Senior TypeScript Developer with Production Xero API Experience

We are looking for a senior full-stack developer to take ownership of the next stage of development of an established SaaS application, with an immediate focus on accounting and Xero integration. (Australian region focus) We specifically want someone who has previously implemented a production Xero API integration. This is not a role for someone learning the Xero API during the project. The initial engagement will be a paid design and discovery phase. You will review the existing application, understand its finance architecture, assess the relevant Xero APIs and help finalise the integration design. We already have a high-level direction. The application will remain the operational source of truth, with Xero used as the connected accounting platform. We need an experienced developer to challenge and refine that approach and turn it into a detailed, practical implementation plan. The design should cover areas such as authentication and token management, organisation connections, data mapping, Xero entity mapping, synchronisation strategy, idempotency, webhooks, retries, reconciliation, error handling, auditability and production rollout. Following the design phase, the intention is for the same developer to build and implement the integration. You should have strong experience with TypeScript, Node.js, React, PostgreSQL, REST APIs, OAuth 2.0, external API integrations, asynchronous processing, error handling and working within an existing production codebase. Xero experience is mandatory. We are particularly interested in developers who have worked directly with the Xero Accounting API, OAuth and token refresh, invoices and bills, contacts, payments, tax mappings, webhooks, reconciliation, duplicate prevention and production integration failures. Experience only using Zapier, Make, n8n or other pre-built Xero connectors is not sufficient. You should also have enough accounting knowledge to understand how an operational system should integrate with an accounting platform without creating duplicate or conflicting financial records. When applying, please describe one Xero integration that you personally designed or built. Explain what system was integrated, what information moved between the systems, whether it was one-way or two-way, how authentication was handled and how duplicate transactions were prevented. Also describe one real production problem you encountered with a Xero or accounting API integration and how you resolved it. Please include a FIXED PRICE for phase one PLUS your hourly rate, availability per week, timezone and earliest start date. Individual experienced developers are preferred over agencies. The initial engagement will be relatively small and focused on architecture and planning. If successful, it will move directly into implementation and potentially ongoing development work.