SymFynity exists because the loop doesn't look like a bug.
Every other kind of runaway failure announces itself. A memory leak gets OOM killed. An infinite loop pins a CPU. A bad query takes the database down and someone gets paged. The system's own pain is the alarm.
An agent burning money does none of that. It looks like a healthy service making successful API calls. Every response is a 200. Latency is fine. Nothing is saturated, nothing restarts, no page fires. The only symptom is an invoice, and invoices arrive after the fact — hourly at best, monthly at worst.
That gap is the whole problem. Every tool that watches spend watches it from the outside, after the money is already committed. Dashboards report. Alerts notify. Neither can refuse. By the time anything tells you, the only decision left is how to explain it.
Enforcement has to live in the request path.
A thing that can only report is a thing that can only apologise. To actually stop spend, you have to be standing in the traffic, willing to refuse — which means being a proxy, and means being fast and boring enough that people will put you in the path of production traffic.
Hence SymFynity: a small Rust binary that forwards your requests upstream unmodified, counts what comes back, and cuts the stream the moment a tenant would go over budget. It never stores your credentials. It never sees a request it doesn't forward. It is deliberately unambitious about everything except the one thing it does.
The picture we kept coming back to is a weir: a low dam across a river that doesn't stop the water, but regulates the flow and measures the discharge. Nothing about it is clever. It just stands there, in the current, and holds the line. That's the job.
Three commitments we designed around, not toward.
The proxy owes us nothing
SymFynity has no account check, no phone-home, and no degraded mode. It was built to work standalone before there was anything to sell, and that hasn't changed because the moment enforcement depends on a vendor being reachable, it isn't enforcement.
Your prompts aren't our business
Prompt content, response content, and tool arguments never leave your process — not by policy but by construction. SymFynity doesn't expose them, so nothing downstream can forward them. The source of the agent that talks to us is published so you can check rather than trust.
Boring where it counts
This sits in the path of your production traffic. That earns a specific kind of restraint: no clever rewriting of your requests, no opinion about your prompts, no features that would justify a bigger blast radius.
A small, founder-led shop.
SymFynity is built by a small independent team who spend their working lives in other people's cloud infrastructure. SymFynity was written because the tools for this problem all reported and none refused.
SymFynity and its agent are on GitHub, source-available under BSL 1.1. The design documents behind them are checked in alongside the code, so you can read the reasoning as well as the result.