How to Cap an AI Agent's Spending at Exactly $50

When your AI agent needs a strict spending cap, the question is not just how to enforce the limit. It is how to contain the blast radius if something goes wrong.

Here is how to create a card where $50.01 is impossible because the money simply is not there.

The API call

curl -X POST https://api.usesignets.ai/v1/cards \
  -H "Authorization: Bearer sk_live_xxx" \
  -H "Content-Type: application/json" \
  -d '{
    "agent_id": "agent_abc123",
    "limit": 50.00,
    "currency": "USD"
  }'

The response

{
  "id": "card_7x9k2m",
  "agent_id": "agent_abc123",
  "last_four": "4829",
  "limit": 50.00,
  "balance": 50.00,
  "currency": "USD",
  "status": "active",
  "created_at": "2026-01-15T14:32:00Z"
}

The card has a ceiling of $50. Not a policy. Not a rule. A balance.

What happens at $50.01

Your agent attempts a purchase that would exceed the limit:

{
  "event": "authorization.declined",
  "card_id": "card_7x9k2m",
  "amount": 12.99,
  "merchant": "api-provider.com",
  "reason": "insufficient_funds",
  "balance_before": 47.50,
  "balance_after": 47.50
}

The decline happens at the card network level. Visa rejects the transaction because the funds do not exist on this card. The balance is the limit.

Blast radius: the real question

Both dedicated virtual cards and network tokens (like those from Mastercard's MDES) can have network-enforced spending limits. The technology for enforcement is similar. The key difference is not how limits are enforced. It is what those limits protect.

A network token attached to your credit line exposes that entire credit line. Even with a $50 limit on the token, a misconfiguration, a bug, or a compromised credential elsewhere could expose thousands of dollars. The blast radius is your full credit limit.

A dedicated prepaid card with $50 loaded has a blast radius of exactly $50. The money beyond that does not exist on this card. There is nothing else to expose.

Consider two scenarios:

Network token on credit line:

• ▸Token has $50 limit (network-enforced)
• ▸Your credit line is $10,000
• ▸If the token limit fails for any reason, exposure is $10,000

Dedicated prepaid card:

• ▸Card has $50 balance
• ▸No credit line attached
• ▸Maximum possible exposure is $50, regardless of what fails

The question is not whether limits can be enforced at the network level. They can, in both cases. The question is what happens when something goes wrong.

Nekuda's Mandates and Mastercard's Consumer Portal both offer sophisticated spending controls. These can be network-enforced. But if those controls protect access to a larger credit line, the blast radius remains the full line.

When to use each approach

Network tokens with limits are useful for:

• ▸Agents using your existing payment methods
• ▸Category restrictions (only allow SaaS merchants)
• ▸Velocity controls (max 10 transactions per hour)
• ▸Approval workflows (human sign-off over $500)

Dedicated prepaid cards are useful for:

• ▸Minimizing blast radius to exactly the funded amount
• ▸Untrusted or experimental agents
• ▸Situations where exposure must be capped regardless of what else fails
• ▸Clear accounting separation per agent or task

The code, one more time

curl -X POST https://api.usesignets.ai/v1/cards \
  -H "Authorization: Bearer sk_live_xxx" \
  -H "Content-Type: application/json" \
  -d '{"agent_id": "agent_abc123", "limit": 50.00, "currency": "USD"}'

One request. One isolated balance. Blast radius contained.

Your agent cannot spend $50.01 because the money is not there. No larger credit line at risk. No exposure beyond what you funded.

Get API access to start issuing capped cards for your agents.