Structured Output Contracts

Problem

Prompt-only parsing fails under drift. A small response change can break deserialization, violate required fields, or produce semantically invalid values.

Solution

Add a contract boundary between model output and application state. The boundary admits only structured, valid outputs.

Mechanism

• schema checks for structural validity
• invariants for domain-level correctness
• repair transforms for common failures
• bounded retries for controlled regeneration

Example

const result = await contract.run({
  prompt,
  schema: invoiceSchema,
  invariants: [hasPositiveTotal, hasCurrencyCode],
  maxRetries: 2,
  repair: [stripMarkdownFence, trimTrailingText],
});

if (!result.ok) {
  throw result.error;
}

return result.value;

This makes acceptance explicit and failure classes observable.