HTTP Headers

Available-Dictionary header

HTTP Header

The HTTP Available-Dictionary request header enables the browser to specify the best matching dictionary it possesses, allowing the server to utilize Compression Dictionary Transport for a resource request.

Clients can send an Available-Dictionary header if they support dcb or dcz encodings. The header consists of a colon-surrounded Base64-encoded SHA-256 hash of the dictionary’s contents.

For more information, see the Compression Dictionary Transport guide.

Syntax

Available-Dictionary: :<base64-hash>:

Directives

<base64-hash>: A Base64-encoded SHA-256 hash of the dictionary’s contents.

Example

Accept-Encoding: gzip, br, zstd, dcb, dcz
Available-Dictionary: :pZGm1Av0IEBKARczz7exkNYsZb8LzaMrV7J32a2fFG4=:

How to Modify Header using Requestly

Requestly is a powerful Chrome extension that allows you to modify HTTP headers, including the Available-Dictionary header. This header is used in shared dictionary compression to indicate which compression dictionaries the client has available. It is helpful when testing advanced compression workflows during development or debugging. Steps to Modify the Available-Dictionary Header:

Install and open the Requestly Chrome extension. You can find it on the Chrome Web Store.
Create a new rule: Click on “Create Rule” and choose “Modify Headers” from the list of available rule types.
Add a new header modification:
- Under “Action”, select “Add” or “Override”.
- In the “Header Name” field, enter Available-Dictionary.
- In the “Header Value” field, enter the list of dictionary identifiers available on the client (for example, dict-v1, dict-v2).
Set the URL condition: Specify the URL or pattern where this header should apply (for example, https://your-api.com/*).
Save the rule.

Once configured, Requestly will inject the Available-Dictionary header into all matching requests. This allows you to simulate a client advertising specific shared compression dictionaries.

You might need to modify the Available-Dictionary header to test shared dictionary compression behavior, validate compatibility between client and server dictionaries, or debug performance optimizations without changing your production environment.