QT4 extensions

A W3C Community Group is working on proposals for extensions to XPath, XSLT, and XQuery, and has produced a version 4.0 set of specifications. Details are at https://qt4cg.org.

Saxon 13 implements many of the new features in these drafts. Specifically, the Saxon 13.0 release aims to conform to the drafts dated 28 January 2026. (These drafts are a snapshot of the state of development at the time they were published. They are used as a baseline because they have a persistent URL, and the documents at that location will not change. However, the specifications themselves should not be considered to be stable: they will change as the design progresses.)

Because the features in the 4.0 specifications cannot be considered stable, they are not enabled by default, and in most cases they are not available in Saxon-HE. See the following page for details on:

Enabling 4.0 features

Generally, Saxonica aims to avoid making incompatible changes except in major releases. But since the 4.0 features are not yet entirely stable, this policy does not apply: the aim instead will be to respond rapidly to changes in the developing specification. Maintenance releases in the 13.x series may therefore reflect changes in the draft specifications. See the Changes history section for detailed information about changes in the Saxon implementation for 4.0 features.

Further information and details of the current Saxon implementation has been integrated into the relevant sections of this documentation. Content is highlighted (like this page) to indicate details about 4.0 features, using a yellow colored side-bar and the QT4 lightbulb image. The main sections with new information are the reference sections:

XSLT elements - for details on all of the new 4.0 XSLT instructions, 4.0 extensions to existing elements, and information about patterns in XSLT 4.0.
XPath syntax - updated to include information on 4.0 extensions, including new types, new expressions, and new function syntax.
Function library - for details on all of the new 4.0 functions, and information about changes to existing functions.