Your Agent Returns 200 and Lies. Verify Before You Trust

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 — "200 OK Is the Most Dangerous Response in Production" : "The most dangerous failures look like success." And the measurements back it up. The Berkeley Function-Calling Leaderboard (BFCL v3) puts frontier-model structurally invalid tool calls at 2–5% even on clean benchmark prompts — higher in noisy production ( via Future AGI ). The arXiv paper Agent Behavioral Contracts reports that across 1,980 sessions , contracted agents caught 5.2–6.8 soft violations per session that uncontracted baselines missed entirely. So the question is not how do I see failures sooner . It's how do I stop accepting a success the agent never actually achieved. TL;DR A 200 and a "status": "done" are claims, not proof. Agents return both while doing the wrong thing — or nothing. Observability is tracking : it tells you a call happened. It can't tell you the result was correct. That's a control problem. verify() runs three checks before you accept success: (1) schema/contract (shape, types, enums/allow-list), (2) claim-vs-evidence (did the agent actually call th