Show HN: An assertion library for E2E testing and real user monitoring
Faultsense Agent is a new assertion library designed for end-to-end testing and real user monitoring, allowing developers to validate user interactions and application behavior in production and test environments.
Background
- Faultsense is a new open-source assertion library designed for both end-to-end (E2E) testing and Real User Monitoring (RUM) — two distinct phases of web development: testing catches bugs before release, while RUM tracks how real users experience a live site.
- What makes this unusual is that most assertion libraries (like Jest's `expect` or Chai) are built solely for testing. Faultsense aims to reuse the same assertion patterns in production monitoring, letting developers write one set of checks that works in CI pipelines and also runs against live user traffic.
- The project was just posted on Hacker News as a "Show HN" — a category where founders share new side projects or startups for feedback from the tech community.
- For readers unfamiliar: "assertions" are the pass/fail checks inside tests (e.g., "expect(page).toHaveText('Login')"). E2E testing tools like Playwright or Cypress simulate a user clicking through a browser. RUM tools like Sentry or Datadog RUM capture real page loads, errors, and performance data from actual users.