Implements HTMX interactions, configures swap behaviors, debugs hx-* requests, and builds hypermedia-driven UI components. Use when tasks involve hx-* attributes, HTMX AJAX requests, swap strategies, server-sent events, WebSockets, or hypermedia-driven UIs.
HTMX
Use this skill for HTMX implementation and integration. Covers htmx 2.x (current stable) and htmx 4.x (beta). Reference files are annotated with [htmx 4], [htmx 4 change], and [htmx 4 removed] admonitions. Read only the reference file(s) needed for the task.
Quick Start
Identify the domain of the task (attributes, requests, swapping, events, patterns).
Open the matching file from references/.
Implement using HTML-first, hypermedia-driven patterns.
Validate that server responses return HTML fragments, not JSON.
Minimal example — a button that loads content via GET:
The server endpoint (/contacts) must return an HTML fragment, not JSON:
<!-- Server response (HTML fragment, no <html>/<body> wrappers) --><ul><li>Alice</li><li>Bob</li></ul>
The default swap is innerHTML — the response replaces the target's children. Use hx-swap="outerHTML" to replace the target element itself.
Critical Rules
HTML responses - HTMX expects HTML responses from the server, not JSON
Attribute inheritance - (v2) Most attributes inherit to children; use hx-disinherit or unset to stop. (v4) Inheritance is explicit — use :inherited suffix to opt in. Not inherited in either version:hx-trigger, hx-on*, hx-swap-oob, hx-preserve, hx-history-elt, hx-validate
Default swap is innerHTML - Always confirm the intended swap method
Form values auto-included - Non-GET requests automatically include the closest enclosing form's values
Progressive enhancement - Use hx-boost="true" — pages must work without JS
Escape user content - Escape all user-supplied content server-side to prevent XSS. Wrap areas rendering user-generated content with hx-disable to prevent HTMX processing of injected attributes
CSS lifecycle classes - HTMX adds/removes CSS classes during requests — use for transitions and indicators
data-prefix supported - All hx-* attributes can also be written as data-hx-* for HTML validation compliance
Stop polling with HTTP 286 - Server returns status 286 to stop every or load delay polling. Always use 286 for poll termination, not conditional client-side logic
Error swaps - (v2) Non-2xx responses are not swapped by default — add an htmx:beforeSwap listener to enable. (v4) All responses swap by default except 204/304 — use hx-status for per-code control
Decouple with HX-Trigger headers - Use HX-Trigger response headers to fire client-side events instead of hardcoding DOM element IDs in server responses
Detect HTMX requests server-side - Check the HX-Request header to serve HTML fragments for HTMX requests and full pages for direct browser requests. Set Vary: HX-Request for caching
Reference Map
All hx-* attributes, values, and modifiers: references/attributes.md
