AI agent policy enforcement: runtime controls for autonomous system actions

Where static policy tools stop

Static policy tools enforce access control at call time against a fixed rule set:

-- IAM policy: agent can call Stripe payment_intents endpoint
{
  "Effect": "Allow",
  "Action": "execute-api:Invoke",
  "Resource": "arn:aws:execute-api:*:*:*/*/POST/stripe/v1/payment_intents"
}

-- Stripe Restricted Key: only payment_intents create permission
{
  "permissions": ["payment_intents:write"],
  "metadata": { "agent": "billing-agent" }
}

Both rules evaluate correctly at every call: does the caller have permission to make this request? What they cannot evaluate: how much has this caller spent so far in this run, and should this call be denied because the total has exceeded the run's budget? That evaluation requires a counter that increments with each call — a stateful artifact that neither IAM nor Stripe Restricted Keys maintain. The Stripe Restricted Key doesn't know it has been used to make 499 previous charges today. It will approve call 500 the same way it approved call 1.

The three enforcement layers for AI agent vendor calls

Why OPA and Cedar don't enforce AI agent spend caps

OPA (Open Policy Agent) and Cedar (AWS's policy language) evaluate policies against request attributes: caller identity, requested resource, method, time. A Cedar policy like permit(principal == Agent::"billing-agent", action == Action::"POST", resource == Resource::"stripe/payment_intents") evaluates deterministically at each call. What it cannot express: a policy that says "deny if the cumulative spend for this run exceeds $500" — because OPA and Cedar evaluate each request independently against a static policy, they have no concept of this run's accumulated spend across prior calls. Implementing spend-cap enforcement in OPA would require an external data source that OPA queries at decision time — a live counter of spend for the current vault key. At that point, you've built a custom spend counter system that OPA is querying, which is equivalent to what a proxy with built-in spend tracking provides, plus the OPA evaluation overhead.

Implementing runtime policy enforcement in code

// Policy-as-code: runtime policies defined at run instantiation
async function issueRunPolicy(runContext: AgentRunContext): Promise<string> {
  const res = await fetch('https://proxy.keybrake.com/vault/keys', {
    method: 'POST',
    headers: {
      'Authorization': `Bearer ${process.env.KEYBRAKE_API_KEY}`,
      'Content-Type': 'application/json'
    },
    body: JSON.stringify({
      vendor: 'stripe',

      // Policy: total spend for this run cannot exceed the approved budget
      daily_usd_cap: runContext.approvedBudgetUsd,

      // Policy: only the exact endpoints this run is authorized to call
      // (narrower than the Stripe Restricted Key's general write permission)
      allowed_endpoints: runContext.allowedStripeEndpoints,

      // Policy: credential expires when the run is expected to complete
      // (even if someone captures the vault key, it's useless after expiry)
      expires_in: runContext.expectedDuration,

      // Policy: every call is attributed to this run in the audit log
      agent_run_label: `${runContext.agentName}/${runContext.runId}`
    })
  });

  const { vault_key } = await res.json();
  return vault_key;
}

// Different run types get different policies
const nightlyBillingKey = await issueRunPolicy({
  agentName: 'billing-agent',
  runId: crypto.randomUUID(),
  approvedBudgetUsd: 50_000,          // high cap: charges many customers
  allowedStripeEndpoints: [
    'POST /v1/payment_intents',
    'GET /v1/customers'
  ],
  expectedDuration: '4h'
});

const refundProcessorKey = await issueRunPolicy({
  agentName: 'refund-agent',
  runId: crypto.randomUUID(),
  approvedBudgetUsd: 5_000,           // lower cap: refunds on specific tickets only
  allowedStripeEndpoints: [
    'POST /v1/refunds'                 // cannot create new charges
  ],
  expectedDuration: '30m'
});

Each run type has its own policy: the nightly billing agent gets a $50,000 cap and 4 hours; the refund processor gets $5,000 and 30 minutes and is explicitly restricted to the refunds endpoint — it cannot create new payment intents even though the billing agent can. These policies are checked at call time against a live counter, not evaluated statically once. When the refund processor's $5,000 cap is exhausted, the next refund call is rejected before it reaches Stripe. The billing agent's policy doesn't change — it's a separate vault key with its own counter.

Revoking a policy mid-run without redeployment

// Emergency revocation: takes effect on the next proxied request
// No redeployment, no secret rotation, no infrastructure change
async function revokeAgentRun(keyId: string, reason: string): Promise<void> {
  const res = await fetch(`https://proxy.keybrake.com/vault/keys/${keyId}`, {
    method: 'DELETE',
    headers: {
      'Authorization': `Bearer ${process.env.KEYBRAKE_API_KEY}`,
      'Content-Type': 'application/json'
    },
    body: JSON.stringify({ reason })
  });

  if (!res.ok) throw new Error(`Revoke failed: ${res.status}`);

  // From this point, any Stripe call using this vault key returns 401.
  // Other runs using their own vault keys are unaffected.
  // The audit log records the revocation event with the reason.
  console.log(`Vault key ${keyId} revoked: ${reason}`);
}

// Wire into your anomaly detection alerting
async function onSpendAnomalyDetected(runLabel: string, spend: number): Promise<void> {
  const keys = await listActiveKeysForRun(runLabel);  // query audit log
  await Promise.all(keys.map(key =>
    revokeAgentRun(key.id, `spend_anomaly: $${spend} in last hour`)
  ));
}

How Keybrake fits

Keybrake is the proxy layer that enforces dynamic runtime policies between your agent and Stripe, Twilio, or Resend. It issues vault keys per run with run-specific policies (spend cap, endpoint allowlist, expiry), maintains the cumulative spend counter for each vault key, enforces the cap before forwarding calls to the vendor (pre-call enforcement, not post-hoc alerting), and stores every call in a queryable audit log with policy verdict. It does not replace IAM or Stripe Restricted Keys — those handle the static access control layer. Keybrake handles the dynamic layer: run-level spend bounds, per-run expiry, and mid-run revocation that takes effect in milliseconds without touching your infrastructure configuration.

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Related questions

Further reading

• AI agent observability — how the audit log produced by policy enforcement answers the four spend visibility questions that APM tools miss.
• AI agent zero trust — how zero-trust network access (ZTNA) and vendor API policy enforcement operate at different layers and why you need both.
• AI agent governance tools — the broader governance landscape for autonomous agent systems, including policy enforcement as one pillar among model behavior, access control, and audit.
• AI agent API key best practices — the seven operational controls, with spend-cap enforcement as the most important one for agents that make vendor API calls autonomously.