Rules engine

Guide to using the Tracer rules engine to build dynamic, expression-based validation logic.

The rules engine is what risk and fraud teams use to change how transactions are approved or blocked, without touching application code. Each rule is a small expression that runs on every transaction Tracer validates — “block this MCC for this segment”, “send anything over R$ 50k to manual review”, “deny if the account is suspended”.

What changes in your operation: rule changes ship through an API endpoint, not through a release. An analyst can publish a new rule in the morning and see it evaluating real transactions within seconds. Every match is recorded, so a denied customer call six months later can be traced back to the exact rule that fired.

Trade-off to be honest about: you have to think in CEL (Common Expression Language) instead of Go, Python, or Java. The learning curve is short — most rules are one line — but the team writing them is no longer your application developers. The upside is no deploys, full audit, and the people closest to the policy own the policy.

Who is this guide for? Risk and fraud analysts who will write rules, developers integrating the validation call, and compliance officers reading the audit trail. The CEL examples get technical further down, but the lifecycle and decision logic are useful for anyone evaluating the product.

The Tracer rules engine evaluates validation logic written in — a type-safe expression language from Google. Expressions are compiled at rule creation and run during every transaction validation; you change behavior by updating rules through the API, without redeploying code.

Why use the rules engine

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• Flexibility: Create and modify rules without code deploys
• Performance: Compiled expressions evaluate in under 1ms each
• Type safety: Expression syntax validated at rule creation
• Every active rule runs: All matching rules are evaluated so the audit trail records every trigger, not just the winning category
• Scope-based: Apply rules to specific segments, accounts, or transaction types

By the end of this guide, you will:

• Understand rule engine concepts and evaluation flow
• Create and test expression-based rules
• Manage the rule lifecycle (DRAFT, ACTIVE, INACTIVE, DELETED)
• Apply best practices for rule management

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What is the rules engine

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The rules engine is the Tracer component responsible for evaluating expressions during transaction validation. It enables fraud analysts and risk managers to configure business logic that executes in real time—without requiring code deployments or engineering support.

How it works

In this flow:

• Load rules fetches all active rules from cache (or database on cache miss)
• Evaluate expressions runs all CEL expressions against the transaction context
• Collect matches gathers all rules that matched and determines the decision

Evaluation pattern

All active rules are evaluated against every transaction. There is no priority ordering or short-circuit evaluation. This ensures:

• Complete audit trail (all matching rules recorded)
• No information loss (analysts can see all triggers)
• Simple logic (no priority conflicts)

Decision precedence (highest to lowest):

1. DENY — any matching DENY rule wins outright.
2. Limit exceeded — if no DENY rule matched but any applicable limit is exceeded, the decision is DENY (rule precedence applies first; limits come in only when no DENY rule matched).
3. REVIEW — if no DENY rule matched and no limit was exceeded, any matching REVIEW rule wins.
4. ALLOW — if only ALLOW rules matched, the decision is ALLOW.
5. Default — if no rule matched at all, Tracer returns the configured DEFAULT_DECISION_WHEN_NO_MATCH (ALLOW unless explicitly set to DENY for fail-closed deployments).

matchedRuleIds in the response contains every rule that matched, regardless of the winning category, so audit consumers can see all triggers.

Why DENY beats REVIEW beats ALLOW. Precedence is fixed and not configurable, on purpose: it removes the “which DENY rule wins?” ambiguity at runtime and makes audit trivial — the response always identifies the strictest action that fired. The cost is that you can’t write “ALLOW rules that override DENYs”; if you need that pattern, the right answer is to make the DENY rule more specific instead.

Tracer returns decisions; it does not block transactions directly. Your system receives the decision and is responsible for taking the appropriate action (e.g., blocking, allowing, or queuing for review).

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Core concepts

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Before creating rules, understand the foundational elements.

Rules

A rule is a unit of business logic composed of:

• Expression - A type-safe expression that evaluates to true or false
• Action - What decision to return when the expression is true
• Scopes - Which transactions the rule applies to
• Status - The rule’s lifecycle state

Expressions

Expressions are written in CEL (Common Expression Language), a type-safe language that evaluates transaction context and returns a boolean value (true or false). CEL provides compile-time validation, so syntax errors are caught when you create the rule—not when transactions are being processed.

Example expressions:

amount > 10000

segment.segmentId == "high-risk-segment-uuid" && amount > 5000

merchant["category"] == "7995"

(merchant.category is the 4-digit ISO 18245 MCC code — "7995" is the MCC for betting/casino. Both merchant.category and merchant["category"] are accepted; the production examples use bracket notation by convention. If you need to match on a string label like "gambling", store it in metadata and match on that instead.)

Expressions have access to the full validation request context. The most commonly used variables are listed below. For the complete catalog of every nested field type and edge case (UUID format, enum values, omitempty behavior), see the ValidationRequest schema in the API reference.

