How to Prevent Runaway LLM API Costs with a Self-Hosted Gateway

Why LLM API costs can get out of control

Building applications with OpenAI, Anthropic, or other AI providers is becoming easier. Controlling the cost of those API calls is a different challenge.

A retry loop, an autonomous agent running without limits, or an application with unexpected traffic can consume a significant amount of API credits before anyone notices.

For applications serving multiple users or tenants, this creates an important question:

How can you enforce spending limits before an API request reaches the AI provider?

The self-hosted gateway approach

One possible solution is to place a gateway between your application and the AI providers.

Instead of sending requests directly to OpenAI or Anthropic, your application sends them through a gateway that can enforce quotas, monitor usage, and manage provider connections.

A self-hosted architecture also gives the operator more control over where application prompts and responses are processed.

Four important capabilities

A practical LLM gateway can provide several layers of cost control:

1. Pre-flight budget enforcement

Check the tenant’s spending limit before forwarding a request upstream.

If the budget has been exceeded, reject the request immediately with HTTP 429. This prevents that request from consuming additional upstream API tokens.

1. Streaming usage metering

Many AI applications use Server-Sent Events (SSE) to stream responses.

Usage tracking should account for streaming responses too, rather than monitoring only completed requests.

1. OpenAI-compatible API

An OpenAI-compatible endpoint can make integration easier.

For example, applications can use a familiar /v1/chat/completions interface while routing requests through the gateway.

1. Provider failover

A gateway can also support fallback between providers when the primary provider encounters certain errors or rate limits.

This helps applications handle provider interruptions more gracefully.

An example implementation

AI Cost & Usage Gateway is a self-hosted implementation built with:

• Python and FastAPI
• HTTPX for asynchronous HTTP communication
• PostgreSQL for usage and quota data
• Server-Sent Events for streaming responses

The project focuses on tenant-level spending limits, streaming usage metering, API compatibility, and provider failover.

Who might benefit?

This architecture may be useful for:

• SaaS companies using multiple AI providers
• Developers building autonomous AI agents
• Multi-tenant applications
• Teams that need more control over API spending
• Organizations that prefer a self-hosted gateway

Final thoughts

AI usage costs should be controlled by architecture, not just by checking invoices at the end of the month.

Putting budget enforcement between your application and the AI provider can help prevent unexpected usage before requests are forwarded.

Project details: AI Cost & Usage Gateway

I’d be interested to hear how other developers handle per-user or per-tenant LLM budgets, especially when streaming responses are involved.