CriterionX

The Problem

Business logic is often scattered throughout codebases—conditionals in controllers, rules in services, validations everywhere. This makes systems hard to test, debug, and audit. When stakeholders ask "why did this decision happen?", there's no clear answer.

My Role

Creator & Maintainer — Designed and built the entire engine:

• Architected the core decision engine
• Built framework integrations (React, Express, Fastify, tRPC)
• Implemented observability with OpenTelemetry
• Created CLI and testing utilities

The Solution

A TypeScript-based decision engine that encodes business logic as pure, testable functions:

1. Declarative Decisions — Define rules for risk levels, eligibility, discounts, etc.
2. Explainable Results — Every decision includes a complete audit trail
3. Profile-Driven — Different behaviors by region, tier, or context
4. Framework Agnostic — Works with React, Express, Fastify, tRPC

Core Principles

Tech Stack

Monorepo Structure

@criterionx/core        # Core decision engine
@criterionx/server      # Server utilities
@criterionx/react       # React integration
@criterionx/express     # Express middleware
@criterionx/fastify     # Fastify plugin
@criterionx/trpc        # tRPC integration
@criterionx/cli         # Command-line tools
@criterionx/testing     # Test utilities

Use Cases

• Risk Assessment — Determine transaction risk levels with full audit trail
• Eligibility Rules — User qualification for features, tiers, or programs
• Pricing Logic — Dynamic discounts and pricing rules
• Compliance — Auditable decisions for regulated industries

What I Learned

• Developer Experience matters — Good APIs get adopted; bad ones get worked around
• Monorepo complexity — Managing multiple packages requires discipline
• Observability from day one — OpenTelemetry integration should be built-in, not bolted on