`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