Browser DevTools extension for debugging HTMX applications.
Works with both htmx 2.x and htmx 4.0 alpha.

Features

Request Inspector

Capture the full HTMX request lifecycle with timing breakdown, headers, request/response body, and event trace.

• HTTP verb, URL, status, swap strategy
• Trigger and target element identification
• Visual timeline bar (Config > Send > Wait > Swap > Settle)
• HX-* request and response headers
• Record / Pause / Clear controls
• HTTP error detection (4xx/5xx shown as errors even on htmx 4 where they swap by default)

Element Inspector

Live DOM tree showing all HTMX elements with their hierarchy, attributes, and resolved targets. Auto-refreshes in real time.

• Collapsible DOM tree filtered to HTMX-relevant nodes
• Click to inspect: shows hx-* attributes, resolved targets, internal data
• Element picker (Chrome DevTools style): click any element on the page
• Hover highlighting on the inspected page
• Request history per element (click to jump to Requests tab)

Event Timeline

Filterable timeline of all HTMX events with category color coding and expandable detail payloads.

• Category filters: Init, Request, XHR, Response, Swap, OOB, History, Transition, Error
• Timestamps relative to first event
• Click to expand full event.detail JSON
• Request correlation (linked request ID)
• Per-tab search (filters reset when switching tabs)

Swap Visualizer

Record DOM swaps with before/after snapshots and LCS-based diff view.

• Record / Pause controls
• Response HTML view
• Before / After DOM snapshots
• Line-by-line diff with add/remove highlighting (LCS algorithm)
• Swap strategy and target element info

Error Panel

Surface silent HTMX failures grouped by error type with badge counts.

• HTTP errors (4xx, 5xx) detected automatically
• Target not found errors
• Network timeouts and swap errors
• Click to jump to associated request

---

htmx 4.0 Support

Version 0.2.0 adds full support for the htmx 4.0 alpha. The extension auto-detects which version is running and adapts transparently.

	htmx 2.x	htmx 4.0
Event format	htmx:configRequest	htmx:config:request
Request tracking	XHR WeakMap	ctx object WeakMap
Detail structure	detail.elt, detail.xhr	detail.ctx.sourceElement, detail.ctx.response
Error events	10 separate events	Unified htmx:error + synthetic HTTP errors
Attributes	hx-ext, hx-request	hx-action, hx-method, hx-config, hx-status
Version badge	Blue pill	Purple pill

Both versions work simultaneously. No configuration needed.

---

Installation

From source

git clone https://github.com/atoolz/htmx-devtools.git
cd htmx-devtools
npm install
npm run build:chrome

Chrome / Edge / Brave / Arc:

1. Open chrome://extensions
2. Enable Developer mode
3. Click Load unpacked
4. Select the dist/ folder

Firefox:

npm run build:firefox

1. Open about:debugging#/runtime/this-firefox
2. Click Load Temporary Add-on
3. Select dist/manifest.json

---

Live Demo

Try the extension with interactive demo pages (no server needed):

• htmx 2.x Demo (XHR mock)
• htmx 4.0 Demo (fetch mock)

Install the extension, open a demo, press F12, and go to the HTMX tab.

---

How it works

Page Script          Content Script       Service Worker       DevTools Panel
(MAIN world)         (isolated)           (background)         (Preact UI)

Captures htmx    ->  Relays via        -> Routes messages,  -> Renders 5 tabs
events on            postMessage +         maintains state      with real-time
document             runtime.sendMessage   per tab              updates

• Page Script runs in the page's JS context via "world": "MAIN" content script. Listens to all htmx:* events (both 2.x and 4.x names), serializes element data, tracks requests via XHR/ctx WeakMaps, and batches messages every 50ms.
• Content Script bridges page and extension contexts via window.postMessage and chrome.runtime.sendMessage.
• Background Service Worker manages per-tab state, tracks request lifecycles with canonical event name mapping, correlates events to requests, synthesizes HTTP errors for 4xx/5xx, and routes data to the panel.
• Panel is a Preact + Signals app (~55KB) rendered inside chrome.devtools.panels.create().

---

Development

npm run dev            # Watch mode (rebuilds on changes)
npm run build          # Production build
npm run build:chrome   # Build + copy Chrome manifest + icons
npm run build:firefox  # Build + copy Firefox manifest + icons
npm run typecheck      # TypeScript type check

Local test server

node test/e2e/fixtures/test-server.js
# Open http://localhost:3456

Covers: GET/POST/PUT/DELETE, error scenarios (404, 500, timeout), all swap strategies, OOB swaps, polling, search with delay, contact editor (click-to-edit), and todo list.

---

Tech Stack

Language	TypeScript
Build	Vite (multi-entry, IIFE outputs for content/page/background scripts)
UI	Preact + @preact/signals (3KB gzipped)
Manifest	Chrome MV3 (also Firefox MV3 128+)
Target	Chrome, Edge, Brave, Arc, Opera, Firefox
Dependencies	7 devDependencies, 0 runtime dependencies

---

License

MIT