Faultsense. Catch the bugs that don't throw.

Add assertions to your HTML. Real user sessions validate them in production, so you see which feature broke, for which users, on which release.

“Are we sure checkout isn't broken?”

Error tracking is quiet. RUM looks normal. The funnel dipped but won't say why. So you click through on desktop, then on mobile, then ask the user for a screen recording. Faultsense answers from the assertions you wrote, validated in real user sessions, per release, per cohort. Not a chart you have to read. A structured fact: assertion, release, cohort, pass/fail.

Written like a test. Reported like a production signal. Per release, per cohort.

Instrument in the HTML you already ship

Test-shaped on the way in. Observability on the way out. Per release, per cohort.

Your HTML What your agent gets

<button
  fs-assert="checkout/submit-order"
  fs-trigger="click"
  fs-assert-added-success=".order-confirmation"
  fs-assert-added-failure=".error-message[text-matches=try again]">
  Place Order
</button>

{
  "assertion": "checkout/submit-order",
  "status": "degraded",
  "release": "6.0.0",
  "pass_rate": 0.88,
  "previous_pass_rate": 0.99,
  "failures": {
    "total": 312,
    "breakdown": {
      "safari_ios":     { "rate": 0.105, "count": 287 },
      "chrome_desktop": { "rate": 0.98,  "count": 12 },
      "firefox":        { "rate": 0.97,  "count": 13 }
    }
  }
}

Works with any web framework that renders HTML

Thousands of sessions

checkout/submit-order

degraded

One fact

Demo

Assert what should happen. Get structured insights when it doesn't.

Interact with the list. Watch the dev tools panel capture each assertion live.
(In production, assertions stream to your agent. The panel is just for local debugging.)

Your agent writes the attributes.

Install the skill. It picks the right trigger, the right assertion type, the right modifiers.

View on GitHub · Marketplace

From failure to shipped fix.

Here's what one incident looks like end to end.

Deploy

Agent deploys v6.0.0 → Real users interact → Events flow to Faultsense

Detect

Faultsense detects regression → checkout/submit-order degraded on release 6.0.0

Diagnose

Agent calls diagnose_assertion("checkout/submit-order"). One tool call:

Fix

The agent opens a draft PR. Your reviewer ships. Faultsense sees the assertion pass again.

Partners, not users (yet).

We're not taking signups. We're taking design partners. About a dozen teams shipping real traffic, ready to battle-test Faultsense in production. Founder-led instrumentation, direct Slack access, and the paid backend is free while you do.

Tell me:

what you're shipping
what broke last week
what agent tools your team uses (bonus: Claude Code, Open Code, Cursor or Codex)

Email Mitch →

I read every one. Expect a reply within 48 hours, usually same day.