Variable	Type	What you typically match on
amount	number	Decimal value converted to float64 for CEL evaluation (max ±2^53).
transactionType	string	One of CARD, WIRE, PIX, CRYPTO.
subType	string	Free-form (e.g., "international", "debit"). Empty string when not provided.
currency	string	ISO 4217 code (e.g., "BRL").
account.status	string	One of active, suspended, closed.
merchant.category	string	ISO 18245 MCC code, 4 digits.
merchant.country	string	ISO 3166-1 alpha-2 (e.g., "BR").
segment.segmentId	string UUID	Segment scope of the transaction.
metadata.<key>	any	Custom fields your integration passes in the request payload.

segmentId and portfolioId live on the top-level segment and portfolio variables, not on account. To match by segment, use segment.segmentId == "...", not account.segmentId == "...". The segment, portfolio, and merchant maps are only present when the request includes them — guard with has(segment) if your rule must run even when the request doesn’t carry one.

Expression evaluation is bounded by CEL_COST_LIMIT (default 10000). Expressions exceeding this cost are rejected at activation time with TRC-0085 / TRC-0088.

Expression examples by use case

Here are practical examples organized by business scenario:

Amount-based rules

// Block transactions above a threshold
amount > 10000

// Block high-value international transfers
transactionType == "WIRE" && subType == "international" && amount > 50000

// Review large cryptocurrency transactions
transactionType == "CRYPTO" && amount > 5000

Merchant-based rules

// Block gambling merchants
merchant.category == "7995"

// Block high-risk merchant categories
merchant.category in ["7995", "5967", "5966"]

// Review transactions from new merchant countries
merchant.country != "BR" && amount > 1000

Account-based rules

// Block suspended accounts
account.status == "suspended"

// Review transactions from newly created accounts
metadata.accountAgeDays < 30 && amount > 500

// Block closed accounts
account.status == "closed"

Combined conditions

// High-value transaction from high-risk segment
segment.segmentId == "high-risk-segment-uuid" && amount > 5000

// International PIX above threshold
transactionType == "PIX" && subType == "international" && amount > 10000

// Large card transaction to foreign merchant
transactionType == "CARD" && merchant.country != "BR" && amount > 3000

Using metadata

// Block transactions from untrusted devices
metadata.deviceTrust == "untrusted"

// Review first-time purchases above threshold
metadata.isFirstPurchase == true && amount > 1000

// Block transactions outside business hours (using metadata)
metadata.isBusinessHours == false && amount > 5000

// VIP customers bypass certain restrictions
metadata.customerTier == "vip" && amount < 50000

Metadata fields are provided by your integration. Design your payload to include the context your rules need.

Actions

Actions determine the decision when an expression evaluates to true:

Action	Description
ALLOW	Allow the transaction
DENY	Deny the transaction
REVIEW	Route to manual review

Scopes

Scopes define which transactions a rule applies to. A rule with no scopes is global and evaluates against every transaction. A rule with one or more scope objects evaluates only when the transaction matches at least one of them (OR semantics across scope objects).

Within a single scope object, the supported fields are:

• segmentId - Match transactions from a specific segment
• portfolioId - Match transactions from a specific portfolio
• accountId - Match transactions from a specific account
• merchantId - Match transactions to a specific merchant
• transactionType - Match specific transaction types (CARD, WIRE, PIX, CRYPTO)
• subType - Match specific subtypes (debit, credit, instant, etc.)

Matching semantics:

• Within one scope object: fields combine with AND. A field that is not specified is treated as a wildcard (matches any value). At least one field must be set — empty scope objects ({}) are rejected with TRC-0111.
• Across multiple scope objects on the same rule: they combine with OR. The rule matches if any scope object matches the transaction.

For example, a rule with two scopes — one targeting transactionType: CARD and another targeting transactionType: PIX — runs for both card and PIX transactions. A single scope with both segmentId AND accountId requires the transaction to match the segment AND the account.

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Rule lifecycle

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Rules progress through a defined lifecycle to ensure safe deployment.

States

State	Description
DRAFT	Not evaluated; expression can be modified freely
ACTIVE	Evaluated during validations; expression is immutable
INACTIVE	Not evaluated; preserved for audit trail; can be reactivated. The expression is still immutable in this state — to edit it, move the rule back to DRAFT via POST /v1/rules/{id}/draft.
DELETED	Soft-deleted; not returned by listings and cannot be recovered through the API, but the row is preserved in the database for audit trail.

Transitions

Transition	From	To	Description
activate	DRAFT, INACTIVE	ACTIVE	Start evaluation (validates expression)
deactivate	ACTIVE	INACTIVE	Stop evaluation
draft	INACTIVE	DRAFT	Re-edit a previously deactivated rule before reactivating
delete	DRAFT, INACTIVE	DELETED	Permanent removal (cannot delete ACTIVE rules)

Active rules must be deactivated before deletion. This prevents accidental removal of rules that are currently being evaluated.

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Create a rule

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Create rules using POST /v1/rules. Rules are created in DRAFT status by default.

A rule requires:

• name: A unique, descriptive name
• expression: A CEL expression that evaluates to true or false
• action: The decision to return when the expression matches (ALLOW, DENY, or REVIEW)
• scopes (optional): Limit which transactions the rule applies to

For complete payload structure and field details, see the API reference.

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Activate and deactivate rules

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After creating a rule, activate it to start evaluation. Deactivate rules to stop evaluation without deleting them.

