Thanks for the bump. Given that this only ran for a few weeks before
being disabled (and importantly, the fact that it's not currently
possible to register for that version
<https://developer.chrome.com/origintrials/#/view_trial/619807898716864513>
of the OT) - LGTM to try again from M149 to M155.
On 3/11/26 3:01 p.m., Thomas Lively wrote:
Hi API owners,
I just wanted to bump this. Does a new OT sound good to you?
Thanks,
Thomas
On Mon, Feb 23, 2026 at 9:09 AM Thomas Lively <[email protected]> wrote:
Hi API owners,
It turned out that we had to disable this origin trial in early
October due to security issues reported to us by an external
security researcher. As a result, the OT was only live for about
three weeks. Since then, we've been hard at work fixing those bugs
and improving our fuzzing for this feature.
We are almost ready to try again, so I'd like permission to run a
new OT for the same feature from M149 to M155.
Please let me know if you have any questions.
Thomas
On Tue, Aug 19, 2025 at 7:37 AM Mike Taylor
<[email protected]> wrote:
LGTM to experiment from M141 to M147 inclusive.
On 8/13/25 4:40 p.m., 'Thomas Lively' via blink-dev wrote:
Good question! The first toolchain to support custom
descriptors is j2cl <https://github.com/google/j2cl>, which
is an open source Java-to-JS/Wasm compiler developed by
Google. Developers will enable the feature via an option
<https://github.com/google/j2cl/commit/a8cdfa7aa36d4c46e8b794139a38ccefcd99e5fb#diff-be4e99d1c21ff16eff4ab47ef7d35d297118d455e283f292745218a53d853553R147-R151>
passed
to the compiler. We hope that once the feature is available
via an origin trial, other toolchains will be motivated to
start experimenting with it as well.
Note that in the initial experiments we don't expect
toolchains to expose any new developer-facing functionality
based on this feature. The use of custom descriptors will
only be an internal implementation detail in the compiler
that results in less memory usage at runtime. Later
experimentation will additionally enable the JS interop
features built on top of custom descriptors, and it will vary
from toolchain to toolchain whether this enables new
developer-facing functionality or enables a performance
improvement in existing functionality.
Also note that this feature will generally only be usable
from managed-memory languages that compile to WasmGC such as
Java, Kotlin, and Dart. It's unlikely that C/C++/Rust and
similar languages will ever make use of custom descriptors.
On Wed, Aug 13, 2025 at 12:24 PM Daniel Bratell
<[email protected]> wrote:
I might have missed it, but how is this new feature going
to be used by web developers? Do you have custom C
compiler for the experiment? And do you need to do
something in C to enable it? Or whatever language people
prefer.
/Daniel
On 2025-08-13 20:04, Thomas Lively wrote:
Yes, apologies. The explainer link should be the same as
the specification link:
https://github.com/WebAssembly/custom-descriptors/blob/main/proposals/custom-descriptors/Overview.md
On Wed, Aug 13, 2025 at 7:58 AM Daniel Bratell
<[email protected]> wrote:
The explainer link is dead, but maybe it was meant
to be the same as the Specification link?
/Daniel
On 2025-08-12 00:50, 'Thomas Lively' via blink-dev
wrote:
Contact emails
[email protected], [email protected],
[email protected]
Explainer
https://github.com/WebAssembly/custom-descriptors/blob/main/proposals/custom-rtts/Overview.md
<https://github.com/WebAssembly/custom-rtts/blob/main/proposals/custom-rtts/Overview.md>
Specification
https://github.com/WebAssembly/custom-descriptors/blob/main/proposals/custom-descriptors/Overview.md
Summary
Allows WebAssembly to store data associated with
source-level types more efficiently in new "custom
descriptor" objects. These custom descriptors can
be configured with prototypes for the WebAssembly
objects of that source-level type. This allows
methods to be installed on a WebAssembly object's
prototype chain and called directly from JS using
normal method call syntax. The prototypes and
methods can be configured declaratively using an
imported builtin function.
Blink component
Blink>JavaScript>WebAssembly
<https://issues.chromium.org/issues?q=customfield1222907:%22Blink%3EJavaScript%3EWebAssembly%22>
TAG review
None
TAG review status
Pending
Origin Trial documentation link
https://github.com/WebAssembly/custom-descriptors/blob/main/proposals/custom-descriptors/Overview.md
Risks
Interoperability and Compatibility
None
/Gecko/: No signal
/WebKit/: No signal
/Web developers/: No signals
/Other signals/:
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?
None
Goals for experimentation
Initially we are looking to measure real-world
performance, heap usage, and OOM rate impact of
just the core Wasm part of the proposal. We want to
verify that heap usage and OOM rates will decrease
as expected and that the performance impact is
negligible. If we do measure a performance impact,
that will inform additional optimization work we
would need to do in the implementation. Notably,
the JS interop features of the proposal are out of
scope for this initial experimentation. They will
likely be evaluated in follow-on experiments.
Ongoing technical constraints
None.
Debuggability
None
Will this feature be supported on all six
Blink platforms (Windows, Mac, Linux,
ChromeOS, Android, and Android WebView)?
No
Is this feature fully tested by
web-platform-tests
<https://chromium.googlesource.com/chromium/src/+/main/docs/testing/web_platform_tests.md>?
No
Flag name on about://flags
None
Finch feature name
None
Non-finch justification
None
Requires code in //chrome?
False
Estimated milestones
Origin trial desktop first 141
Origin trial desktop last 147
Link to entry on the Chrome Platform Status
https://chromestatus.com/feature/6024844719947776?gate=5062088692858880
This intent message was generated by Chrome
Platform Status <https://chromestatus.com/>.
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