htmx 4.0 moves to fetch() and makes attribute inheritance explicit

The htmx library shipped version 4.0.0 on 28 August 2026, after eight months of work. The deepest change is invisible to end users: the internal engine drops XMLHttpRequest in favour of fetch(). That foundation unlocks streaming of HTML fragments, but it comes with breaking changes that every 2.x project must absorb before switching.

#From XMLHttpRequest to fetch(): a rewritten engine

Moving to fetch() simplifies the implementation and enables progressive rendering: fragments returned by the server can appear as they arrive, without waiting for the full response. The trade-off is clear: any extension, interceptor or library that hooked into htmx 2.x’s request layer will stop working until it is adapted to the new API.

#Three behavioural breaks to plan for

The first affects attribute inheritance, now explicit. An attribute is no longer propagated to child elements by default; it must carry the :inherited suffix. The hx-disinherit attribute, which used to block that propagation, is gone because it no longer has a purpose.

<!-- htmx 2.x: inherited implicitly -->
<div hx-confirm="Are you sure?">…</div>

<!-- htmx 4.0: inheritance is declared -->
<div hx-confirm:inherited="Are you sure?">…</div>

The second concerns events, standardised on the htmx:phase:action pattern. Scripts listening for the old names have to be updated.

The third is about history. htmx 4.0 no longer uses localStorage by default: on back navigation, the page is re-fetched from the server. Projects that relied on the local cache can restore a close behaviour through the new hx-history-cache extension, backed by sessionStorage.

The heart of htmx 4.0 is not new syntax but an engine rebuilt around fetch() that makes HTML streaming possible.

#Native morphing, streaming and new extensions

htmx 4.0 brings swap morphing in without an external dependency, based on the idiomorph algorithm: a fragment replaces the target while preserving existing DOM state. A new <hx-partial> tag appears to orchestrate out-of-band style swaps more legibly. Around the engine, a family of extensions supports the transition.

Notable extensions include hx-preload for preloading, hx-download for file downloads, hx-sse and hx-ws for event-stream and WebSocket streaming, hx-multipart for composite responses, and hx-alpine-compat to coexist with Alpine.js. An htmax.js bundle packages htmx with a ready-made set of extensions.

#Migration: tooling and timeline

A command-line checker reviews templates and flags implicit inheritance, renamed or removed attributes, old event names and deprecated APIs.

npx [email protected] upgrade-check -- ./templates

The team also ships skill files aimed at coding assistants (htmx-guidance, htmx-debugging, htmx-extension-authoring, htmx-upgrade-from-htmx2) to assist diagnosis and the migration of a 2.x codebase.

#The bottom line

htmx 4.0.0 is a deliberate breaking release: the engine moves to fetch(), inheritance becomes explicit, events are renamed and history stops relying on localStorage. Nothing forces an immediate migration, since 2.x remains the default install, but teams aiming for fragment streaming now have a mapped path, verification tooling included.

#Further reading

Official announcement and release notes: htmx 4.0.0 has been released.

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