Operation	Endpoint	Description
Activate	POST /v1/rules/{id}/activate	Start evaluating this rule
Deactivate	POST /v1/rules/{id}/deactivate	Stop evaluating (preserves for audit)

Deactivating a rule preserves it for audit purposes. Use delete only when you want to permanently remove a rule.

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List and query rules

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Query rules for management and auditing using GET /v1/rules.

Query parameters

Parameter	Type	Description
name	string	Filter by name (case-insensitive partial match)
status	string	Filter by status (DRAFT, ACTIVE, INACTIVE). DELETED is not a valid filter value.
action	string	Filter by action (ALLOW, DENY, REVIEW)
account_id	UUID	Filter by scope: account ID
segment_id	UUID	Filter by scope: segment ID
portfolio_id	UUID	Filter by scope: portfolio ID
merchant_id	UUID	Filter by scope: merchant ID
transaction_type	string	Filter by scope: transaction type (CARD, WIRE, PIX, CRYPTO)
sub_type	string	Filter by scope: subtype (e.g., debit, credit)
limit	integer	Items per page (default: 10, max: 100)
cursor	string	Pagination cursor from previous response
sort_by	string	Sort field: created_at, updated_at, name, status (default: created_at)
sort_order	string	Sort direction: ASC, DESC (default: DESC)

Get a specific rule

Use GET /v1/rules/{id} to retrieve the full rule definition including expression and scopes.

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Update a rule

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Update rules using PATCH /v1/rules/{id}. Rules can be updated in any status, with one important restriction:

The expression field is immutable in ACTIVE and INACTIVE states — deactivating a rule is not enough. To edit an expression, move the rule from ACTIVE → INACTIVE (POST /v1/rules/{id}/deactivate), then from INACTIVE → DRAFT (POST /v1/rules/{id}/draft). Only DRAFT rules accept expression updates. Once edited, reactivate with POST /v1/rules/{id}/activate.

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Delete a rule

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Delete rules that are no longer needed. Only DRAFT and INACTIVE rules can be deleted. ACTIVE rules must be deactivated first.

DELETE /v1/rules/{id}
X-API-Key: {api_key}

Deletion is permanent. Deleted rules cannot be recovered and do not appear in any listings.

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Best practices

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Follow these practices for effective, maintainable rules.

Naming

• Use descriptive names - The name should clearly state what the rule does
• Include context - Mention the scenario or transaction type
• Avoid abbreviations - Prefer clarity over brevity

Less clear	More clear
Rule 1	Block night transactions above BRL 5,000
Block high	Deny high-value weekend transactions
PIX rule	Review PIX transfers to new recipients

Expression design

• Keep expressions simple - Complex logic is harder to maintain
• Use scopes for filtering - Don’t repeat scope conditions in expressions
• Test edge cases - Consider boundary values and null fields

Lifecycle management

• Start in DRAFT - Test before activating
• Return to DRAFT before editing the expression - The expression is immutable in ACTIVE and INACTIVE; move the rule to DRAFT via POST /v1/rules/{id}/draft to edit, then reactivate
• Archive unused rules - Keep audit trail intact
• Delete only when certain - Deletion is permanent

Monitoring

• Review matched rules - Check which rules are triggering
• Monitor DENY rates - High deny rates may indicate overly aggressive rules
• Audit regularly - Ensure rules still align with business requirements

Common pitfalls when working with rules:

• “I edited the expression but the change didn’t take effect.” The expression is immutable in ACTIVE and INACTIVE states. Move the rule back to DRAFT via POST /v1/rules/{id}/draft, edit, then reactivate. INACTIVE alone is not enough.
• “My rule is ACTIVE but Tracer isn’t evaluating it yet.” The rule cache refreshes every ~10 seconds (RULE_SYNC_POLL_INTERVAL_SECONDS). Newly activated rules take up to that window to start running. Plan integration tests around this delay.
• “I want to delete an ACTIVE rule.” You can’t — POST /v1/rules/{id}/deactivate first, then DELETE /v1/rules/{id}. This forces a visible step where the rule stops affecting traffic before it disappears from listings.
• “My empty scope {} is being rejected with TRC-0111.” Every scope object must have at least one field set. To run a rule globally (against every transaction), omit the scopes array entirely — don’t pass {}.

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Quick reference

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Key endpoints, actions, and status information.

Endpoints

Operation	Method	Endpoint
Create rule	POST	/v1/rules
List rules	GET	/v1/rules
Get rule	GET	/v1/rules/{id}
Update rule	PATCH	/v1/rules/{id}
Delete rule	DELETE	/v1/rules/{id}
Activate rule	POST	/v1/rules/{id}/activate
Deactivate rule	POST	/v1/rules/{id}/deactivate
Draft rule	POST	/v1/rules/{id}/draft

Statuses

Status	Evaluated	Editable	Can delete
DRAFT	No	Yes	Yes
ACTIVE	Yes	Partial (expression immutable)	No (deactivate first)
INACTIVE	No	Partial (expression immutable; return to DRAFT first)	Yes
DELETED	No	No	N/A