Tool Contracts for Agents That Touch Money

When an agent can quote, book, or pay, freestyle JSON is a liability. Thin margins make disputes expensive.

The pattern

Give every money-adjacent tool a strict contract—and eval it.

1. Define input/output schemas with required fields, currency, and locale.
2. Enforce idempotency keys and explicit confirmation before irreversible side effects.
3. Refuse when confidence, missing fields, or policy checks fail—then assert those refusals in the golden set.

Minimum viable contract checks

schema valid  →  idempotent retry  →  confirmation gate  →  refusal path tested

No tool ships without a failure mode in CI.

When to use

• Checkout, quoting, collections, or ticket-creation agents
• Ops copilots that write to billing or ERP systems
• Extends Eval Gates into agent tool surfaces

Trade-offs

• More engineering upfront than demo agents
• Prevents the “it worked in the recording” incident that burns trust

If money can move, the contract is the product.