Build Backend

Domain contract

• Trace changed behavior from input through validation, authorization, domain logic, persistence, side effects, and response mapping.
• Preserve published contracts unless the request explicitly changes them.
• Enforce authorization at a trustworthy server-side boundary.
• Validate untrusted input before domain or persistence operations.
• Keep multi-write operations atomic where partial state would violate invariants.
• Design external calls for timeouts, retries, idempotency, and duplicate delivery where applicable.
• Do not query or mutate production data without explicit authorization.
• Do not fabricate migration, framework, or API behavior; verify version-sensitive details.
• Avoid broad service abstractions that are used once.

Execute and verify

Read references/contracts-and-data.md when the task changes a public API, schema, transaction, job, or integration.