WebTransportError API alignment and serialization

Category: Network / Connectivity
Type: Chromium catches up
Status: Proposed (Chrome Proposed)
Intent stage: None

Breaking change: No
Enterprise impact: None
Chromium flag: enable-experimental-web-platform-features
Spec maturity: Published standard
Web Platform Tests: Yes

WPT details: https://wpt.fyi/results/webtransport/idlharness.https.sub.any.html?label=experimental&label=master&aligned

Summary

Updates Chromium WebTransportError to match the current WebTransport specification. Adds the standard (message, options) constructor while preserving the shipped dictionary constructor for compatibility; carries the full unsigned-long stream error code end to end; and makes WebTransportError structured-clonable while preserving its message, source, and nullable error code. Serialization is enabled by default behind a writer-side runtime kill switch, while decoding remains permanently enabled for persisted values.

Motivation

Chromium's shipped WebTransportError implementation predates the current WebTransport specification. It accepts a legacy dictionary-only constructor, narrows outgoing application stream error codes to 8 bits despite the API exposing an unsigned long, and does not support the structured serialization required by the specification. The change aligns Chromium with the standardized API and improves WebTransport interoperability. The legacy constructor remains available so existing Chromium applications continue to work.

Standards & signals

Specification: https://www.w3.org/TR/webtransport/#web-transport-error-interface
Firefox: Shipped/Shipping
Safari: Shipped/Shipping
Web developers: Positive — The Interop 2026 WebTransport proposal received 28 👍 reactions. Developers also reported active WebTransport use and significant cross-engine differences.
Tracking bug: https://issues.chromium.org/issues/577073049

Ecosystem

Web Platform Tests: https://wpt.fyi/results/?q=webtransport%2Fidlharness
Chrome Platform Showcase: https://chrome-platform-showcase.paulkinlan-ea.deno.net/features
GenDN reference: https://gendn.paulkinlan-ea.deno.net/features
chrome.dev demos search: https://www.google.com/search?q=site%3Achrome.dev+WebTransportError%20API%20alignment%20and%20serialization
MDN Web Docs: https://developer.mozilla.org/en-US/search?q=WebTransportError%20API%20alignment%20and%20serialization
Baseline status: https://webstatus.dev/?q=WebTransportError%20API%20alignment%20and%20serialization

Docs: https://developer.mozilla.org/en-US/docs/Web/API/WebTransportError https://developer.chrome.com/docs/capabilities/web-apis/webtransport

Samples: https://developer.mozilla.org/en-US/docs/Web/API/WebTransportError

Explainers: https://github.com/w3c/webtransport/blob/main/explainer.md

View on chromestatus.com