A success gate verifies an AI agent's claimed success before your system accepts it. SuccessGate runs three read-only checks — schema/contract, claim-vs-evidence against the actual tool-call trace, and an optional post-condition probe — and turns a silent 200 into an explicit REJECTED with reasons. It's stdlib Python, needs no API key, moves nothing, and ships with a self-test you run in one command.
Here's the failure that started this for me. An agent in a CRM workflow reported {"status": "sent", ...} for an invoice. Clean run. Green dashboard. 200 OK. The invoice went to a customer id that wasn't on our allow-list — a near-miss hallucination the model was completely sure about. Nothing crashed. No exception, no stack trace. We found it days later, downstream, the expensive way.
That's not a rare bug. It's the default failure mode of agents in production, and it has a name now: silent-success drift. Cycles' writeup put it bluntly — ). The arXiv paper — don't let the agent spend. Part two was ). Tracking is near-universal. The ability to check the result lags by a third of the field. That delta is exactly where silent-success drift lives.
And it compounds. The arithmetic everyone quotes is real: at 95% per-step correctness, a 10-step workflow lands right only ~60% of the time; at 85%, about 20%. Structured output helps a lot — Agentmelt's production data puts action-success at 95–99% with structured output versus 70–85% parsing unstructured text (, don't let them send a bad transaction, and don't accept a success they never achieved.
Written with AI assistance and reviewed/edited by a human. The code in this post was run before publishing; the output block above is from a real run (2026-06-08). External figures — BFCL v3's 2–5% invalid tool calls (via Future AGI), Agentmelt's 95–99% vs 70–85% structured-vs-unstructured action success, the arXiv 2602.22302 figures (1,980 sessions, 5.2–6.8 soft violations/session, 200 scenarios), and Cycles' 89%/62% survey split — are as reported by those sources; check the originals before quoting.
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