How to Choose a Spend Firewall
Choosing a spend firewall is like choosing any control: you're buying decisions, not dashboards. Here are the questions that separate them.
The criteria
1. Decision path: deterministic rules or model-based? (Deterministic can't be injected.)
2. Latency: what's the p95 decision time?
3. Rule types: caps, velocity, allowlists, categories, time-of-day, approvals?
4. Audit: is every decision logged with the rule that fired?
5. Pricing: flat or metered? Any overage tier?
6. Deployment: hosted, self-host, or both?
7. Integration: HTTP, MCP, CLI — and your stack?
How to score
Weight by your priorities. For most teams: deterministic path, flat pricing, audit log.
FAQ
Why does deterministic matter?
A model-based decision path can be argued with or injected; rules can't.
What's the most underrated criterion?
The audit log — it answers questions six months later.
Related
Stop the next $12,400 night.
One API call (or MCP tool) in front of every agent transaction — APPROVED, BLOCKED, or FLAGGED, deterministic, ~5 ms, fully logged.
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