`fetch`: `ReadableStream` in `body`

[benmccann]

Description

Allow using a ReadableStream in the body for fetch. Ideally for both the request and response bodies

Specification

https://fetch.spec.whatwg.org/

web-feature

https://web-platform-dx.github.io/web-features-explorer/features/fetch-request-streams/

Test Links

No response

Additional Signals

I have personally run into the lack of this API as being burdensome as a maintainer of SvelteKit where we implemented "remote functions" (RPCs) and as a maintainer of Immich where we implemented progress tracking for uploads and downloads of photos. We've had to do workarounds using XMLHttpRequest instead of fetch as a result: https://github.com/immich-app/immich/blob/17bbcdf58418b3168f05b4b7aea5912129d86c54/web/src/lib/utils.ts#L85

The lack of this feature is a major driver for adoption of the axios data fetching library, so as a result more bytes get sent to the client as axios must be sent across the wire rather than relying on the platform.

Implemented in a number of platforms: https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API#browser_compatibility

Missing from Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1387483

$15,000 bounty for the feature in webkit: WebKit/standards-positions#24 (comment)

A sample of representative StackOverflow posts on the topic:

• https://stackoverflow.com/questions/35711724/upload-progress-indicators-for-fetch
• https://stackoverflow.com/questions/40939857/fetch-with-readablestream-as-request-body/41222366
• https://stackoverflow.com/questions/47285198/fetch-api-download-progress-indicator
• https://stackoverflow.com/questions/52422662/how-to-get-file-upload-progress-with-fetch-and-whatwg-streams

A number of likes and responses here: https://bsky.app/profile/benmccann.com/post/3lxdgi2kb2s25

[github-actions]

This comment was automatically generated based on the information you provided. Please don't edit it.

Below is additional information about the web feature (from the web-features project) which is referenced in your proposal.
If this doesn't accurately correspond to your proposal, please update your initial comment to include web-features: <feature-id>.
To find feature IDs, use the web-features explorer.

Feature Fetch upload streams

• ID: fetch-request-streams
• Name: Fetch upload streams
• Description: A fetch() request uploads a stream of data to the server when a request's body property is a ReadableStream object.
• Baseline status: Limited Availability
• Docs: Request: body property
• Chrome use counter: chromestatus.com
• WPT tests: wpt.fyi/results/?q=feature:fetch-request-streams
• More information: See the web-features explorer

[MayyaSunil]

Thanks for proposing this focus area @benmccann!

We’ve been running brief surveys on this feature across multiple channels:

• X: https://x.com/mozdevnet
• Bluesky: https://bsky.app/profile/developer.mozilla.org
• Mastodon: https://mastodon.social/@mdn
• GitHub Discussions: https://github.com/orgs/mdn/discussions
• Discord: https://developer.mozilla.org/discord

You can view the aggregated results here: https://reporting.alchemer.com/r/28049_68c0257f09ff33.00740288

We see a strong perceived value for cross-browser support for this feature.

[jakearchibald]

we implemented progress tracking for uploads and downloads of photos. We've had to do workarounds using XMLHttpRequest instead of fetch as a result

This feature does not solve this use-case. Trying to solve this use-case with this feature would be an anti-pattern. I've written details about this here https://jakearchibald.com/2025/fetch-streams-not-for-progress/.

There's a draft PR for progress events which I think is what developers really want here, but of course that isn't ready for interop 2026.

We should still consider this for interop for other use-cases, but we should be mindful that shipping this widely before we have progress events may results in folks misusing this feature and causing future compat issues.

[jgraham]

To be clear, the intended scope of this is fetch(url, {body: ReadableStream}), correct? I ask because the web-feature linked currently covers the body property on Request objects, which has caused some confusion (the latter is already supported in WebKit but not Gecko).

[jakearchibald]

The scope of this is fetches where the request has a body that's a stream. That includes request.body, but it's not limited to that. Supporting just request.body (as Safari does today) isn't enough.

[jgraham]

So both Request.body and fetch(url, {body: ReadableStream})?

[jakearchibald]

Yeah, you can't really have one without the other.

When you do fetch(url, {body: readableStream}), fetch will, under the hood, call new Request(url, {body: readableStream})

It's also expected to be able to do:

const request = new Request(url, {body: readableStream});
await fetch(request);

[benmccann]

Yes. Thanks for clarifying

[wpt-interop]

Thank you for proposing fetch: ReadableStream in body for Interop 2026.

We are pleased to let you know that it has been accepted as part of the Fetch uploads and ranges focus area.

You’ll be able to track progress on this topic throughout the year via the Interop 2026 dashboard.

Posted on behalf of the Interop team.