A $10/month kill switch for GCP

I give AI agents access to cloud providers. That's useful — and dangerous. An agent with GCP credentials can provision resources, and a misconfigured loop can run up a bill before you notice. I wanted a hard ceiling: if total monthly spend crosses $10, shut everything down. No human in the loop. No "warning" email I might miss.

The setup

GCP has budget alerts that can publish to Pub/Sub. A Cloud Function subscribes to the topic and fires when spend hits a threshold. The function has one job: disable billing on the project. No billing = no resources can run.

The implementation is a single Python function deployed to Cloud Functions (gen2). It receives the Pub/Sub budget notification, checks if the cost has crossed the threshold, and if so, calls cloudbilling.projects.updateBillingInfo with an empty billing account — effectively unlinking the project from its payment method.

I set three alert thresholds: 50%, 90%, and 100% of the $10 budget. The 50% and 90% alerts send Telegram messages so I can see spend approaching the limit. The 100% alert triggers the kill.

The IAM

The function's service account needs billing.resourceAssociations.delete at the project level and billing.accounts.get at the billing account level. I also granted the 6 project-level roles the AI agent needs for its actual work — compute, storage, IAM viewer, etc. — on the same SA, so there's one credential set for both "do work" and "kill if too expensive."

The budget itself is created via gcloud billing budgets create with a Pub/Sub notification channel. The whole thing — SA, IAM bindings, budget, Pub/Sub topic, subscription, and Cloud Function — deploys from a single shell script that runs in Cloud Shell.

Testing

The function has a DRY_RUN flag. First deployment runs with DRY_RUN=true: it logs what it would do and sends a Telegram message but doesn't actually kill billing. I triggered a test alert, confirmed the message arrived, then flipped to DRY_RUN=false.

Total time from "I want a cost ceiling" to "kill switch is armed": about 45 minutes. Monthly cost of the kill switch itself: effectively zero (Cloud Functions free tier covers the occasional invocation).

Why this matters

If you're giving AI agents cloud credentials — and you should, because that's where the leverage is — you need a circuit breaker that doesn't depend on you being awake. A budget alert email is not a kill switch. A Cloud Function with billing.resourceAssociations.delete is.