Validation

Criterion uses Zod for schema validation at every step.

What Gets Validated

1. Input — Before rules are evaluated
2. Profile — Before rules are evaluated
3. Output — After a rule emits a result

Validation Flow

Input → Validate → Profile → Validate → Rules → Output → Validate → Result

If any validation fails, the engine returns an error result instead of throwing.

Input Validation

const inputSchema = z.object({
  amount: z.number().positive(),
  currency: z.string().length(3),
  customerId: z.string().uuid(),
});

Invalid input returns:

{
  status: "INVALID_INPUT",
  data: null,
  meta: {
    explanation: "Input validation failed: amount must be positive"
  }
}

Profile Validation

const profileSchema = z.object({
  threshold: z.number().min(0),
  maxRetries: z.number().int().positive(),
});

Output Validation

const outputSchema = z.object({
  risk: z.enum(["HIGH", "MEDIUM", "LOW"]),
  score: z.number().min(0).max(100),
});

If a rule emits invalid output, you get INVALID_OUTPUT — this indicates a bug in your rule logic.

Best Practices

1. Be specific — Use .positive(), .min(), .max() to catch edge cases
2. Use enums — z.enum() for finite sets of values
3. Document with descriptions — z.string().describe("ISO currency code")
4. Validate early — Stricter schemas catch bugs faster