Intent to Prototype: BYOB support for Fetch

Skip to first unread message

Nidhi Jaju

Apr 17, 2023, 3:03:11 AM4/17/23
to blink-dev, Adam Rice

Contact emails





Makes Response.body be a readable byte stream instead of a "default" readable stream. This enables it to be used with bring-your-own-buffer (BYOB) readers, reducing garbage collection overhead and copies.

Blink component



Using readable byte streams for Response.body allows more precise memory allocation, minimizes buffer copies, and reduces GC overhead.

Initial public proposal


TAG review


TAG review status

Not applicable


Interoperability and Compatibility

Low risk because streams and fetch have already been standardized for a long time. Other browsers have implemented other parts of the standard, and Firefox has already shipped this behavior for many months and others will most likely also adapt this feature as well soon.

Gecko: Shipped/Shipping ( Already shipped in Firefox in 2022.

WebKit: Positive ( @annevk from Apple approved the PR to update the spec with relevant changes and expressed interest as an implementer on behalf of Apple.

Web developers: No signals

Other signals: Deno is also interested in, and somewhat shipped, this behavior (


Currently, to clone a body, we tee the body's stream, but teeing always returns two "default" streams, where the chunks are not cloned for both streams. Making Response.body into a byte stream, will mean that cloning it will result in cloning the chunks for the second stream, which is different behavior. In order to mitigate activation risks, we are splitting this change into two releases. One where the default teeing behavior will also start to clone for the second stream behind the "ReadableStreamTeeCloneForBranch2" feature flag, and then make Response.body a readable byte stream behind the "FetchBYOB" feature flag.

WebView application risks

Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?


No special support needed.

Is this feature fully tested by web-platform-tests?


Flag name

ReadableStreamTeeCloneForBranch2 and FetchBYOB

Requires code in //chrome?


Tracking bug

Estimated milestones

Shipping on desktop115
Shipping on Android115
Shipping on WebView115

Link to entry on the Chrome Platform Status
Reply all
Reply to author
0 new messages