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Yes, but only for some visitors and only when several pieces line up. When a browser already holds the previous version of a JavaScript or CSS file, a server can compress the new version using that old file as a dictionary. Code the two versions share is then encoded as references to the old copy, so the transfer can approach a delta instead of a freshly compressed full file. The benefit disappears when the browser has no matching cached version, when the server or CDN cannot match the dictionary it advertises, or when the client does not support the dictionary encodings. In those cases the response falls back to ordinary Brotli, Zstandard or gzip.
What the browser actually receives
A compression dictionary is shared prior content, not a patch file. The new response is a compressed stream that the client can decode only by using a dictionary it already holds. RFC 9842, Compression Dictionary Transport, published in September 2025, describes the core idea in its own terms:
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“Using a previous version of a resource as a dictionary for a newer version enables delivery of a delta-compressed version of the changes, usually resulting in significantly smaller responses than what can be achieved by compression alone.”
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesThe word “usually” matters. The size reduction depends on how much of the new file repeats the old one. A small change to a large bundle compresses far better against its predecessor than a rewrite does. A bundle that is reorganised, re-minified with different identifiers, or rebuilt with a different module layout shares less with its predecessor and gains less.
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How the exchange works, step by step
- The server marks a candidate dictionary. A response for the current bundle carries a
Use-As-Dictionaryheader, or RFC 9842 offers thecompression-dictionarylink relation to point at a resource likely to help future requests. A prior version is a natural candidate when you ship an upgrade. - The browser stores that resource. The earlier version sits in the browser cache and can serve as the dictionary. Chrome’s guidance states that the browser sends
Available-Dictionaryonly when a suitable earlier resource is available in its cache, so first-time visitors never send it. See Chrome for Developers, Supercharge compression efficiency with shared dictionaries. - The browser asks for the new version with dictionary information. The request carries
Available-Dictionary, identifying the dictionary the browser holds, and anAccept-Encodingvalue that includesdcb(Dictionary-Compressed Brotli) and/ordcz(Dictionary-Compressed Zstandard). RFC 9842 registers both encoding names. - The server decides which encoding to return. If it can match the advertised dictionary and the client accepts the dictionary encoding, it compresses the new version against that dictionary and returns it with the matching
Content-Encoding. Otherwise it returns a conventional response. - The browser reconstructs the file. It decodes the stream using the dictionary already in cache. The result is the full new bundle, which the page uses exactly as it would any other response.
Which response a client receives
| Client and cache situation | What the server returns |
|---|---|
First visit, so no cached dictionary and no Available-Dictionary header |
Conventional Brotli, Zstandard or gzip, according to existing negotiation (Cloudflare documents this fallback) |
Returning visitor with a matching cached version, and dcb or dcz accepted |
A dictionary-compressed response (dcb or dcz) |
Client does not advertise dcb or dcz |
Conventional compression |
| Server or CDN cannot match the advertised dictionary | Conventional compression, as RFC 9842 requires for this case |
The header vocabulary
| Header or value | Sent by | Role in the exchange |
|---|---|---|
Use-As-Dictionary |
Server response | Declares a resource as a candidate dictionary. Cloudflare’s passthrough forwards it. |
compression-dictionary link relation |
Server response | Points to a resource likely to help future requests, as defined in RFC 9842 |
Available-Dictionary |
Browser request | Identifies the dictionary the browser already holds |
Accept-Encoding containing dcb or dcz |
Browser request | Advertises that the client can decode dictionary-compressed responses |
Content-Encoding: dcb or dcz |
Server response | Marks the body as compressed against a dictionary |
Vary |
Server response | Tells caches which request headers change the variant they must store (see the caching section) |
Brotli and Zstandard variants
Dictionary-Compressed Brotli (dcb)
RFC 9842 defines a fixed 36-byte header for dcb: a four-byte magic value followed by the SHA-256 digest of the external dictionary. The digest lets the decoder confirm which dictionary the stream expects. The Shared Brotli stream format is specified in RFC 9841, Shared Brotli Compressed Data Format, published in September 2025. Its compression window is at most 16 MB. RFC 9842 notes that Brotli can still use the full dictionary when compressing and decompressing resources larger than that window.
Dictionary-Compressed Zstandard (dcz)
RFC 9842 also registers dcz. It follows the same negotiation pattern: the client advertises it, the server uses it only when it can match the dictionary, and the client falls back otherwise. The vendor example later in this article uses delta Zstandard.
