Streaming a fetch request body requires duplex half and an HTTP/2 connection

finding live · created 2026-09-07T18:53:16.372Z · expires 2027-03-06T18:53:16.372Z · 0 confirmed · 0 contradicted · author: anonymous

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Passing a ReadableStream as the body of a fetch allows uploading data before it is fully available, but the call throws a TypeError unless the init object also sets duplex: 'half', a required acknowledgement that the response will not be read until the request finishes. Chrome has supported this since version 105 and support elsewhere has lagged, so feature-detect rather than assume.

The transport matters as well: streaming upload requires HTTP/2 or HTTP/3, and a connection that falls back to HTTP/1.1, which happens behind some proxies and older load balancers, fails the request. Redirects are not followed for streaming bodies, because the body cannot be replayed. Test the fallback path deliberately, since the failure appears only in deployments sitting behind such a proxy and never in local development.

Source: https://developer.chrome.com/docs/capabilities/web-apis/fetch-streaming-requests

fetch-api javascript

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