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Scenario: Merchant Category Drift

A research agent is approved to buy datasets — and starts buying compute. Nobody changed the code; the agent just drifted.

What happens without a firewall

The compute purchases settle under the same key as the datasets. The bill grows; the categories blur; procurement finds out at month-end.

The category rule says research agents may spend on data only. The compute purchase → BLOCKED.

category_rule (data = allowed, compute = blocked for this agent) + per-agent daily ceiling.

{
  "decision": "BLOCKED",
  "reason": "Category compute not allowed for this agent",
  "rule_id": "rul_cat_03",
  "transaction_id": "txn_77aa01"
}

Category rules keep agents in their lane.

Is category drift common?

Yes — agents pick tools and services based on task, not policy. Category rules make the policy explicit.

What if the agent legitimately needs compute?

Change the rule deliberately — the point is that drift requires a decision, not an accident.

One API call (or MCP tool) in front of every agent transaction — APPROVED, BLOCKED, or FLAGGED, deterministic, ~5 ms, fully logged.