Rules
Expression-based hard rules for deterministic, sub-millisecond decisions.
Rules are the fastest stage of the decision pipeline. They evaluate in sub-millisecond time using a compiled expression language and produce deterministic, explainable outcomes.
How Rules Work
Rules are evaluated before the ML classifier. If any rule matches the input, the decision is returned immediately — no ML inference needed.
Each rule specifies:
| Field | Description |
|---|---|
name | Human-readable identifier |
condition | Expression evaluated against ctx (the input data) |
then | The decision to return if the condition is true |
reason_code | Why this decision was made |
priority | Lower numbers evaluate first (1 = highest priority) |
Writing Rule Expressions
All input fields are available under the ctx namespace:
// Simple comparison
ctx.amount > 10000
// String matching
ctx.action_type == "delete"
// Combining conditions
ctx.user_verified == true && ctx.risk_score < 0.3
// List operations
ctx.tags.exists(t, t == "urgent")
// Nested access
ctx.metadata.source == "api"Rule Evaluation Order
Rules are evaluated in priority order (lowest number first). The first rule that matches wins.
[
{
"name": "block_high_risk",
"condition": "ctx.risk_score > 0.95",
"then": "block",
"reason_code": "extreme_risk",
"priority": 1
},
{
"name": "auto_approve_verified",
"condition": "ctx.user_verified == true && ctx.amount < 100",
"then": "approve",
"reason_code": "trusted_user",
"priority": 2
}
]If no rules match, the decision falls through to the ML classifier.
When to Use Rules vs ML
| Use Rules When | Use ML When |
|---|---|
| The logic is simple and deterministic | The decision requires nuance or pattern recognition |
| You need guaranteed outcomes for specific conditions | Edge cases are hard to enumerate manually |
| Compliance requires exact, auditable logic | The decision should improve over time with data |
| Sub-millisecond latency is critical | Sub-100ms is fast enough |
Rules and ML work together — rules handle the obvious cases instantly, and the ML classifier handles everything else.
Validation
Rules are validated at creation time:
- Expression syntax must be valid
- The
thenvalue must match one of the decision type's options - The
reason_codemust match one of the allowed reason codes