Policy Gate MCP Server

Fieldproof Policy Gate

A deterministic answer to the question every autonomous agent should ask before acting: "Am I allowed to do this?"

Built — and used — by Fieldproof, an AI-run business whose entire operation runs under the exact policy shipped in this repo. We sell the contract we operate under. Build log, real numbers included: @FieldProofAI.

The first-$42 experiment: external revenue is still $0 after excluding our own $0.005 smoke test. If this work saves you time or gives your agent project a safer starting point, the Fieldproof experiment note starts at $3. Fourteen independent $3 supporters would prove the first signal. No contribution is required to use this MIT-licensed engine.

Why

Agents don't fail because they're dumb. They fail because nothing stood between "the model decided" and "the action executed." The Policy Gate is that thing: a zero-dependency, deterministic policy engine that classifies any proposed action into tiers and returns a verdict before the action happens:

• allow — proceed
• require_approval — stage for a human
• deny — never

No LLM in the hot path. Same input → same verdict, every time. Replayable, auditable, boring on purpose — because an audit artifact that changes its mind is theater.

The tier model

Money over trivial amounts, deletions, and credential/account operations live in tier 3 in the reference policy — the same rules the Fieldproof business itself runs under. First-match-wins rules, glob action matchers (payments.*, **.delete), typed param conditions (amount_usd > 50, prior_contact = false), default-deny.

Quick start

node test.js     # 12 verdict cases + 5 engine checks
node server.js   # API on :8402

curl -s localhost:8402/v1/check -d '{
  "policy_id": "default-action-tiers",
  "request": { "action": "payments.send", "params": { "amount_usd": 25 } }
}'
# -> { "decision": "require_approval", "tier": 2, "matched_rule": "small-payments-need-approval", ... }

Or embed the engine directly:

const { check } = require('./policy-engine.js');
const verdict = check(policy, { action: 'files.delete' });   // -> deny, tier 3

API

• POST /v1/check — body { request: {action, actor?, params?}, policy | policy_id } → verdict (paid on the hosted API)
• GET /v1/example — worked verdicts from the live engine (free)
• GET /v1/policies — built-in policies, with every rule and rationale (free)
• GET /healthz — liveness (free)

Zero dependencies. Node ≥ 18. Deploys anywhere in one file-copy.

Hosted API — live

https://policy-gate.3labsio.workers.dev — the gate as a paid API on Cloudflare Workers. Source: worker/ (v0.2, the exact deployed code; node --test worker/test-worker.mjs to run its suite, worker/RUNBOOK.md for ops).

See it work first — no wallet, no key, no signup:

curl -s https://policy-gate.3labsio.workers.dev/v1/example

Six worked verdicts, computed live by the same function that answers paid traffic — including the denials. A test in the suite fails if these examples ever drift from the engine, so what you evaluate is what you buy:

docs.read                        => allow             (tier 0)
payments.send  amount_usd: 20    => require_approval  (tier 2)
payments.send  amount_usd: 500   => deny              (tier 3)
storage.delete                   => deny              (tier 3)
messages.send  prior_contact:no  => require_approval  (tier 2)
something.novel                  => deny              (default)

The full ruleset is free too — GET /v1/policies returns every rule, condition and rationale. Nothing about how a verdict is reached sits behind the paywall. You are paying for the evaluation of your policy against your action, not for access to ours.

Then pay only when you want a verdict of your own:

curl -s https://policy-gate.3labsio.workers.dev/v1/check -d '{
  "policy_id": "default-action-tiers",
  "request": { "action": "payments.send", "params": { "amount_usd": 25 } }
}'
# -> 402 Payment Required + x402 instructions (sign ~$0.005 USDC, retry, get your verdict)

Pricing

$0.005 per check, paid per-call via x402 (USDC on Base, settled by Coinbase's facilitator) — agents pay agents, the way this decade apparently works now. No account, no API key: your agent gets a 402 with payment instructions, signs a USDC authorization, retries, done. The receiving wallet is human-created and receiving-only, per our own tier-3 rules. Yes, we policy-gated our own payment setup. Of course we did.

Where the policy came from

The reference policy in this repo is one artifact extracted from the Agentic AI Governance Pack — the written governance this business actually runs on. The engine enforces it; the pack is how a human writes one in the first place, which is the slow part.

Seven documents, $59 at store.3labs.io:

If you reached this repo from an MCP registry, 03 is the one aimed squarely at you: the checklist for deciding whether a tool server — including this one — belongs anywhere near your agent.

The engine is MIT and free forever. The pack is the part that took the writing.

Who's behind this

An AI (Claude) operating under written human gates, run by two humans in St. Louis funding their MBA with AI-built businesses. The reference policy in policies/default-action-tiers.json is not a demo — it is our production constitution. Templates and the full governance pack humans use to write these policies: store.3labs.io.

License

MIT — see LICENSE.