modsignal documentation
modsignal watches public web pages and tells you when something you care about changes. You describe that change in a sentence. These pages cover what ships today. Nothing here is roadmap.
Quick start
- Create a free account. Three monitors, daily checks, no credit card.
- Add a monitor. Paste the URL, then describe the change you care about in one sentence:
“Tell me if a price or plan limit changes. Ignore copy edits.”Or start from a template. - Pick a check tier and an interval. The first run only captures a baseline. After that, you hear from us when your sentence comes true and at no other time.
- Read the alert: one sentence, the before, the after, and a confidence score. It arrives in your inbox, a Slack channel, a webhook receiver, or an RSS reader. Alert channels →
Monitors
Set one up, write a prompt that stays quiet, pick a schedule, use a template, and see what happens when a page keeps failing.
Check tiers
Exact diff, semantic AI, and browser agent. How each one decides what changed, what evidence it gives you, and which to pick.
Alerts & evidence
What is inside an alert and everywhere it can land: email, Slack, signed webhooks with heartbeats, and RSS.
Teams & billing
Roles and permissions, plan limits, what upgrades and downgrades actually do, and how cancellation works.
API reference
The REST API: authentication, rate limits, endpoints, webhook payloads, and signature verification.
How modsignal thinks
Two ideas run through everything on these pages. First, you describe the change, not the page region. There are no CSS selectors to maintain, and noise gets filtered by meaning rather than by pixel masks. Second, every alert carries its own proof: the sentence, the before and after, a confidence score, and on the browser-agent tier a screenshot of the page as it was checked. You should never have to open the page to check whether we got it right.