Slack Alerts When a Website Changes — the No-Code Way
Point Rendex Watch at any URL, then let Zapier's native Website Changed trigger post to Slack the moment the page actually changes. Each alert already carries a plain-English summary of what changed and a cropped image of the exact spot — so there's no webhook to host, no signature to verify, and nothing to render yourself.
Last updated 2026-07-21
The Problem
You want to know when a competitor's pricing page, a supplier's terms, or your own status page changes — and you want it in Slack, not in one more monitoring dashboard nobody opens. Doing it yourself means a schedule to re-fetch the page, a headless browser to render the JavaScript-heavy markup, somewhere to store each snapshot, a pixel-diff step to compare them, and a way to mute the noise from rotating banners. That's a small service to build and babysit — and a plain HTTP check can't even see a visual change.
The Solution
Rendex Watch is that whole service, managed, and Zapier already speaks to it natively. Create a watch on the page, then build a Zap with the Rendex 'Website Changed' trigger — Zapier polls your watches for you and starts the Zap for each new change. Because Rendex does the change detection, the trigger hands Zapier a ready-to-use plain-English summary, a cropped image of just the part that moved, and full before/after/overlay images. Drop a Slack step after it, map the summary into the message text and the crop into an image, and you're done. No webhook endpoint, no signature check, no headless browser — one watch and one Slack step.
How the Workflow Runs
Rendex Watch checks
Rendex re-renders your URL on a schedule — daily on Free, as fast as every 30 min on Pro. You never run a cron.
Website Changed fires
The native Zapier trigger polls your watches and starts the Zap for each new change — carrying a plain-English summary, a crop of the change, and before/after images.
Post to Slack
A Slack 'Send Channel Message' step posts the summary and attaches the crop image — the team sees what moved without opening the site.
Input → Rendered Output
Rendered by Rendex
What You Need
- A Rendex API key. Watch runs on Free (1 watch, a daily check); Starter and Pro add more watches and faster checks.
- The Rendex app connected in Zapier — search "Rendex" in the app directory and paste your key once.
- At least one Rendex Watch created on the page you care about (from the dashboard, the API, or the Create Website Watch action).
- A Slack workspace connected in Zapier and the channel you want the alerts posted to.
What This Recipe Uses
Real-Chrome visual diff
Watch re-renders the full page in real Chrome and compares it to the baseline — it catches a swapped price or a broken layout that an HTTP status check never could.
Plain-English 'what changed'
Every change arrives with a one-line summary (e.g. "The Pro price rose from $19 to $24"), so your Slack alert reads like a sentence, not a diff score.
A crop of just the change
The trigger includes a cropUrl — a tight image of the exact region that moved — so you can attach the change itself to Slack, not the whole page.
Native polling, one credit per check
The Website Changed trigger is native to Zapier — no webhook to host, no signature to verify. You set the interval; Rendex owns the schedule, render, storage, and diff.
Build It
# Zapier — no code
Trigger: Rendex — Website Changed
(fires per change on the watches connected to your key)
Action: Slack — Send Channel Message
Channel: #market-intel
Message: ":eyes: *{{ Rendex: Name }}* changed\n{{ Rendex: Ai Summary }}\n{{ Rendex: Url }}"
# Attach the crop of exactly what changed:
Image URL (or Upload File): {{ Rendex: Crop Url }}
# Tip: no Ai Summary on a run? Fall back to {{ Rendex: Diff Overlay Url }}.# Create a watch on the page you want to monitor. After this, the Zapier
# "Website Changed" trigger polls it automatically — no webhook needed.
# diffMode "visual" = pixel diff + a highlighted overlay; threshold is the
# 0..1 noise floor the diff must EXCEED to count as a change.
curl -X POST https://api.rendex.dev/v1/watches \
-H "Authorization: Bearer rdx_YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"url": "https://competitor.com/pricing",
"name": "Competitor pricing",
"intervalMinutes": 1440,
"diffMode": "visual",
"threshold": 0.02
}'
# => { "success": true, "data": { "id": "<uuid>", "lastStatus": "queued", ... } }
#
# intervalMinutes floor by plan: Free 1440 (daily), Basic 180, Starter 60, Pro 30.
# You can also create the watch with the "Create Website Watch" Zapier action,
# or from the Rendex dashboard — the trigger picks it up either way.100 free API calls/month — no credit card required. Get your API key and start building.
Reach for this whenever a human needs to see that a page moved — a competitor's pricing, a vendor's SLA or terms, a regulator's notice board, your own marketing site after a deploy. On Zapier the appeal is how little there is to it: the Website Changed trigger is native polling, so unlike a raw webhook you don't stand up an endpoint or verify a signature — Zapier just checks your watches and fires. And because Rendex has already done the diff, the event arrives pre-enriched: a one-line summary of what changed, a crop of exactly the region that moved, and before/after images. That means your Slack message can be genuinely useful — the summary as the text, the crop as the picture — instead of a bare 'something changed' ping. Watch runs on the free tier (one watch, a daily check), and each scheduled check draws a single credit from the same pool as your screenshots and PDFs, so a basic monitor costs almost nothing to run.