supports compression dictionary transport ↗. Today, this is Chrome 130 or later, Edge 130 or later, or another Chromium browser at the same version.

The browser request includes dcb or dcz in Accept-Encoding and an Available-Dictionary header.

Your origin returns a delta-compressed response with Content-Encoding: dcb or dcz and a Vary header that includes Accept-Encoding, Available-Dictionary.

The dictionary, the delta response, and the request are served over HTTPS from the same origin. Per RFC 9842, Section 8 ↗, compression dictionary transport is HTTPS-only.

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How shared dictionaries work


The protocol uses two new request and response headers and two new content encodings:


HeaderDirectionPurpose
Use-As-DictionaryOrigin → browserMarks a response as usable as a dictionary for future requests matching the supplied match value.
Available-DictionaryBrowser → originAdvertises the SHA-256 hash of a dictionary the browser already has for the request URL.
Content-Encoding: dcb or dczOrigin → browserDelta-compressed against the advertised dictionary, using Brotli (dcb) or Zstandard (dcz).



The first response for a versioned asset includes Use-As-Dictionary, and the browser stores the response. On subsequent requests for assets matching the pattern, the browser sends Available-Dictionary: :<sha256>: and adds dcb, dcz to Accept-Encoding. Your origin compresses the new asset against the dictionary and returns it with Content-Encoding: dcb or dcz. The browser uses its stored copy to reconstruct the full response.

The match value in Use-As-Dictionary is a WHATWG URL Pattern ↗, not a regular expression. Match patterns operate on the percent-encoded URL path and are scoped to the same origin as the dictionary.

The Available-Dictionary value is a Structured Field ↗ byte sequence: the base64-encoded SHA-256 hash wrapped in colons (for example, :pZGm1Av0IEBKARczz7exkNYsZb8LzaMrV7J32a2fFG4=:). The colons are part of the syntax.

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Enable shared dictionaries


Enabling shared dictionaries is a two-part task:

Turn on passthrough for your zone in Cloudflare. This tells Cloudflare to forward dictionary headers and vary cache entries correctly.

Update your origin server to mark assets as dictionaries and return delta-compressed responses against them.

The work of creating dictionaries and compressing new responses against them happens at your origin, not at Cloudflare.


1. Enable passthrough in Cloudflare







To enable shared dictionaries in the dashboard:


In the Cloudflare dashboard, go to the Speed Settings page.
Go to Settings ↗


Go to Content Optimization.



Toggle Shared Dictionaries to On.




Use the following PATCH request to enable shared dictionaries:
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" }'
To turn shared dictionaries off, set value to "disabled".

Valid values for this setting are:


ValueBehavior
passthroughCloudflare forwards shared dictionary request and response headers, accepts dcb/dcz responses from the origin, and varies cache entries.
disabledCloudflare strips shared dictionary headers and does not cache dcb/dcz variants.





You can configure shared dictionaries using the cloudflare_zone_settings_override resource. For more details, refer to the Terraform documentation ↗.





Note


Because shared_dictionary_mode is a zone setting, it is not compatible with Cloudflare Version Management. You can enable passthrough in a non-production version, validate that origin responses are correct end to end, and promote to production with a single deploy on a new zone.




2. Mark assets as dictionaries at your origin


For each versioned asset you want to use as a dictionary, include a Use-As-Dictionary header on the first response:
Use-As-Dictionary: match="/static/app-*.js", type="raw"Cache-Control: public, max-age=31536000, immutableContent-Encoding: br
The match value tells the browser which future request URLs should advertise this dictionary. It is a WHATWG URL Pattern, does not support regular expressions, and must resolve to the same origin as the dictionary.


3. Compress new versions against the advertised dictionary


When a request arrives with an Available-Dictionary header, look up the dictionary by its SHA-256 hash. If you have it, compress the response against it and return:
Content-Encoding: dczVary: Accept-Encoding, Available-DictionaryCache-Control: public, max-age=31536000, immutable
RFC 9842, Section 6.2 ↗ requires the Vary: Accept-Encoding, Available-Dictionary response header so that browser caches do not serve the wrong variant. Cloudflare's cache also varies on these headers when passthrough is on.


4. Fall back when no dictionary is available


When the browser does not advertise Available-Dictionary, the hash does not match a dictionary you have, or the browser does not advertise dcb/dcz, return the response with normal Brotli, Zstandard, or Gzip compression.


Implementation options


Cloudflare does not prescribe a specific origin implementation. Common starting points include:

A reverse proxy. Configure NGINX, Caddy, or a similar proxy to attach Use-As-Dictionary headers and produce delta responses with a sidecar process.

Native support in your application server. Extend your existing compression middleware to read Available-Dictionary and emit dcb or dcz.

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Test shared dictionaries


To confirm a request is using a shared dictionary, request the asset twice. The second request advertises the dictionary you received in the first response.
# Prime the dictionary.curl -sI -H "Accept-Encoding: br, gzip, zstd, dcb, dcz" \ https://example.com/static/app.v1.js# Request the next version, advertising the dictionary you just received.# Replace <hash> with the base64-encoded SHA-256 of the first response.# The surrounding colons are part of the Structured Field syntax# and are required by RFC 9842, Section 2.2.curl -sI -H "Accept-Encoding: br, gzip, zstd, dcb, dcz" \ -H "Available-Dictionary: :<hash>:" \ https://example.com/static/app.v2.js
The second response should include Content-Encoding: dcz (or dcb), Vary: Accept-Encoding, Available-Dictionary, and a Content-Length significantly smaller than a non-delta response.

You can also use canicompress.com ↗ to confirm your browser supports shared dictionaries and to inspect a working delta-compressed response.

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Limitations


Origin-side work is required. In passthrough mode, Cloudflare does not generate dictionaries or compute deltas. If your origin does not produce dcb/dcz responses, no compression savings occur.

Body-modifying features are incompatible. Cloudflare features that rewrite response bodies do not work on delta-compressed responses. Turn these features off on dictionary-compressed paths, or set cache-control: no-transform on the origin response. For details, refer to Content compression.

Browser support is partial. Visitors on browsers that do not request dcb or dcz continue to receive Brotli, Zstandard, or Gzip per your existing Compression Rules and default compression behavior.

Same-origin only. Per RFC 9842, Section 9.3.1 ↗, dictionaries are scoped to the response origin. Cross-origin dictionary use is not supported.


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On this page

Overview

Availability

Requirements

How shared dictionaries work

Enable shared dictionaries

1. Enable passthrough in Cloudflare

2. Mark assets as dictionaries at your origin

3. Compress new versions against the advertised dictionary

4. Fall back when no dictionary is available

Implementation options

Test shared dictionaries

Limitations


Edit pageReport issue














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