Intent to Ship: Fetch API: Forward reason from AbortController to fetch Response
Chromestatus
Gecko: Shipped/Shipping
WebKit: No signal
Web developers: No signals
Other signals:
Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?
No information providedhttps://wpt.fyi/results/fetch/api/abort/general.any.html Specifically the tests: * response.arrayBuffer() rejects with abort reason if already aborted (and other response methods such as body()) * Stream errors once aborted with abort reason. Underlying connection closed.
| Shipping on desktop | 153 |
| Shipping on Android | 153 |
| Shipping on WebView | 153 |
| Shipping on iOS | 153 |
Open questions about a feature may be a source of future web compat or interop issues. Please list open issues (e.g. links to known github issues in the project for the feature specification) whose resolution may introduce web compat/interop risk (e.g., changing to naming or structure of the API in a non-backward-compatible way).
No information provided
Dan Clark
> https://wpt.fyi/results/fetch/api/abort/general.any.html Specifically the tests: * response.arrayBuffer() rejects with abort reason if already aborted (and other response methods such as body()) * Stream errors once aborted with abort reason. Underlying connection closed.
Daniel Clark
Thanks for pointing out the specific spec algos.
> @ricea had suggested using an experiment, I assume because the use of fetch is so widespread and changing the type of error that's thrown could have consequences, even if it's the intended behavior. But I'll let him comment on that himself if he has anything to add.
We have generally been recommending against gradual rollout in cases like this: https://chromium.googlesource.com/chromium/src/+/main/docs/flag_guarding_guidelines.md#prefer-waterfall-rollout-for-platform-changes. In short, an experimental rollout makes any breakage hard to track for developers because it won’t show up consistently. The usual approach would be to just ship normally via canary/dev/beta/stable and be ready to roll it back via finch if there are issues.
We shouldn’t do either without trying to understand the risk, though. What do you expect potential compatibility issues to look like for this? Do developers often have logic branching on specific abort values for these promises versus just logging them? Do we have Use Counters that could put a high bar on the risk, like a counter for how often already-completed fetches are aborted with a reason, or how often the Response methods are called for a fetch in this state?
From: Taylor Brandstetter <dead...@google.com>
Sent: Wednesday, August 12, 2026 2:30 PM
To: blink-dev <blin...@chromium.org>
Cc: Daniel Clark <dan...@microsoft.com>; Chromestatus <ad...@cr-status.appspotmail.com>; dead...@google.com <dead...@google.com>; Adam Rice <ri...@google.com>
Subject: Re: Intent to Ship: Fetch API: Forward reason from AbortController to fetch Response
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> Can you please give a more specific link into the part of the spec that defines this behavior?
Sure thing; the body methods point to the steps for consuming a body: https://fetch.spec.whatwg.org/#concept-body-consume-body
Which point to the steps for fully reading a body: https://fetch.spec.whatwg.org/#body-fully-read
Which say to read all the bytes from the reader.
And regarding the reader, the main abort steps (https://fetch.spec.whatwg.org/#http-network-fetch) say:
- If fetchParams is aborted, then:
- Set response’s aborted flag.
- If stream is readable, then error stream with the result of deserialize a serialized abort reason given fetchParams’s controller’s serialized abort reason and an implementation-defined realm.
So, both the readable stream and all the body methods consuming it should be errored with the abort reason.
> Can you request a WebKit signal?
I didn't realize it was necessary just for filling a gap in the implementation, but the wide review documentation does call this case out, so I will request a signal and update the feature later.
> I'm still unclear on which tests cover this change.
I called them out there; "response.X() rejects with abort reason if already aborted" (for each body method) and "Stream errors once aborted with abort reason". They fail on everything but Firefox (well except for the latter which doesn't run on Firefox); I'm assuming the dashboard wasn't updated when you responded but you can see the failures now.
> Can you say more about this roll out plan? This doesn't seem like the kind of thing that would need to be rolled out gradually via experiment.
@ricea had suggested using an experiment, I assume because the use of fetch is so widespread and changing the type of error that's thrown could have consequences, even if it's the intended behavior. But I'll let him comment on that himself if he has anything to add.
> Lastly please request the other "Prepare to Ship" reviews in the chromestatus entry (Privacy, WP Security, etc).
Will do. Thanks for your review, Dan.
Taylor Brandstetter
Which point to the steps for fully reading a body: https://fetch.spec.whatwg.org/#body-fully-read
Which say to read all the bytes from the reader.
And regarding the reader, the main abort steps (https://fetch.spec.whatwg.org/#http-network-fetch) say:
If fetchParams is aborted, then:
Set response’s aborted flag.
If stream is readable, then error stream with the result of deserialize a serialized abort reason given fetchParams’s controller’s serialized abort reason and an implementation-defined realm.
Alex Russell
Taylor Brandstetter
I don't see any use counters specifically for providing an abort reason, no. Branching logic on the error from the promises would be the risk, although there's a good chance that we were just being overly cautious. I have no reason to believe this would be particularly common. If others are ok with it, I would suggest just shipping normally with 154. You mentioned being ready to roll back via finch; does that mean we would keep the change gated behind a flag, but roll it out to 100% immediately for 154 when it branches? I'm not very familiar with finch (or this process) so bear with me.
I requested the WebKit signal (https://github.com/WebKit/standards-positions/issues/711) and the other Prepare to Ship reviews, which I assume should be fairly trivial. I haven't requested Prepare to Ship for Adoption yet as it requires the feature to be stable for four weeks, and I just changed the milestone to 154.
Dan Clark
Taylor Brandstetter
The Adoption gate should be filled out before shipping...where did you see a reference to "stable for four weeks"?
One of the checkboxes is "Announcement lead time. This feature's name, summary, and milestones have been unchanged for the past four weeks".
Which I assumed didn't apply since I changed the milestone (153->154). However, if we ship it by enabling the feature flag on stable immediately—rather than going through canary/dev/beta first—it could actually ship in 153, as the code is ready. Shall I revert that change and proceed with the Adoption prepare to ship? The feature entry hasn't even existed for four weeks, though, is that a problem?
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Daniel Clark
Date: Wednesday, August 19, 2026 at 12:37 PM
To: Daniel Clark <dan...@microsoft.com>
Cc: blink-dev <blin...@chromium.org>; sligh...@chromium.org <sligh...@chromium.org>; Chromestatus <ad...@cr-status.appspotmail.com>; Adam Rice <ri...@google.com>
Subject: Re: [blink-dev] Re: Intent to Ship: Fetch API: Forward reason from AbortController to fetch Response
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The Adoption gate should be filled out before shipping...where did you see a reference to "stable for four weeks"?
Taylor Brandstetter
Daniel Bratell
LGTM3
/Daniel
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