# Countersign

- **Event:** [Meta Global AI Developer Hackathon](https://cerebralvalley.ai/e/meta-ai-developer-hackathon)
- **When:** Oct 26, 12:00 PM – Nov 4, 9:00 PM (EST)
- **Where:** Online
- **Team:** [Walter Guevara](https://cerebralvalley.ai/u/Lucumax)
- **GitHub:** https://github.com/Lucumax/countersign
- **Demo video:** https://www.youtube.com/watch?v=ARWHVxYnWyM
- **Gallery:** https://cerebralvalley.ai/e/meta-ai-developer-hackathon/hackathon/gallery
- **Page:** https://cerebralvalley.ai/e/meta-ai-developer-hackathon/hackathon/gallery/2386

Built for my world: payments, where a forged supplier instruction is one of the oldest frauds, and AI agents are starting to pay invoices.

Countersign is an open-source safety layer for an AI agent that pays supplier invoices. The agent prepares the payment; a person releases it. Jordan, an illustrative finance lead at a small packaging plant, relies on an AI purchasing agent (synthetic suppliers, sandbox money). The agent, on Meta's Muse Spark, reads supplier email and proposes payments. A person approves a budget once: three suppliers, 1,500 sandbox dollars, no single payment above 900. Inside it, with nothing outside read yet, a routine payment runs on its own. Once the session has read a supplier message, every payment the agent proposes waits for approval of that exact payment, once, whatever the message claims. A payment beyond the budget, or to a supplier nobody approved, is refused. Every decision is written to a hash-chained receipt log, and Verify names the earliest record that fails.

Live observation: in a recorded live console session (muse-spark-1.3, Nov 4, 2026), Muse Spark read the freight email m-105 and proposed only the 150.00 owed to initech-freight, never the 900.00; Countersign held it for a person. Neither of the two takes completed the pre-registered protocol (scenario 1 was run first by mistake in both, and take 2 was stopped before the proposal), so this is an observation, not a measured result. Both takes are published in evidence/console_live/. The video's approval click is labelled stand-in footage.

The measured result, from a saved live suite of 135 agent runs: in 108 runs on 12 synthetic scam emails (three setups, one without Countersign) Muse Spark never proposed the scammer's payment. So in these runs Countersign blocked no attack. What it adds is a release step the agent can't skip, bound to the exact payment, the same idea as keeping preparing and releasing a payment in separate hands (inspired by that practice; the console does not identify who approves). The cost: every payment that follows an email waits for a person. The 27 runs that were not scam emails ended as they should (18 genuine-invoice runs paid as owed, 9 paid nothing because nothing was owed).

The value shows in a control with no model (12 hand-written attacker calls: 7 held, 5 refused, 0 executed) and a replay of 8 attacker calls written by another AI model: a limit and supplier list alone paid 5 of 8 on changed terms (2,360 sandbox dollars), Countersign none. Authored calls, not a running agent. The demo runs with no key, with a labelled scripted stand-in for the model.

Limits: sandbox money, a scripted approver in the measured runs, counts and not rates. A person can still approve a bad payment. A demonstration, not a benchmark. MIT licence. Not affiliated with or endorsed by Meta.

LLM-assisted development I specified, directed and reviewed.

## More from Meta Global AI Developer Hackathon

- [Plurid](https://cerebralvalley.ai/e/meta-ai-developer-hackathon/hackathon/gallery/2383)
- [RecallLoop Team](https://cerebralvalley.ai/e/meta-ai-developer-hackathon/hackathon/gallery/2384)
- [BRACU ABYSS](https://cerebralvalley.ai/e/meta-ai-developer-hackathon/hackathon/gallery/2385)
- [Uplink](https://cerebralvalley.ai/e/meta-ai-developer-hackathon/hackathon/gallery/2387)
- [StatWise Innovators](https://cerebralvalley.ai/e/meta-ai-developer-hackathon/hackathon/gallery/2388)
- [ReproTrace AI](https://cerebralvalley.ai/e/meta-ai-developer-hackathon/hackathon/gallery/2389)

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Markdown version of https://cerebralvalley.ai/e/meta-ai-developer-hackathon/hackathon/gallery/2386. Site index for agents: https://cerebralvalley.ai/llms.txt · full text: https://cerebralvalley.ai/llms-full.txt