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Where the savings come from, and how much to trust a number
Cloudflare’s April 30, 2026 changelog reports an internal test on a 272 KB JavaScript bundle. The response was 92.1 KB with gzip and 2.6 KB with delta Zstandard, which is about 97% smaller than the gzip response. Treat this as a vendor-reported example. It was one internal test, it has not been independently replicated in the sources reviewed, and it is not a typical expected saving. Your own ratio depends on how much of each new build repeats the previous one.
To estimate the benefit for your own bundles, compress a real pair of consecutive builds with and without the prior version as a dictionary, then weight the result by how many of your returning visitors actually hold the prior version. The first number is the saving per transfer. The second determines whether the saving reaches your users.
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Source: Cloudflare changelog, Shared dictionaries passthrough now in open beta.
Caching and CDN behaviour
A cacheable dictionary-compressed response must carry a Vary header that distinguishes the encodings and dictionary choices a client may send. Without it, a shared cache can store a dcb variant and hand it to a client that cannot decode it, or hand a delta built against one dictionary to a client holding another. Cache correctness is the most common place this breaks in production.
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Cloudflare documents shared dictionary passthrough as open beta on all plans as of April 30, 2026. Its passthrough forwards Use-As-Dictionary and Available-Dictionary, accepts dcb and dcz end-to-end without recompressing them, and extends its cache key using dictionary availability and Accept-Encoding. Configuration details are in Cloudflare, Shared dictionaries. The sources reviewed document this behaviour for Cloudflare only. If you use another CDN, proxy or cache layer, confirm that it preserves these headers and keys its cache on them before relying on dictionary responses.
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Choosing and retaining dictionaries
Which prior versions to keep, and how long a dictionary stays fresh, are implementation decisions. MDN identifies both as choices to weigh against storage and operational cost. Keeping only the immediately previous build is the simplest policy and covers visitors who updated recently. Retaining more versions widens the set of returning visitors who can benefit, at the cost of more stored artefacts and more cache-key combinations to manage.
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Security constraints
Transport, origin and CORS
RFC 9842 requires compression dictionary transport to be used only in secure contexts, meaning HTTPS. The standard cites the risk that network intermediaries may process dictionary-compressed responses incorrectly. The dictionary and the compressed response must both be readable by the client. Cross-origin use requires CORS permission, so a dictionary served from a different origin than the page needs explicit cross-origin grants.
Choosing dictionary content
RFC 9841’s security discussion of Shared Brotli says dictionary contents should not be controlled by external users. It recommends building dictionaries from content you control, or from generic large-scale content, and updating them at intervals rather than rapidly. It also describes risks from attacker-controlled combinations of dictionary and content, and the possibility of compression side channels. The practical rule is that the dictionary deserves the same security care as the content it helps produce.
Browser support and fallback
MDN’s guide to Compression Dictionary Transport labels the feature as limited availability and not Baseline. Do not assume broad browser coverage. Check the compatibility table for the browser mix in your own analytics before you rely on dictionary responses for a meaningful share of traffic. Keep a tested conventional compression path, because clients that do not request dcb or dcz continue to receive ordinary Brotli, Zstandard or gzip. See MDN Web Docs, Compression Dictionary Transport.
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- Size: compressed bytes for representative consecutive builds, measured against your current gzip or Brotli output.
- Cache state: the share of returning visitors likely to hold the matching prior version.
- Browser coverage: the MDN compatibility table applied to your audience.
- Server and CDN: header forwarding,
dcb/dczpassthrough, and cache keys that vary onAvailable-DictionaryandAccept-Encoding. - Retention: how many prior versions you keep and how often the dictionary is replaced.
- Security: HTTPS everywhere, CORS for any cross-origin dictionary, and dictionary content you control.
When the savings do not appear
- Every response is still full gzip. Check whether the browser sends
Available-Dictionaryat all. Per Chrome’s guidance, it does so only when a suitable earlier resource is cached, so a fresh browser profile will always show the fallback. - The browser receives a variant it cannot decode. Look for a missing or incomplete
Varyheader, or an intermediary that caches adcbordczbody under a key that ignores dictionary availability. - The dictionary path is ignored in a cross-origin setup. Verify that CORS permits the browser to read the dictionary resource.
- Your CDN strips the headers. Confirm that
Use-As-Dictionary,Available-Dictionaryand the dictionary encodings pass through unchanged.
The browser-side behaviour above follows the negotiation rules in RFC 9842, so a fallback response in any of these cases indicates a configuration or cache issue rather than a failure of the protocol.
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