What problem does it solve? Express API routes often drift out of sync with their React clients, skip input validation, or mishandle authentication tokens, leading to security gaps and broken contracts. This Skill enforces consistent invariants when implementing or reviewing Express 5 endpoints so server and client stay aligned. ## Core Features & Use Cases - Endpoint Implementation & Review: Guides changes to Express 5 routes while inspecting the affected route and the shared client service layer (src/services/api.ts). - Validation & Security Invariants: Enforces validation of body, params, query, files, and model output, correct handling of Firebase identity tokens versus Google OAuth access tokens, and bounds on uploads and AI request costs. - Contract Compatibility: Ensures error responses stay structured and secret-free, and that any contract change is applied to both server and client together. - Use Case: When adding a new upload endpoint that calls an AI model, use this Skill to validate file size and count limits, verify the correct token type, and update the React client's API service in the same change. ## Quick Start Ask the assistant to implement or review an Express endpoint following the express-api-contracts invariants, including validation, token handling, and the matching client contract update.
