What problem does it solve? Adding a new API error in this NestJS codebase requires coordinated edits across the ErrorCode enum, a dedicated exception class, and the throwing call site — and mistakes break the API contract that the verification harness asserts exactly. This Skill walks through that workflow so every error renders as the contract-mandated { error: { code, message }, serverTime } envelope. ## Core Features & Use Cases - Contract validation: Checks the code against API_CONTRACT.md before adding it, since the harness asserts the contract's spelling exactly. - Enum and exception generation: Appends the UPPER_SNAKE member to error-code.enum.ts and creates a one-level-deep exception class whose Nest base class carries the HTTP status (401/404/409/422). - Call-site migration: Replaces bare Nest exception throws with the named exception that takes facts, not prose messages. - Use Case: A service throws new ConflictException('not enough stock') and the harness expects INSUFFICIENT_STOCK — run this Skill to add the enum member, create InsufficientStockException, and rewire the throw. ## Quick Start Ask the assistant to add the error code INSUFFICIENT_STOCK as a 409 in the holdings module and replace the bare ConflictException throw.
