Cost monitoring without a proxy — an honest comparison

When a proxy is the right tool

• Per-request logging. You need to see every prompt and completion, attribute them to users, or tag them with metadata. A proxy captures this at the request level because it sits in the request path.
• Prompt inspection. You want to scan inputs for PII, enforce content policies, or log prompts for compliance. A proxy can read and transform the payload before it reaches the provider.
• Real-time rate limiting. You need to reject requests immediately when a budget is exhausted — not after a polling interval. A proxy can enforce this per-request because it controls the path.

When a proxy is the wrong tool

• Latency matters. Every proxy hop adds latency — typically 20–100ms per request, more under load. For latency-sensitive workloads, this is a direct cost to user experience.
• Prompt exposure is a concern. A proxy sees every prompt and completion by definition. If your threat model excludes third-party prompt access, a proxy is a non-starter. Even with encryption in transit, the proxy operator can read your data at the application layer.
• You already have production keys deployed. Migrating traffic through a proxy means changing every API endpoint in your application. For teams with keys deployed across services, this is an integration project, not a configuration change.

The ModelBrake tradeoff

ModelBrake never sits in the request path. You give us a read-only admin credential — the one that can see spend but cannot call models. We poll the provider's usage API on a schedule: every 15 minutes for OpenAI, Anthropic, and Vercel, every 6 hours for Bedrock (constrained by Cost Explorer data availability).

The tradeoff is a detection gap. Spend that occurs between polls is not visible until the next poll completes. For most teams, 15 minutes is fast enough to catch a four-figure spike before it becomes a five-figure invoice. For teams that need per-request enforcement, a proxy is the right tool and ModelBrake is not.

Side-by-side