Senior or Staff Fullstack Software Engineer to work with one of our most important clients in London. Hybrid work model based in Fitzrovia (London), with four days per week in the office and one day working from home. Responsibilities include designing and implementing services for revenue workflows, payment logic, invoicing pipelines, building APIs and full-stack features, integrating with internal and external systems, implementing financial logic, and leveraging AI tools. Must-have: 7+ years software engineering, TypeScript-native ORM (Drizzle/Prisma), Node.js, GraphQL, PostgreSQL, AWS, event-driven systems, data modeling, secure service architecture. Nice-to-have: experience with financial systems, Salesforce/NetSuite/Concur, serverless, multi-party payments, Claude API and Cursor, AI tooling proficiency. Benefits include High-Impact Environment, professional development, flexible culture, global opportunities, etc.